Instr 12205 Elements, Transmitters, Transducers, Displacers

Elements, Transmitters, Transducers, DisplacersModule 12205 Instrumentation Trainee Task Module 12205 ELEMENTS, TRANSMITTERS, TRANSDUCERS, Objectives Upon completion of this module, the trainee will be able to: 1. Given the basic instrument channel, describe the major functions of the detectors, transducers, and transmitters. 2. Define the following commonly encountered measurement terms: a. Accuracy b. esolution c. e!roducibility d. "ensitivity e. es!onsiveness #. $ist four classifications of errors associated with instrumentation. %. Given a ty!ical measurement system a!!lication, determine whether the !rocess variable is being measured directly or whether it is being inferred. &. "tate the significance of a calibration stic'er on a device. (. )e able to e*!lain the !rinci!le of o!eration of an orifice !late. +. Describe the relationshi! between flow and differential !ressure in a fluid system. ,. -*!lain at least four methods of measuring !ressure or differential !ressure. .. Discuss three common methods for measuring tem!erature. 1/.$ist the advantages and disadvantages of thermocou!les and 0D1s. 11.Given a diagram, describe the o!eration of a current to !ressure or a !ressure to current transducer. 12.12. Describe the basic function of a transducer. 1#.$ist the standard in!ut and out!ut voltages and currents for most transmitters. 1%.Given a diagram, e*!lain the o!eration of a transmitter in a system. 2rere3uisites "uccessful com!letion of the following 0as' 4odule5s6 is re3uired before beginning study of this 0as' 4odule: 788- 8ore 8urricula9 788- 0as' 4odule 122/1, Craft-Related Mathematics; 788- 0as' 4odule 122/2, Instrumentation Drawings and Documents II; 788- 0as' 4odule 122/#, Principles of Welding; 788- 0as' 4odule 122/%, Process Control Theory. e3uired "tudent 4aterials 1. "tudent 4odule 2. e3uired "afety -3ui!ment :nstrument 0rainee 0as' 4odule 122/& 2 8ourse 4a! :nformation 0his course ma! shows all of the Wheels of earning tas' modules in the second level of the :nstrument curricula. 0he suggested training order begins at the bottom and !roceeds u!. "'ill levels increase as a trainee advances on the course ma!. 0he training order may be adjusted by the local 0raining 2rogram "!onsor. 8ourse 4a!: :nstrument, $evel 2 $-;-$ 2 8O42$-0- -lements, 0randucers, and transmitters < 4odule 122/& # / eview of )asic :nstrument and 8onstrol 8hannels >>>>>>>> + 1.% "econdary "tandards >>>>>>>>>>>>>>>>>>>>>>>> 1. 1.. 1% 1.#..1.2 0ransducer?8onvertor >>>>>>>>>>>>>>>>>>>>>>>./ 4easurement "tandards and -lements >>>>>>>>>>>>>>.# 4easurement -rrors >>>>>>>>>>>>>>>>>>>>>>>>./ Orifice 2lates >>>>>>>>>>>>>>>>>>>>>>>>>>>> 22 :nstrument 0rainee 0as' 4odule 122/& % .0A)$.2. .% 0ransmitter >>>>>>>>>>>>>>>>>>>>>>>>>>>>>. 1. 1..2.1 Direct vs. 1. Accuracy >>>>>>>>>>>>>>>>>>>>>>>. 12 1./.1. 11 1.( 2rimary and "econdary -lements >>>>>>>>>>>>>>>>>. 22 2.1 Detector 5"ensor6 >>>>>>>>>>>>>>>>>>>>>>>>>.1 Accuracy >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>. 1+ 1./.2 2recision vs. 1.1. 1...& @or'ing "tandards >>>>>>>>>>>>>>>>>>>>>>>>>.#.#.( 1. 21 1.#... 1.. 1.# Am!lifier 5"ignal 8onditioner6 >>>>>>>>>>>>>>>>>>>. Page 1../ Detectors >>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 22 2. :nferred 4easurements >>>>>>>>>>>>>>>>>.1.#. 2/ 1.+ 8alibration >>>>>>>>>>>>>>>>>>>>>>>>>>>>>.1.& "ensitivity and es!onsiveness >>>>>>>>>>>>>>>>>>.2 4easurement "tandards >>>>>>>>>>>>>>>>>>>>>>.# 2rimary "tandards >>>>>>>>>>>>>>>>>>>>>>>>>.#.1.#. 11 1../ :ntroduction >>>>>>>>>>>>>>>>>>>>>>>>>>>>. 1. . 2/ 1.2.#. 1/ 1. 1.2.O= 8O70-70" Section Topic ………………………………………………………………….2...#../ eview of 4easurement 0erminology >>>>>>>>>>>>>>>. 1. 1/ 1.2.% e!roducibility and Drift >>>>>>>>>>>>>>>>>>>>>>. "ignificant =igures >>>>>>>>>>>>>>>>>>>>>>>>>. 1+ 1.. / 8a!acitance 0y!e 2ressure "ensor >>>>>>>>>>>>>>>>../. 2.1/./ 0ransducer =unctions >>>>>>>>>>>>>>>>>>>>>>>.O= 8O70-70" Section Topic …………………………………………………………………. ## 2. &2 2. Page -lements..10hermoelectric 2ower >>>>>>>>>>>>>>>>>>>>>>>. &% 2.2.1/./ ./ Altrasonic =lowmeters >>>>>>>>>>>>>>>>>>>>>>>... #/ 2..#. +2 0A)$./ )ellows 2ressure Devices >>>>>>>>>>>>>>>>>>>>>> (+ #.. +1 %.#0hermocou!le $aws >>>>>>>>>>>>>>>>>>>>>>>>.(.1.1/. 0randucers. (/ #../ Dia!hragm "eals >>>>>>>>>>>>>>>>>>>>>>>>>>..#./ Dia!hragm 2ressure Devices >>>>>>>>>>>>>>>>>>>./0hermocou!les >>>>>>>>>>>>>>>>>>>>>>>>>>> %./ Altrasonic $evel 4easurement >>>>>>>>>>>>>>>>>>>.1/./.%. and transmitters < 4odule 122/& & .&../ )ourdon 0ube >>>>>>>>>>>>>>>>>>>>>>>>>>>> (/ #. 2.2.enturi 0ube >>>>>>>>>>>>>>>>>>>>>>>>>>>>./ "econdary -lements >>>>>>>>>>>>>>>>>>>>>>>>..1/.(.+./ )imetallic "tri! 0hermometers >>>>>>>>>>>>>>>>>>> %% 2.2.20hermocou!le 4etals>>>>>>>>>>>>>>>>>>>>>>>>./ 0ransducers >>>>>>>>>>>>>>>>>>>>>>>>>>>>> +2 %./ Annubar 0ubes >>>>>>>>>>>>>>>>>>>>>>>>>>>..%0hermocou!le 0ables>>>>>>>>>>>>>>>>>>>>>>>>./ 2ressure 8a!sules >>>>>>>>>>>>>>>>>>>>>>>>>. %2 2./ 8a!acitance 0y!e $evel Detectors >>>>>>>>>>>>>>>>> #.%.1/. &/ 2./ 4agnetic =lowmeters >>>>>>>>>>>>>>>>>>>>>>>> #/ 2. (.1/. (( #. #.&. #.(0hermocou!le 8onstruction >>>>>>>>>>>>>>>>>>>>. 2. &.1. 2./ 0he 2ilot 0ube >>>>>>>>>>>>>>>>>>>>>>>>>>>. &2 2.&Designations for 0hermocou!le @ire >>>>>>>>>>>>>>>. &( 2. (& #. 2+ 2. .%..2 D2 8ell 2ressure 4easurement >>>>>>>>>>>>>>>>>>..% )alancing "ection >>>>>>>>>>>>>>>>>>>>>>>>>. %. %./ 2?: 0ransducers >>>>>>>>>>>>>>>>>>>>>>>>>>..1 &.( &./Accelerometer >>>>>>>>>>>>>>>>>>>>>>>>>>>.12.#. ../ 0ransducer 0y!es >>>>>>>>>>>>>>>>>>>>>>>>>.oltageBdivider 2ressure 0ransducer>>>>>>>>>>>>>>>> +./../ 0ransducer O!eration >>>>>>>>>>>>>>>>>>>>>>>.2 &.1. +./ :?2 0ransducers >>>>>>>>>>>>>>>>>>>>>>>>>>.1 D2 8ell $i3uid $evel 4easurement >>>>>>>>>>>>>>>>>./ 0ransmitters >>>>>>>>>>>>>>>>>>>>>>>>>>>>. .2 2rocess 4easuring "ection >>>>>>>>>>>>>>>>>>>>./$inearB. +# %.1.1.. +# %...& 2neumatic :n!ut?Out!ut "ection >>>>>>>>>>>>>>>>>. . ..1.2. &.( 2neumatic =orce )alance 0ransmitters A!!lications >>>>>>>.1A!!lications >>>>>>>>>>>>>>>>>>>>>>>>>>>>> .1 =orce )alance Differential 2ressure 2neumatic 0ransmitters >>>./ 4etallic "train Gauge >>>>>>>>>>>>>>>>>>>>>>>> ++ %. ..+. +.2./ &.2..&. %.% &.1./ 2neumatic 0ransmitters >>>>>>>>>>>>>>>>>>>>>>../ D2 8ell =low 4easurement >>>>>>>>>>>>>>>>>>>>> . ..# &./ 2ressure "train Gauges >>>>>>>>>>>>>>>>>>>>>>. +2 %.oltageBGenerating 0ransducers >>>>>>>>>>>>>>>>>> +./ 2?: 0ransducer O!eration >>>>>>>>>>>>>>>>>>>>>. %.# =orce )ar "ection >>>>>>>>>>>>>>>>>>>>>>>>>. .2 &..../ 2neumatic =orce )alance 0em!erature 4easurement >>>>>>.. %. .(..11. &. &.1 "emiconductor "train Gauges >>>>>>>>>>>>>>>>>>>./ :?2 0ransducer O!eration >>>>>>>>>>>>>>>>>>>>>..12ieCoelectric 0ransducers >>>>>>>>>>>>>>>>>>>>>.1 %..%.1.2 &../ %. . +& %.& &.1/...1/...#.11./. . .. .. .1 .ariable Differential 0ransformer >>>>>>>>>>>>>. +% %./ :nstrument 0rainee 0as' 4odule 122/& ( .1. +. +% %.2. ( (.. Drift: 0he gradual change of an instrument out!ut from the correct value.% A!!lications of 4otion )alance 0ransmitters >>>>>>>>>>>./. .2 &.1.& 2neumatic 4otion )alance 0em!erature 4easurement >>>>>>.1 4easuring "ection >>>>>>>>>>>>>>>>>>>>>>>>>. +. or chec'ing the absolute values corres!onding to graduations on a measuring instrument.# &. .# Out!ut "ection >>>>>>>>>>>>>>>>>>>>>>>>>>>.%./. .1. Noble "etal: A metal that is so inert that it is usually found as uncombined metal in nature.1.&.# )ellows and elay "ection >>>>>>>>>>>>>>>>>>>>> . gold. .+ (.% &.% &. .1 2hotodectors >>>>>>>>>>>>>>>>>>>>>>>>>>>>./ 4otion )alance 2neumatic 0ransmitters >>>>>>>>>>>>>> . the metal which does not corrode. . 0randucers. 2latinum.( 2neumatic 4otion )alance 2ressure 4easurement >>>>>>>>.2 $in' and =la!!erB7oCCle "ection >>>>>>>>>>>>>>>>>> .%.. +.& (./ =iberBO!tics >>>>>>>>>>>>>>>>>>>>>>>>>>>>> .1 &.%. -lements.( (.%. Mea !re"ent error: 0he difference between a measured value and an actual value. Also.% Out!ut Device >>>>>>>>>>>>>>>>>>>>>>>>>>>. ./ -lectronic 0ransmitters >>>>>>>>>>>>>>>>>>>>>>> .1 =orce )alance Differential 2ressure -lectronic 0ransmitters>>>> .1. 0rade 0erms :ntroduced in this 4odule Calibrate: 0he action of chec'ing the readings of an instrument against a 'nown standard and adjusting the instrument to correct any errors..# &./ O!tical 0ransmitters >>>>>>>>>>>>>>>>>>>>>>>>. correcting. and transmitters < 4odule 122/& + ..%.1.. +.%.2 "ensor Assembly >>>>>>>>>>>>>>>>>>>>>>>>>> . and silver are noble metals.& (. Calibration: 0he !rocedure laid down for determining.%. ..1.. Secon%ar# ele"ent: 0he element of a measurement device which ta'es the out!ut from the !rimary element and sends a signal !ro!ortional to it to the controller. 0he more commonly used devices and the !rinci!les involved in sensing and measuring a !arameter are covered in this module.:-@ O= )A":8 :7"0 A4-70 A7D 8O70 O$ 8FA77-$" :n 4odule 122/%. reliable way !ossible. the instrumentation industry has become a highly diversified field. 1. 0o !erform these tas's you must understand the meaning of !rocess measurement and be familiar with the devices that !rovide measurements. 1. ensure the !roduction of high 3uality !roducts or services. Eou must also be familiar with the basic !rinci!les involved in detecting and sensing these !arameters./ :70 ODA80:O7 0O 2 O8-"" 4-A"A -4-70B 0he industrial !lants of today re3uire the measurement and control of numerous !arameters to o!erate in the most efficient. Pri"ar# ele"ent: 0he element in a measurement device which is acted on directly by the !rocess./ -. Relati$e error: 0he e*!ression of an error as a !ercent of the value being measured. Tran %!cer: A device that !rimarily functions to convert its in!ut signal to an out!ut signal of a different form. :nstrument 0rainee 0as' 4odule 122/& . . Re pon i$ene : 0he ability of an instrument to follow changes. 0hey are each shown here in !igure ". Tran "itter: A device that !rimarily functions to !re!are and send in!ut information to a remote location. and maintain the devices used to measure and control these various !rocess !arameters. 2rocess 8ontrol 0heory. o!erate.Preci ion: 0he degree of re!roducibility of measurement by an instrument. the basic instrument and !rocess control channels were introduced and described in general terms./.1. Duality technical !ersonnel are re3uired to install. )ecause there are such a wide range of !arameters monitored in a ty!ical !lant. calibrate. and to limit emissions from the !lant that could have adverse effects on the environment. 8onditions must be constantly monitored to !rovide a safe and comfortable atmos!here for the wor'ers in the !lant. :n many cases. :t is very common for instrument manufacturers to combine both this am!lification and signal conditioning function with one of the other bloc's !hysically. 0hese devices are often referred to as the G!rimaryG. 0his is more cost effective. Transducer: converts the out!ut signal of the detector to a signal that can be used easily. Amplifier: increases the !rocess signal to a usable magnitude. . @hen it is used.=igure 1. 0randucers. a transducer can be !art of the !rimary element or the transmitter or it can stand alone. this GconversionG setu! is not needed. 0hey are: Detector (or Sensor): senses the !arameter being monitored 5the !rocess variable or the controlled variable6 and changes that !arameter to a mechanical or electrical signal which is !ro!ortionally related to the measured variable. it is most often found in the GfieldG in close !ro*imity to the measuring element. :n many cases it is in the same housing or case as the measuring element and a!!ears to be one device. N&TE: 2ro!er handling storage and !rotection is critical. :f the detector signal can be used directly. 0hese bloc's are common to almost all instrumentation a!!lications. De!ending u!on the a!!lication. you will be focusing on the first four 5%6 bloc's in each of these channels. :t is very im!ortant that you have a good understanding of them and the variety of ways they can be a!!lied. or GmeasuringG element of the instrument channel. $et1s review and elaborate the functions of each bloc'. )asic :nstrument and 2rocess 8ontrol 8hannels :n this module. -lements. signal conditioning also occurs along with am!lification. and transmitters < 4odule 122/& . :t is ty!ically found as a !art of either the transducer or the transmitter. the detector receives energy from the !rocess and !roduces an out!ut that is de!endent on the measured 3uantity.1. and detection !rinci!les. let1s discuss these four bloc's individually. "ome metals. 0wo dissimilar metals. 2arameter 0em!erature Detector esistance 0em!erature Detector 5 0D6 0hermocou!le Detection 2rinci!le esistance of certain metals varies linearly with tem!erature. Detectors are also selected for measurement systems on the basis of the !arameter being sensed. A good instrument is designed to minimiCe this loading or disturbance of the !arameter being measured. will e*!and or distort in !ro!ortion to -*!ansion of a 4etal :nstrument 0rainee 0as' 4odule 122/& 1/ . when heated. :t may contain the detector. :t is im!ortant for you to 'now what to e*!ect when reading the s!ecification or data sheets in these instrument1s vendor manuals. :t is im!ortant to realiCe that the sensing element always e*tracts some energy from the !rocess: the measured 3uantity is always disturbed by the act of measurement. )efore going on. 0hey need not a!!ear in the order of !igure " and not all of the com!onents described may be re3uired. range of measurement.1 Detector 5"ensor6 0he first contact that a measurement channel of instrumentation has with the !rocess !arameter to be measured is through the action of the detector or sensing device. the desired accuracy. :n measuring systems made u! of mostly electric or electronic com!onents. A s!ecific instrument channel may involve these basic com!onents in any number and any combination. As you go through the e*am!les in this module you will see these variations. 4ost of the time all four functions are !erformed but it is !ossible with modem instruments to have one or more devices do them all. 0he reason for such a variety of !otential variations is that the manufacturers of instrumentation are !roducing 5and naming6 devices in inconsistent ways with regard to these four basic functions. transducer and the am!lifier 5signal conditioning6 functions. and the !articular ty!e of out!ut it su!!lies. !arameters monitored. 0hese detection !rinci!les and associated detectors are discussed in greater detail in the following sections of this module. 0his is one major advantage of electronic measurement and control channels. !igure # lists some ty!ical detectors.Transmitter: transmits data from one instrument com!onent to another when com!onents are !hysically se!arated. the loading of the signal source 5!rocess variable6 is almost e*clusively a function of the detector. 0o sense the !rocess !arameter. !roduce a voltage !ro!ortional to their tem!erature. Other com!onents in the electronic instrument channel receive most of the energy or !ower they need from !ower su!!lies inde!endent of the !rocess itself. when joined. 1. 0his effect is referred to as loading. As !igure # suggests. it is also !ossible that a second or a third transducer may be found in some instrument and control channels. 0he detector1s out!ut is usually not directly usable in the control or instrument channel. 1. "ame as for measuring !ressure. oval tube will attem!t to achieve a straight cylindrical sha!e when internal !ressure is a!!lied. @herever a signal conversion must occur. tem!erature. :t is !ossible to have one transducer convert the detector out!ut to a form that can be am!lified and also !ossible to have another transducer -lements. A curved. 0his is the function of the GtransducerG. you will find a transducer.1. )ourdon 0ube $evel Differential 2ressure 8ell =loat =low =low estrictor 8ombined with a Differential 2ressure 8ell =igure 2. 2ressure -*!ansion of a $i3uid Differential 2ressure 8ell $i3uids will also e*!and when heated or contract when cooled. Often it must be converted. fluid level and li3uid flow. A material less dense than the fluid being monitored will float on the fluidHs surface. and transmitters < 4odule 122/& 11 . 0he differential !ressure cell is used to measure the !ressure dro!.2 0ransducer?8onvertor Almost immediately after being sensed by a detector the out!ut of the detector must be changed or converted to a more easily used form. :t is almost always !hysically connected to the !rimary detecting element. 0he !ur!ose of the transducer is to convert any in!ut it receives to another ty!e of out!ut signal more readily usable by the ne*t com!onent or !ortion of the instrument or control channel. 0randucers. "ystem !ressure can be a!!lied to the internal volume of the bellows with a fi*ed !ressure 5normally atmos!heric6 bellows. Once the measured variable out!ut of the transducer is converted to some usable form it can be mani!ulated by the instrument channel com!onents as necessary without loading the !rocess variable which !roduced it. $ist of 2arameters 4onitored and 0y!ical Detectors Detectors measure !rocess variables such as !ressure. @hile this conversion is easily seen as necessary at the out!ut of the detector 5few detectors !rovide out!uts directly usable6. A bellows will e*!and when the internal !ressure is greater. 0he !ressure dro! across the flow restriction is !ro!ortional to the s3uare of the flow. the most common detector out!ut is a very small dis!lacement or distance moved that is !ro!ortional to the measure of the !rocess variable. am!lified or conditioned in some way before it can be used to indicate or control the !rocess !arameter.the amount of heat absorbed. :t is im!ortant that you understand that a transducer functions to convert signals 5i. mechanical. am!lification is !erformed also. Again. Obviously. 1. in the control room for e*am!le6. valve or heater. if other conditioning or modification of the signal is re3uired. and it will !rovide the out!ut in a form ready for direct transmission to a remote location. Actually. so in most diagrams you will find a transducing element also directly connected to the !rimary sensing element. "e!arate !hysical instruments !erformed this function in the !ast9 however. 0he am!lifier bloc' indicates this usually ha!!ens somewhere in the channel. it may occur several times before the signal is able to be used. :t will cost less to manufacture ultimately.# Am!lifier 5"ignal 8onditioner6 4ost measured variable signals must be increased in either am!litude or !ower so they can be used by indicators or controllers directly or so they can be transmitted to them.e. even as !art of another bloc' li'e the transmitter. 0he term GtransmitterG has now grown to the !oint where it could embrace that function and all of the others above. the transmitter or wherever the instrument manufacturer finds it is most economical.1. 0he transmitter very often will contain the transducing element. :nstrument 0rainee 0as' 4odule 122/& 12 . :t !re!ared a signal to be sent from one location to another involving some distance. etc. or detector.e. this has evolved due to instrument manufacturers becoming more innovative. 0here are advantages to this. am!lifiers and signal conditioning element.1. :t usually does not include the detector 5but it could6. 4uch of the time now in industry you will find that a detector is connected to the in!ut side of a transmitter directly and the out!ut side can be connected to the final indicator or controller. an am!lifier bloc' may not a!!ear on your instrument channel diagrams. 0herefore. electrical.convert the am!lified signal to a form where it can easily be transmitted to an indicator or a controller elsewhere in the !lant 5i. :t has less !arts so it may be easier to maintain. and that a transducer can be found anywhere in a channel. 1. Often.6. inside another element of the channel. 0he am!lification can ta'e !lace in the transducer. :t is just as li'ely that the controllers out!ut might need conversion or transducing again for transmission bac' into the !lant to o!erate a control element 5i. but rest assured am!lification is most li'ely being !erformed somewhere.% 0ransmitter 0he transmitter is a device that originally was very s!ecific in function. 4ost detector out!uts must be converted for use by the channel.6 from one form to another that is needed at that !oint in the channel. etc.e. !neumatic. it is easier to wor' with or install. more often now instrument manufacturers are including the am!lification device or stage as !art of other !hysical elements in the channel. :f the &/Binch to 1&/Binch level indicator discussed !reviously has a reference accuracy of L1J of actual reading and the tan' !resently has 12& inches of water the indicator should be reading 12& inches L1. the reference accuracy would be e*!ressed sim!ly as L1M=. 0his can be e*!lained by using the following e*am!le: A meter is used to indicate the water level in a tan' between &/ inches and 1&/ inches. =or a tem!erature measuring device. and transmitters < 4odule 122/& 1# . this can be e*!ensive. #. :t can be difficult to maintain if you are not s!ecially trained to wor' on such a com!le* device. it usually must be re!laced. &. I/.2. eference accuracy can be e*!ressed in !ercent of the u!!er range value.:-@ O= 4-A"A -4-70 0.2& inches. 2.4:7O$OGE 0he following terms a!!ly to any and all of the bloc's in the basic instrument channel.# inches 5about &?1( inch6./ -. and the reference accuracy is L/. the reference accuracy of the gauge would be /..1 !si. Eou will see e*am!les of all of the above in this module. :f the u!!er range value of a !ressure gauge is 1// !si. %. 0here is no absolute accuracy9 however. 0he reference accuracy of the indicator is LNJ of s!an. =inally. when used as a !erformance s!ecification for an instrument.1J of whatK As the definition states.1J . the reference accuracy of the indicator is N inch of level. GaccuracyG means reference accuracy. 0herefore. eference accuracy is a number or 3uantity that defines the limits that errors will not e*ceed when the device is used under referenced conditions. often you will hear someone say that a detector is accurate to within I. "ome have been !resented in !ast modules.2. eference accuracy can be e*!ressed in a number of ways: 1. @hile relatively easy to do. often their usage is not !recise. eference accuracy can be e*!ressed in !ercent of s!an. -lements. 1. the reference accuracy would be /.1 Accuracy Accuracy is the degree to which the out!ut of an instrument a!!roaches an acce!ted standard or true value.0here are also disadvantages for the user. reference accuracy can be e*!ressed in !ercent of actual out!ut reading.2J of scale length. :t can be e*!ressed in terms of the measured variable. "ome have only been mentioned in !assing. =or an indicating meter with a (Binch scale length and a reference accuracy of I1. the out!ut of a device is com!ared or referenced to some value or standard to determine whether the instrument is !erforming as re3uired. 1. since the areas they deal with overla!.. @hen it brea's. 0hese terms have very s!ecific meanings but. $et1s loo' more closely at several terms and how they a!!ly to the instrumentation in this module. 0randucers. :t can be e*!ressed in !ercent of scale length. 0herefore.1J of u!!er range value. :nstrument 0rainee 0as' 4odule 122/& 1% . a high degree of !recision.@hen stating the accuracy of an instrument. it may be found that they each have the same internal !arts arranged in e*actly the same manner. @hen they are used. each re!eatedly senses the !rocess variable and !rovides an out!ut measured variable signal that is in e*act agreement with their !revious measures of the !rocess variable. the !recision of the measurement de!ends u!on how closely the individual results agree among themselves. date and model are com!ared. if two detectors of the same ma'e.2 2recision vs. 0o say that a com!onent is accurate to within /. accuracy and !recision have two distinct meanings. 1. therefore. 0he !ercent s!ecification must be related to some s!ecific magnitude. :t should be understood that in instrumentation. Accuracy 0he word !recision means shar!ly or clearly defined.1 J is meaningless.2. =or e*am!le. -ach detector can be considered to have. it is very im!ortant to e*!ress the 3uantity to which the accuracy is referenced. :f an instrument is used to !erform a re!eated set of measurements on a !rocess. "ee !igure $ for an e*am!le of e*!ressing accuracy in various ways. but !recision is no guarantee of accuracy. and transmitters < 4odule 122/& 1& .=igure #. 0here are a number of reasons why this can ha!!en. -*am!les of Accuracy Fowever. 0randucers. 0his is done by reducing the !otential for error where !ractical. 0he two detectors may have been installed in a slightly different manner9 they may have been adjusted or calibrated in different ways 5or to different reference standards69 or one may be e*hibiting more wear or friction than the other internally. )oth can be !recise instruments but. 0he !recision of a device may be high. one is more accurate than the other. -lements. :t is often u! to the instrument installer to ta'e the necessary !recautions to ensure that instruments are functioning !ro!erly and that no controllable outside !henomenon is influencing the accuracy of the measurements ta'en. it is !ossible that each may !rovide a different value for the measured variable or out!ut. obviously. 1. we convert the actual numerical error value into a relative error value which more closely e*!resses the accuracy 5or lac' of it6 of the instrument. is determined by the magnitude of the relative error. it is normal !ractice to ma'e and record a series of observations rather than be content with only one value or reading. :t is one reason why many adjustment forms and !rocedures often re3uire you to chec' and record the !erformance values of a device first when you start wor' on it and. mm B +&.alue* P100% 0rue .alueB 0rue . :f a significant change is noticed. to obtain the various reference forms of the e*!ression the true value must be e*!ressed in those reference values. mm. =or e*am!le.alue O +&. 0he actual error is given by: -rror 5-6 O 4easured . will be noticed by a change in the out!ut of the !recision instrument by your com!aring the readings before and after wor' has been done. then. it is often more useful to convert it into an indication of how accurate the measurement is. due to something you may have done to it. the error of a barometric !ressure measurement might be B1 mm if the barometer read +&.4easurement is the combined result of a human o!eration on instrumentation as well as the functioning of that instrumentation. :n this instance it would be 1?+&.. you will be re3uired to verify or recalibrate the device to a 'nown standard. elative -rrorO 4easured .# 4easurement -rrors 0he error of a measurement is the numerical difference between the measured value and the true value. as the QabsoluteBvalue notation above signifies.alue B 0rue . Accuracy. 0o do this. :f error multi!lied this by 1//J. :nstrument 0rainee 0as' 4odule 122/& 1( . mm O B1mm @hile is it necessary to 'now the actual error. @hen !erforming !recision measurements. Eour judgement is a !art of the nature of measurement. 0his is one reason you !erform instrument Gchec'sG so fre3uently. :n these instances you are using the fact that a change in the accuracy of an instrument. elative error is defined as the ratio of the actual error value to the true value. when you1ve finished wor' on it. it would e*!ress the relative error as a !ercentage of the true value.alue -rror* P1//J 0rue .alue elative -rrorO 0he relative error is always e*!ressed as a !ositive number. 0his is the only way you can ensure the accuracy of the device is acce!table. again. in general. Of course. mm when a more accurate measure indicated the true !ressure was re!resented by +&. :n instrumentation terminology the error of a detector or sensor then would be the difference between the measured variable and the actual !rocess variable.2. and transmitters < 4odule 122/& 1+ . lac' of e*!erience. fouling u! the environment. An e*am!le of ine*!erience can be an error due to !aralla*. or bias on the !art of the wor'er. Data are selected to substantiate the results or to ma'e your job easier. -rror Due to 2aralla* Other errors due to carelessness or ine*!erience might be mathematical errors or not 'nowing where to find or verify something in a !rocedure. or a !lant could be o!erating a little less efficiently for the ne*t year. -lements. but this can be time consuming and e*!ensive. technical manual or s!ecification sheet. 4isreading a measurement can be due to carelessness or the lac' of e*!erience of the wor'er. A common method of overcoming !ersonal errors is to have readings made by more than one wor'er. "oon the instrument will drift further out of tolerance and you will need to reBcalibrate it anyway. rather than being acce!ted with e3ual confidence or objectivity. 2aralla* occurs when a wor'er is not e*!erienced 5or careful6 enough to 'now that the measured value will change with the relative !osition of the eye reading it. you are just fooling yourself. =igure %. )ias must be controlled by the individual wor'er. :n this case. 8arelessness is a common factor in many errors.-rrors in general measurement wor' are classified as: 2ersonal -rrors andom -rror "ystematic -rrors A!!lication -rrors 2ersonal errors are those caused either by carelessness. :t usually is caused by having some !reconceived notion or e*!ectation of the magnitude or 3uality of the variable under measurement or the device measuring it. A common e*am!le of bias occurs when you carefully evaluate a borderline measurement value on a calibration chec' to be within s!ecifications because you 'now someone will ma'e you go bac' and reBcalibrate the device if one measurement !oint is Gout of lineG. :t1s also !ossible a lot of wasted !roduct could have been !roduced in the mean time. 0randucers. )ias results in a more subjective ty!e of error. 2aralla* is demonstrated in !igure %. =igure &. which can be demonstrated by !lotting incremental increases in first ascending ste!s and descending ste!s. friction and bac'lash in gearing. . !igure ' shows this same loo! !lotted as a statement of continuous !ercent accuracy. hysteresis. 0y!ical Fysteresis $oo! =igure (. A!!lication errors occur from im!ro!er use or faulty installation of an instrument. "ystematic errors are what can be considered builtBin errors which result from the characteristics of the materials used in construction of the instrumentation systems. 0he errors !robably e*ist. :naccuracies arising from such causes are more or less regular in character. 0y!ical Fysteresis $oo! 2lot 0hese errors are normally small enough to Glive withG and actually ma'e u! the majority of the tolerance s!ecified by the manufacturer as the !ercent accuracy of the instrument. A!!lication errors can be minimiCed by following the manufacturer1s s!ecifications for use and installation. "ystematic errors are caused by such things as the natural inertia of moving !arts. as well as the design :nstrument 0rainee 0as' 4odule 122/& 1. !igure & shows a ty!ical hysteresis loo!. but they are considered indeterminate. 0hese errors are re!eatable and result in the ty!ical hysteresis loo! of an instrument.andom errors occur when re!eated measurements of the same 3uantity result in differing values. :n s!ea'ing of a thermometer. :ndeed.2. Drift is the !rimary cause for the need to reBcalibrate instruments. "ensitivity and res!onsiveness are fre3uently confused. An evaluation of the !ercent res!onsiveness of the !ressure detector would be: -lements. someone might say it is Gsensitive to /. as well as the design s!ecifications and general rules of industrial Ggood !racticeG standard 5such as those of the :nstrument "ociety of America.s!ecifications for use and installation.% e!roducibility and Drift e!roducibility is the degree of closeness with which the same value can be measured at different times. 1. 0hat way you can antici!ate when the instrument channel is most li'ely to need reBcalibration. after steadyBstate has been reached. -*am!le: A !ressure detector at 2// !si re3uires a change of L2 !si to cause a change to be !erceived at its out!ut. 0he sensitivity of a device is an im!ortant !ro!erty which is determined or set by the designer based on the re3uirements of the a!!lication. 0he term res!onsiveness denotes the amount of change in the !rocess variable needed to cause a !erce!tible change or movement in the measured variable.1M8G. but it was desired to 'now what change in the calibration of device was occurring. 0his usually occurs over a long !eriod of time during which the value of the variable is assumed not to change. 0randucers.6.2. drift is defined above in terms of the variation of the measured value from the Gcalibrated valueG. or general deterioration as a function of time. :t is usually e*!ressed as a !ercentage of s!an of the instrument. Drift can be caused by !ermanent GsettingG of the mechanical or !hysical com!onents of the detector or instrument. . a good way to !lan your normal calibration cycles of instrument channels is to 'ee! a record of the GdriftG accruing with res!ect to time. 1. stress on the e3ui!ment !arts.& "ensitivity and es!onsiveness 0he sensitivity of a device is the ratio of a change in out!ut magnitude to the change of in!ut that causes it. 0he re3uired sensitivity will be decided by the design engineer based u!on the smallest change needed to be measured in the given !rocess variable. :t could have been defined in terms of the change in the measured value from the true value. erosion. and transmitters < 4odule 122/& 1. or the amount of in!ut change that can cause the out!ut to start to change in any device. drift can be due to wear. 2erfect re!roducibility indicates that an instrument or instrument channel has no GdriftG. when it is correct to say that the thermometer will Gres!ond to a change of L/.1M8G. :f you are involved in maintaining instrumentation. Alternatively. Drift is a gradual se!aration of the measured value from the calibrated value. or fatigue in the metals or other materials of construction. :t is a ratio that describes how much the in!ut variable must change to !roduce some change in out!ut magnitude. 0his is a direct measurement.1 Direct vs. tem!erature or flow. direct measurement is not always ade3uate or !ossible. =or the measurement to be really useful it must be reliable and accurate. or !henomenon. A measuring instrument is sim!ly a device used to sense and relay that value to us or another device for !rocessing the information. 1. Direct measurement is much less common than you thin'.#. Eou would com!are the length of the !age to the measurement mar's or increments on the rule. your measurement would !robably be accurate to within 1?. 0he value determined by the instrument is generally. condition. 4easurements for this discussion fall into two general categories: those measurements made directly and those that are inferred. of an inch9 with the 4etric "ystem rule 5or meter stic'6 it would !robably be accurate to within 1B# mm. =or one thing the human senses are not !re!ared to ma'e direct com!arisons of all 3uantities with e3ual facility. :nstrument 0rainee 0as' 4odule 122/& 2/ . Often we re3uire greater accuracy./J at 2// !si 1. 3uantitative./% in. with a !reciseness of about 1 mm 5a!!ro*imately /. :n many cases they are not sensitive enough./ 4-A"A -4-70 "0A7DA D" A7D -$-4-70" 0he !ur!ose of measurement is to determine the value of a 3uantity.:t is 3uite !ossible that the value of res!onsiveness may vary throughout the range of the detector. :nferred 4easurements 0he wide variety of measurements made in industrial !lants and the varying environmental conditions under which these measurements must be made re3uire mention of the basic nature of measurement. Fow would you measure the length of this !ageK Eou would !robably be satisfied to use a ruler or metric rule. Eou have determined the length of the !a!er by direct com!arison of that !arameter to a 'nown or acce!table standard. :n these instances we rely u!on some more com!le* form of measurement system. -*am!les of this would be !ressure. 0he way this is assured is to effectively see that measuring instruments are always functioning to com!are the !rocess variable being sensed to the e3uivalent of a 'nown measure or standard. es!onsiveness may be im!roved by !ro!er lubrication and adjustment of the instrument. just as accuracy can.6. Although to measure by direct com!arison is the sim!lest method.alueof Out!ut 52ressure6 2 !si P1//J 2//!si es!onsive nessO O 1. :n the case of the -nglish "ystem ruler. but not necessarily. Often our senses just don1t detect in a 3uantitative way what we want to measure. !hysical !arameter. es!onsive nessO 8hange in :n!ut 52ressure6 P1//J . @e can ma'e direct com!arisons of small distances using a rule.#. 0he set of ultimate standards is maintained by the A. 0he !arameter of interest is affecting a characteristic or !ro!erty of the material of the measuring system or detector. 7ational )ureau of "tandards 57)"6. 1.# 2rimary "tandards 2rimary or absolute standards are constructed to conform to the legal definitions of different fundamental units of measurement. -lements. 0hese standards are sometimes referred to as !rototy!e standards. D. A standard can also be an instrument of high accuracy.2 4easurement "tandards A standard is an accurate 'nown 3uantity used for calibration of measurement instruments. standards used for calibration !ur!oses are set to the 7)". inde!endently accurate and correct. you can send it !eriodically to the regional laboratory and they will chec' the accuracy of your e3ui!ment against their secondary standards. 0randucers. 0he 7)" chec's these standards against their own standards for accuracy. and gallons.:nferred measurement occurs when there is an indirect com!arison. An e*am!le of a !rimary standard is the standard meter or a set of !recision weights for ounces and !ounds. a voltage is !roduced.. !ints.".#. Other ty!es of standard measures are ultimately traceable to these.#. 0raceability is an im!ortant 3uality of instrumentation in highBtech and?or haCardous industrial a!!lications.% "econdary "tandards "econdary reference standards are devices that are co!ied from e*isting !rimary or absolute standards. 0hree basic levels of measurement standards are common. Often these standards are maintained accurate and traceable to the 7)" !rimary standards by s!ecial com!anies that set u! regional standards laboratories. 0here are: • • • 2rimary or absolute standards "econdary reference standards @or'ing standards 1. :nstead of sending your measuring e3ui!ment to the 7)" in @ashington. On occasion. when a difference in tem!erature e*ists between the junctions of a thermocou!le. therefore. and transmitters < 4odule 122/& 21 . 0he change in the detector is what is actually being measured. 1. 3uarts. Again. =or e*am!le.#. 0hese standards are then said to be traceable bac' to the 7)". it is im!ortant that inferred relationshi!s as this be traceable or com!arable to 'nown standards.8. "ome construction and o!erating com!anies maintain their own secondary standards laboratory onBsite. 0he term GabsoluteG is used to indicate these measures are finite and are. Another e*am!le is a set of containers that hold !recise amounts of li3uids for liters. 0he voltage is the actual !arameter being measured and the tem!erature is inferred or derived from the characteristic voltage measured. "tandards e*ist for every ty!e of measurement. %. :n most cases. conversion 5transducing6.& @or'ing "tandards @or'ing standards are used to calibrate the instruments installed in systems in the field. :n may industries. :t usually does not !resent an out!ut that is !owerful enough to be a!!lied directly for indication or control of e3ui!ment. 0he !rimary elements function is to. to note that many of the mechanical elements !roduce an out!ut that is a !hysical dis!lacement. and many of the electrical elements !rovide an electrical signal out!ut directly. !igure + is only a re!resentative listing. :nstrument 0rainee 0as' 4odule 122/& 22 . $ists common mechanical and electrical !rimary elements and the o!eration they ty!ically !erform. 0he !rimary element may be very sim!le. of course.( 2rimary and "econdary -lements All instrument channels contain various com!onent !arts or GelementsG which !erform the !rescribed measurement. first. =riction is minimiCed. 2. :t. an industrial !rimary element converts the measured variable into a dis!lacement. however. -lectrical elements have several im!ortant advantages: 1. An out!ut signal !ower of almost any magnitude can be !rovided. 0he !a!er wor' associated with the calibration and adjustment of instrumentation in the filed is the documentation of !ro!er installation and setu! of !lant systems. Often this mechanical dis!lacement is converted by a secondary element to an electrical or electronic signal. 4assBinertia effects are minimiCed. consisting of no more than a mechanical s!indle. or the !ublic that the !lant can be o!erated safely and adherence to environmental guidelines or regulations. 4any of these elements will be describe in more detail later. #. may be much more com!le*.1. sense the 3uantity of interest and. Am!lification and signal conditioning is easily !erformed. for without it you would be unable to assure the government regulators. arm or contacting member used to !rovide movement or force to the secondary element. 0his system of traceability is what ensure the reliability of the safe functioning of industrial !lant instrumentation. this !a!erwor' is just as im!ortant as actually installing and calibrating the e3ui!ment. :t is significant. 1. to !rocess the sensed information into a form that is usable by the instrument channel.#. 0he detector or sensor accuracy is thereby traceable through the local wor'ing standards to secondary reference standards to ultimately the !rimary standards at the 7)". 0he !rimary sensing element is the !art of the instrument or channel that first uses energy from the measured medium to !roduce a condition or signal that re!resents the value of the measured medium 5!rocess variable6. which is easily converted 5or transduced6 by a secondary element to an electrical signal. or the !lant o!erators and owners. !igure +. then.#. conditioning 5am!lification6 and transmitting functions described earlier. @hile a bit more cumbersome. 0randucers. =igure +.&.+ 8alibration -very measuring system must be !rovable9 that is it must !rove its ability to measure reliability. 1. 4echanical and -lectrical 4easuring -lements -lements. combinations can often be Of course. and transmitters < 4odule 122/& 2# . 0he !rocedure for establishing an instrument1s accuracy is called calibration. dis!lacement and?or force can also be easily converted to a !neumatic signal for !rocessing and transmission.#. it has a major advantage that a loss of !lant !ower will not immediately inca!acitate the instruments. -lectronic transducer?transmitter miniaturiCed. /.#. 0he orifice !late is a thin circular metal !late with a shar! edged hole. 2. wor'ing standards. given the rigidness of your local calibration !lan and !ractices. L /.e. o!eration must be observed and recorded.#% and the accuracy of the detector is stated to be /. 0hree 'inds of orifice !lates are used: the concentric. :t is also called the G!rimaryG or GmeasuringG element. #./&6. "ignificant =igures 0he documentation of calibration is very im!ortant.2$A0-" Orifice !lates are the most common ty!e of flow measuring element.(. 0he orifice !late is usually mounted between two flanges. Eour a!!lication of a tag or stic'er to an instrument or detector means that it has been tested and adjusted in accordance with acce!ted !rocedures and !ractices and is certified as calibrated by you to its degree of accuracy recorded in accom!anying documentation./& and of the test e3ui!ment is L /. 1. the detector senses the !arameter being measured. 0his means that. :f the reading is inter!reted by the observer as being #. 'nown magnitudes of the !rocess variable 5or in!ut !arameter6 must be a!!lied to the detector and the detector./ D-0-80O "B 0he first major bloc' of a !rocess instrument channel is the detector.1. 2. As we covered earlier. more commonly. :nstrument 0rainee 0as' 4odule 122/& 2% ./ O :=:8. 0he instrument manufacturer will !rovide in the accom!anying manuals sam!le !rocedures for calibrating the devices. the accuracy of the instrument is valid and traceable to a set standard. then the reading should be ta'en to be the least accurate limit involved 5i. secondary or. :t is essential for good !erformance to wor' to achieve a high degree of accuracy and reliability. 0his often must be combined with general good industrial !ractice guidelines and local?com!any !rocedures for testing and calibrating system installations. as well as the whole instrument channel. some of these digits will have an element of doubt associated with them.//1. :n writing a measured value as a series of digits. :t is im!ortant that the values or numbers used in the calibration records be consistent with the !ossible sensitivity of the instrument and testing e3ui!ment being used.. the eccentric and the segmental as shown in !igure (. 0he total number of significant figures is de!endent u!on the !robable error associated with the observation.8alibration is the testing of the validity of the measurements by an instrument in normal o!eration by com!arison with measurements made by !rimary.(. At some !oint during the installation of instruments and systems. thic' de!ending !rimarily on the diameter of the !i!e for which it is manufactured. and transmitters < 4odule 122/& 2& . a relatively small differential !ressure results. to 1?2 in. 0he ratio of the orifice bore to the internal !i!e diameter is called the )eta. Orifice 2lates 0he concentric orifice !late is the most commonly used of the three ty!es. ) O d?D where: ) O orifice bore to internal !i!e diameter ratio d O orifice bore 5inches6 D O internal diameter of !i!e 5inches6 0he flow !attern and the shar! leading edge of the orifice !late that !roduces it are of major im!ortance to the accuracy of the flow measurement when using a concentric orifice !late. At high eynolds numbers. also has a considerable influence on the flow !attern. the velocity !rofile of the fluid reveals that the greatest flow rates occur at the center of the !i!e. 0randucers. 0he !late is usually manufactured with a tab on which !ertinent orifice !late data is stam!ed such as orifice bore or hole siCe. the velocity !rofile is relatively flat so a large energy conversion ta'es !lace and as a result a relatively large differential !ressure is !roduced. Any nic's or rounding of the shar! edge changes the flow !attern significantly and therefore. 8oncentric orifice !lates should be used for clean va!or free li3uids or condensate free va!ors and gases. affects the accuracy of the measurement.=igure . 0he !attern obtained at high eynolds numbers is desirable. 0he ty!e of flow as reflected by the eynolds number. !articles tend to dro! out and collect at the bottom of the u!stream face of the orifice !late9 gases -lements. As the fluid !asses through the orifice only a small energy conversion is re3uired to constrict the flow. :t is usually made of stainless steel from 1?. At low eynolds number. therefore. Other materials such as 4onel or Fastelloy are used for fluids corrosive to stainless steel. :n li3uid flow a!!lications as the fluid converges in order to !ass through the orifice.. laminar flow. however. if the diameter of these holes is less than one tenth of orifice bore diameter. 0he diameter of the o!ening is . =or these a!!lications. the concentric orifice should always be used. the segmental orifice or the eccentric orifice !late is normally used. as indicated in !igure ). =igure . 0his small drain hole also !ermits drainage of a horiContal !i!e that contains the orifice. :n gas or va!or flow a!!lications condensate tends to form a !uddle at the bottom of the horiContal line u!stream of the orifice !late. causes inaccurate flow measurement. 0he o!ening in a segmental orifice is a segment of a circle as shown in !igure (. 0his is also indicated in !igure ). the segmental orifice is used with the circular section at the bottom of the !i!e.and va!ors tend to collect at the to! of the u!stream face. therefore. @hen flow is measured in vertical runs.ent and Drain Foles 0he vent and drain holes have little effect on the flow measurement because. 0he circular section of the segment should be concentric with the !i!e. because it eliminates damming of foreign materials on the u!stream side of the orifice when the orifice !late is mounted in a horiContal !i!e. 0he collection of gases or va!ors can be eliminated by drilling a small vent hole nearly flush with the inside diameter of the !i!e at the to! of the orifice !late. @hen the flow of a li3uid containing solids or of a gas containing moisture is to be measured in a horiContal !i!e... :nstrument 0rainee 0as' 4odule 122/& 2( . Drain and vent holes are inade3uate for li3uid flow a!!lications where large 3uantities of solids or gases are !resent and for gas or va!or flow a!!lications where large 3uantities of condensate are !resent. 0he segmental orifice !late is useful. @hen the flow of li3uid containing gases is to be measured in a horiContal !i!e. 8oncentric Orifice 2late with . then the ma*imum flow through these holes is less than 1J of the total flow. 0he collection of !articles and condensate can be alleviated by drilling a small drain hole nearly flush with the inside diameter of the !i!e at the bottom of the orifice !late. Any of these conditions changes the area of the u!stream fluid stream and. the segmental orifice is used with the circular section at the to! of the !i!e.J of the inside diameter of the !i!e. ".0he eccentric orifice has a circular hole bored tangent to the inside diameter of the !i!e. 0hese ta!s are drilled through the flange so that they sense the !ressures at the edge of the orifice !late as shown in !igure "*.J of the inside diameter of the !i!e. -lements. =lange and 8orner 0a! $ocations . radius ta!s. because the vena contracts may be less than 1 inch from the orifice !late. 0he eccentric orifice !late is used in the same way as the segmental orifice !late.ena 8ontracts ta!s have an u!stream or high !ressure ta! located one !i!e diameter u!stream of the orifice !late and a downstream or low !ressure ta! located at the vena contracts. this is the o!timum location for orifice !late ta!s. 0heoretically. and !i!e ta!s. 0here are five commonly used ta! locations for measuring the differential !ressure across an orifice !late. 0his arrangement is shown in !igure "*. 0he diameter of the o!ening is . vena contracts ta!s. for !i!e siCes of 2 inches or greater. =lange ta!s are the ones most often used in the A. errors are introduced if the orifice !late bore is changed due to erosion. 0hey are not recommended for !i!e siCes less than 2 inches.. 0hese ta!s are drilled through the orifice flanges 1 inch from the surface of the orifice !late. 0herefore. adius ta!s as shown in !igure ""+ are an a!!ro*imation of the vena contracts ta!s. 0randucers. the !oint of minimum !ressure. corner ta!s. and transmitters < 4odule 122/& 2+ . the !oint of minimum !ressure varies with the d?D ratio. 0hese ta!s are indicated in !igure "". 8orner ta!s are in common use in -uro!e. Fowever. =igure 1/. because the greatest differential !ressure is available between these !oints. 0he u!stream or high !ressure ta! is located one !i!e diameter u!stream of the orifice !late and the downstream or low !ressure ta! is located 1?2 !i!e diameter downstream of the orifice !late. 0hey are the flange ta!s. as shown in !igure "$./M from the center of tangency of the o!ening. !i!e diameters downstream of the orifice !late. 0his ta! arrangement is shown in !igure "#. )ecause of the distance from the orifice. =or gas flow. =or best accuracy.ena 8ontracta and adius 0a! $ocations 2i!e ta!s or full flow ta!s measure the !ermanent !ressure dro! across an orifice !late. =igure 12. . $ocation of the ta!s along the side of the !i!e !revents tra!!ed gas bubbles from interfering with the measurement and !revents sludge or !articles from fouling sensing lines. there is some li'elihood of measurement errors created by head loss in the long length of the !i!e. the !ressure ta!s for segmental orifice !lates must be at 1. 0his location of the ta!s allows condensate to drain from the sensing lines. e*act location of the ta!s is not critical. 0he ta!s are drilled into the !i!e 2N !i!e diameters u!stream of the orifice !late and . the ta!s are generally located at the to! vertical centerline of the !i!e.=igure 11. 0his is also shown in !igure "$. . 0he !ressure ta!s for eccentric orifice !lates must be located at 1. Fowever. 2i!e 0a! $ocations 0he !ressure ta!s for li3uid flow are generally located along the horiContal centerline of the !i!e./M to the eccentric o!ening. :nstrument 0rainee 0as' 4odule 122/& 2./M or . Orifice 0a! $ocations Orifice !lates are the most widely used of the !rimary flow elements./ . 0he differential !ressure develo!ed by the venturi is sensed between an u!stream or high !ressure ta! located N !i!e diameter u!stream of the inlet cone and a low !ressure ta! located at the center of the throat.=igure 1#. =luids that contain large amounts of sus!ended solids. as !reviously mentioned. A ty!ical venturi tube is shown if !igure "%. "econdly. so that the !ermanent !ressure loss is only 1/J to 2&J of the differential !ressure develo!ed by the device. or those that are very viscous can be measured by venturi tubes with ma*imum accuracy. -rosion of the shar! edges can cause serious inaccuracies in the flow measurement. they cause a high !ermanent !ressure dro!. . Orifice !lates. the velocity decreases and the !ressure is recovered. however.-70A : 0A)0he venturi tube is the most accurate of all !rimary elements when it is !ro!erly calibrated. :n the diverging recovery cone. :n addition. =irst of all. because they are ine*!ensive and easy to install. a cylindrical throat and a diverging recovery cone.2. -lements. and transmitters < 4odule 122/& 2. more em!irical data has been collected on this device than has been collected for any of the other !rimary elements. 0he recovery cone allows a relatively large !ressure recovery. they are highly susce!tible to erosion because of the shar! edges at the o!ening. At this !oint the velocity and !ressure are neither increasing nor decreasing. 0he low !ressure is measured in the center of the cylindrical throat. 0he ty!e consists of a converging conical inlet section. such as slurries. 0randucers. 2. 0he inlet section decreases the area of the fluid stream causing the velocity to increase and the !ressure to decrease. have two serious disadvantages. 0his design. 7evertheless. faces !er!endicular to the flow stream as indicated in !igure "'. the 2itot tube has limited industrial a!!lication.2:0O0 0A)0he 2itot tube measures fluid velocity at one !oint within the !i!e. A sim!le 2itot tube consists of a cylindrical !robe which is inserted into the flow stream. . shown in !igure "&+ further assures that a buildu! of solids does not occur and !ermits com!lete drainage of horiContal !i!es. faces into the stream9 the second. :t is also commonly used for flow measurement in large !i!es and ducts such as in ventilation systems. :n this ty!e of venturi. :nstrument 0rainee 0as' 4odule 122/& #/ .enturi 0ube -ccentric venturi tubes are occasionally used in systems where the flow of slurries is to be measured. -ccentric . s!ot measurements and laboratory measurements. =or this reason.=igure 1%. the throat is flush with the bottom of the !i!e. :t is a ty!e of head flowmeter. the flow indication obtained from a 2ilot tube can be highly inaccurate. =igure 1&. called the im!act o!ening. !articularly in laminar flow conditions. "ince the velocity of a fluid !assing through a !i!e varies with its distance from the !i!e wall. called the static o!ening. :t has two o!enings9 the first./ 0F. for velocity measurements.enturi 2. the 2itot tube is the best and most often used device.#. elocity readings from the 2itot tube !ositioned at several different distances from the !i!e wall would have to be ta'en. 2itotB. which is the sum of im!act !ressure and static !ressure.enturi 0ube -lements. 0o obtain a true measurement of flow in a !i!e. it is necessary to 'now the average velocity of the fluid. .=igure 1(. 0his is fre3uently done in test wor'. Eou should recall that the !ressure at the throat of a venturi dro!s because of a velocity increase. "everal different variations of the 2itot tube have been designed in order to !rovide a higher differential !ressure than that !roduced by im!act !ressure alone. however. is develo!ed the same as in the conventional 2itot tube. One such variation is the 2itot .. 2ilot 0ube 0he differential !ressure !roduced by the device is measured by a conventional differential !ressure measuring device such as a bellows. :t should be noted. but it is hardly !ractical for industrial !rocess flow measurement. weighted in accordance with a factor based on distance from the wall and finally averaged in order to obtain an accurate flow measurement. =igure 1+. that the high !ressure connection senses the im!act !ressure and the low !ressure connection senses static !ressure. 0he differential !ressure created by this device is measured in the same manner as with the conventional 2itot tube. 0he !ressure at the im!act o!ening is com!ared to the reduced !ressure at the throat of a small venturi that is also sus!ended in the fluid stream.enturi shown in !igure ". 0he !ressure at the im!act o!ening. and transmitters < 4odule 122/& #1 . 0randucers. :t consists of two !robes. its other advantages are similar to the conventional 2itot tube9 easy installation and low cost. 0herefore. 0he major disadvantage is that it can measure velocity at only one !oint within a !i!e or duct..&. :nstrument 0rainee 0as' 4odule 122/& #2 . =irst of all. Annubar 0ube A line inserted into the u!stream !robe senses the average of the im!act !ressures !resent at the four o!enings. :n addition. :t has several advantages.%./ 4AG7-0:8 =$O@4-0. sewage. 2." 0he magnetic flowmeter o!erates on the !rinci!le of =araday1s $aw of :nduction that states: anytime a conductor is moved through a magnetic field at right angles an electrical !otential is develo!ed. =igure 1./ A77A)A 0A)-" 0he Annubar tube is a variation of the conventional 2itot tube that nearly eliminates the major disadvantage of conventional 2itot tubes. 0he second disadvantage is that the im!act o!ening is easily bloc'ed if the 2itot tube is used for the measurement of dirty or stic'y fluids. 0his average !ressure is a result of the average velocity in the !i!e. one that senses fluid velocity and one that senses static !ressure. 0he !robes are sus!ended in the fluid line in much the same way as the conventional 2itot tube. 0he velocity sensing !robe has four o!enings or !orts that face u!stream into the flow stream. :t is !articularly useful for measuring the flow rate of fluids that !resent -ery difficult handling !roblems such as corrosive acids.0he 2itot tube has two serious disadvantages. 2. it !roduces no a!!reciable !ressure dro!. the differential between average im!act !ressure and the static !ressure sensed by the downstream !robe gives an accurate indication of the fluid flow rate. 0he magnetic flowmeter was develo!ed to measure the volumetric flow rate of electrically conductive fluids. :n addition to the fact that this device !rovides average velocity measurement. slurries and !a!er !ul! stoc'. -ach of the !orts are located at !ositions re!resenting e3ual crossBsectional areas to the flow stream. 0hese !orts are shown in !igure "(. it is easy to install and it is ine*!ensive. 0hese com!onents are shown in !igure #*. 0o determine the -4= induced into a conductor the following e3uation is used: -4= induced OR )y where: ) . 0he magnetic flowmeter is com!rised of a tube. the magnetic flu* density is increased. and the thumb !oints in the direction of the motion of the conductor with res!ect to the field. the length of the conductor in the magnetic field and the s!eed or velocity of the conductor. and transmitters < 4odule 122/& ## . As the fluid !asses through the magnetic field. 0he magnitude of the induced -4= is !ro!ortional to the magnetic flu* density. the middle finger !oints toward the negative !otential. 0he laminated core concentrates the magnetic lines of flu* of this field around the tube. 0he units normally used for magnetic flu* density. which is the length of the conductor. 8urrent flow through the coil !roduces a magnetic field. are webers !er s3uare meter. 0he magnetic flu* density is a term which describes the strength of the magnetic field. ). 0he !rocess fluid is the moving electrical conductor. when an electrical conductor moves through a magnetic field in a direction !er!endicular to the magnetic lines of flu* an -4= is induced into the conductor. 0randucers. a coil. that is from the north !ole to the south !ole of the magnet. :f the inde* finger !oints in the direction of the magnetic lines of force. where a weber is the measure of the number of magnetic lines of force. :t is the number of magnetic lines of force !er unit area. R O magnetic flu* density O velocity of the conductor O length of the conductor 0he direction or !olarity of the induced -4= is determined by using the leftBhand rule. the -4= sensed by the electrodes is !ro!ortional to the velocity of the fluid. )y concentrating these lines of flu*. an -4= is induced into it that is sensed by a !air of electrodes. -lements.As was stated !reviously. 0he tube directs the !rocess fluid through the concentrated magnetic field. as illustrated in !igure "). then. :f the magnetic field strength and the distance between the electrodes is constant. a laminated core and the -4= sensing electrodes. 0he electrodes sense the average -4= of the fluid. @hen stainless steel is used the interior of the :nstrument 0rainee 0as' 4odule 122/& #% . the magnetic flowmeter measures the average velocity of the fluid regardless of whether the flow is laminar or turbulent. @e should remember from that discussion. $eftBFand ule for :nduced -4= =igure 2/. 0y!ical materials used in the construction of the flowmeter tube are fiberglass reinforced !lastic or nonB magnetic stainless steel. that the velocity of the fluid close to the !i!e wall is less than the velocity of the fluid flowing at the center of the !i!e. 0his also allows biBdirectional flow measurement.. :n addition. the characteristics of fluid flow were discussed. 0he tube carrying the !rocess fluid must be made of a nonBmagnetic material to allow the field to !enetrate to the li3uid. 0hese variations in the velocity !rofile do not affect the flow measurement accuracy of the magnetic flowmeter. 4agnetic =lowmeter At the beginning of this cha!ter.=igure 1. it must be made of a nonBconducting material so that the !otential induced into the fluid is not short circuited by the !i!e. 0herefore. the crystal converts electric energy into mechanical energy. A device ca!able of ma'ing both of these conversions is the !ieCoelectric crystal. 0heir a!!lication to field installations has awaited the availability of costBeffective electronics ca!able of accurately measuring small changes in time or fre3uency./// FC6. :t is essential that these electrodes remain free of dirt. when an -4= is a!!lied across the two !lates on either side of a crystal. :t is as if the crystal became a battery for an instant. the transmitter must convert an electrical signal to a mechanical motion. !olyurethane. 2. before the use of sound waves to determine flow can be e*!lained. !latinum electrodes are often used. =urthermore. the normal sha!e of the crystal is distorted. the crystal converts mechanical energy into electrical energy. !ieCoelectricity is a !ro!erty of nonconducting solids that have a nonBsymmetrical crystal lattice structure. @hen an -4= of o!!ositeB!olarity is a!!lied. Fowever. Dirt acts as an electrical insulator and reduces the accuracy of measurement. "ound is the transmission of very small !ressure variations through a medium to a receiving device. and transmitters < 4odule 122/& #& . 0his section will discuss the two most common a!!lications of ultrasonic flow measurement9 they are the time difference method and the fre3uency shift method of flow measurement. a small -4= is develo!ed between the two !lates. the crystal s!rings bac' to its original sha!e and an o!!osite!olarity -4= is develo!ed between the two !lates. -lements. 0hese two reci!rocal effects that occur in a crystal are 'nown as the !ieCoelectric effect. :n this way. 0he ty!ical fre3uencies used by ultrasonic flowmeters are in the range of 1 4FC to 1/ 4FC. 0hey actually come in contact with the !rocess fluid. 0randucers. "!ecifically. @hen the !lates are released." Altrasonic flowmeters are actually a grou! of devices based on well documented theory. or glass. =or instrument a!!lications. Altrasonic sound waves are above the range of fre3uencies audible by the human ear 52/. but for highly corrosive service.tube is lined with a nonBconducting material such as teflon. 0he electrodes are flush with the tube interior.(. :n this way. and the receiver must convert this motion bac' to an electrical signal./ A$0 A"O7:8 =$O@4-0. 0y!ical deformations that can occur in a crystal are shown in !igure #". some characteristics of sound and the method of detecting and transmitting it must be understood. the !hysical distortion of the crystal is reversed. @hen a crystal is held between two flat metal !lates and the !lates are !ressed together. 0he electrodes must be made of good conducting material. 0hey are usually made of ty!e #1( stainless steel. 0he determination of fluid flow by the use of ultrasonics im!lies the transmission and rece!tion of sound waves. we determined that flowrate was !ro!ortional to the density of the fluid. 0he electronic signal a!!lied to the transmitting crystal should be of the same fre3uency at which the crystal resonates in order for the crystal to sustain its mechanical oscillations. :n a !revious discussion. "imilarly. the crossBsectional area of the !i!e through which the fluid flows and the velocity of the flowing fluid. the receiving crystal should be matched to the transmitting crystal. where: . Asing the following e3uations.=igure 21. sound is the transmission of small !ressure variations through a medium. m v A S O O O O O volumetric flow rate 5ft1?min6 mass flow rate 5lbm?min6 fluid velocity in 5ft?min6 flow area in 5ft26 fluid density in 5lbm?ft#6 O v * A or m O * A * v :f the flow area and the fluid density were 'now constants. its siCe and its sha!e. 0he molecules do not travel an a!!reciable distance. only the variations in !ressure actually move. flowrate could be determined by measuring the velocity of the flowing fluid. 0he method :nstrument 0rainee 0as' 4odule 122/& #( . 2ieCoelectric 8rystal Deformation -very crystal has a resonant fre3uency that is de!endent u!on its structure. Altrasonic flowmeters are used to measure velocity. As !reviously mentioned. then. then flowrate can be determined: . @hat actually occurs is that the molecules of the transmission medium are alternately com!ressed and rarefied 5s!read out6. /// ft?sec. 0his means that if sound is transmitted in the direction of the fluid flow. =igure 22. 0he o!eration of this instrument re3uires that the velocity of sound in the fluid. 8onversely. 0he detectors used are !ieCoelectric crystals identical to those discussed in the !receding !aragra!hs. 0he velocity of the fluid can be calculated using the following e3uation: vf then: vf O where: vf vm 8 $ t* O O O O O velocity of fluid measured velocity of sound with flow s!eed of sound with no flow distance from transmitter to receiver time for fluid to travel from transmitter to receiver $ B8 t* O vm O 8 "ince $ and 8 are constants and t can be measured. :n this a!!lication the difference in time between transmission with the flow and transmission against the flow is measured./0. 7ow the increment of -lements. at rest. be accurately 'nown. if the sound is transmitted in a direction o!!osite the fluid flow the actual velocity will be the difference between the two velocities. Altrasonic =lowmeters A similar method of determining the flow rate is illustrated in !igure ##. and transmitters < 4odule 122/& #+ . the absolute velocity of the !ressureBdisturbance !ro!agation is the algebraic sum of the two. the actual velocity of the sound is the sum of the sound1s velocity when the fluid is at rest !lus the velocity of the fluid.em!loyed to measure the velocity of the fluid is rather sim!le. 0hen when flow e*ists. 0randucers. 0he rate at which sound is !ro!agated through a given medium at rest is constant9 for water is &. the velocity of the fluid and thus. !igure ## is a sim!le s'etch of the detector !lacement for ultrasonic measurement of flow. the sound will travel from transmitter to receiver at a greater velocity. the flow rate can be determined. :f the fluid also has a velocity. As the t increases. 0he e*ternal mounts re3uire the sound to be transmitted through the walls of the !i!e.time can be measured directly and is larger than the difference between vm and 8 in the first e*am!le. 0he fluid velocity is calculated as shown below: tw O $ $ andt a O 8Iv 8B v and: so: Tt O ta B tw ∆t O $ $ B 8B v 8Iv or: if: then ∆t O 2$v 8 B v2 2 v2 UU c2 ∆t O 2$v 82 or: where: vO ∆t82 2$ $ 8 v tw ta O O O O O distance between sensors s!eed of sound with no flow velocity of fluid sound transit time with flow sound transit time against flow 0his e3uation shows that fluid velocity is linearly changing with the measured time difference. if the fluid velocity is multi!lied by the cosine of the angle /. 0he two devices analyCed thus far offer little resistance to fluid flow. 0he !reviously develo!ed e3uation is still valid. the fluid flow rate is also increasing. Also. An e*am!le of an ultrasonic flow measuring device that re3uires no !i!ing !enetrations is illustrated in !igure #$. both devices re3uired that the detectors be !laced within the fluid stream. . however. Fowever. and therefore. :nstrument 0rainee 0as' 4odule 122/& #. :n most cases the device is calibrated for only one schedule !i!e so the transit time of the sound through the !i!e is 'nown. Fere the detectors are mounted e*ternal to the !i!e and function as both the transmitter and the receiver 5transceiver6 for the sound. 0his adds another variable for which an adjustment must be made. cause a negligible !ermanent head loss. the sound is no longer traveling in a !ath !arallel to the fluid. Altrasonic =low 4easurement 0he fre3uency shift method of ultrasonic flow measurement uses detectors !laced as shown in !igure #%. 0he am!lifiers are actually selfB e*cited oscillators. in turn. 0he transient time. 0he o!eration of this system is based on measuring a difference in fre3uency. 0he actual relationshi! of the fre3uency difference to fluid velocity is develo!ed below: =igure 2%. 0he fre3uency of either oscillator is e3ual to the inverse of its signals transient time through the fluid. the fre3uency of oscillation is a function of the signal transient time through the fluid. =re3uency "hift Altrasonic =low 4easurement =a O 1?ta and fb O 1?tb )ut: ta O $ $ andtb O 8 I vcosV 8 B vcosV 0herefore: fa B fb O1?ta 1 / tb O $ 8 B vcosV -lements. 0he advantage of this system over the time difference method is that it can be made to be inde!endent of the s!eed of sound in the fluid at rest.=igure 2#. 0herefore. =ollowing the initial !ulse. and transmitters < 4odule 122/& #. . each succeeding !ulse is triggered by the recei!t of the !revious !ulse. Am!lifier A oscillates at a fre3uency that is greater than the fre3uency of am!lifier ). 0randucers. is a function of the magnitude of the fluid flow. 0E2. the ca!acitance is calculated using the following e3uation: 8 O /.-$ D-0-80O " A ca!acitor consists of two conductors se!arated by an insulator. =igure 2&. 8a!acitance is measured in farads. W A d where: 8 O ca!acitance in !icofarads A O !late area 5* X y6 in s3uare inches d O distance between !lates in inches W O dielectric constant /.22%. 8a!acitors =or the !late ca!acitor. A one farad ca!acitor is ca!able of storing one coulomb of charge for each volt a!!lied. 2. O conversion factor :nstrument 0rainee 0as' 4odule 122/& %/ .+. !igure #& is a sim!lified s'etch of a !late and a cylindrical ca!acitor.$-. 0he insulator is referred to as the GdielectricG and the conductors are referred to as the G!latesG of the ca!acitor.so: ∆f O vO 2vcos $ or: ∆f$ 2cos V where: fa O fre3uency of am!lifier A fb O fre3uency of am!lifier ) ta O sound transient time across fluid tb O sound transient time across fluid tb O s!eed of sound in fluid at rest v O velocity of the fluid $ O distance between detectors 0his illustrates that fluid velocity is directly !ro!ortional to the difference in am!lifier fre3uencies.22%./ 8A2A8:0A78. / .0he dielectric constant is a factor that com!ares any material to a !erfect vacuum.(1% O conversion factor =or the measurement of level in nonBconductive materials.// %. A ca!acitor with a vacuum dielectric has a dielectric constant of one.# 2. 0he actual measured ca!acitance is the sum of 8 1. 8v and 8$. efer to !igure #' for a diagram of this ty!e of installation. 8a!acitor 8$ re!resents the ca!acitance of the !ortion of the !robe e*!osed to the li3uid././ %. it re!resents the ca!acitance of the leads and measuring system. 0randucers. and transmitters < 4odule 122/& %1 . the tan' walls serve as the other !late. ). 8a!acitance 81 is unaffected by tan' level. Ta/le " lists the dielectric constants of some common materials..//( 2. 8a!acitor 8 v re!resents the ca!acitance of the !ortion of the !robe e*!osed to the va!or./ 0able 1. its value is a function of the tan' level and the li3uid1s -lements./0 in inches W O dielectric constant /.. Dielectric 8onstants for 8ommon 4aterials 0he ca!acitance of the cylindrical ca!acitor in !igure #& is calculated using the following e3uation: 8 O /.Y6 @ater 5212Y6 Dielectric 8onstant 1. and 8 are the dimensions indicated in !igure #&./ . e!lacing vacuum with !a!er doubles the ca!acitance9 !a!er has a W of two.. 0he !robe serves as one !late of the ca!acitor. its value is largely determined by the tan' level and dielectric constant of the va!or. Wv .(1% W where: A. a bare metal !robe is used.// 1. 4aterial .acuum Air 2a!er DuartC 0eflon @ater 5#2Y6 @ater 5(. =igure 2(. there is a definite advantage to ma'ing the measurement at high fre3uencies. 4easured ca!acitance is at its ma*imum value of 8 1 I 8$ ma*. efer to the !revious e3uations as necessary during the following discussion. the rate at which 8 $ increases is greater than the rate at which 8 v decreases because of the difference in their dielectric constants. 81 I 8v ma*. measured ca!acitance is minimum. 8$ starts to increase and 8v begins to decrease. but still small because W v is small. the insulated !robe has been develo!ed. is a diagram of an insulated ca!acitance !robe level measurement system. as shown in the e3uation below: "!an O u!!er range value B lower value "!an O 81 I 8$ ma* B 581 I 8v ma* 6 "!an O 8$ ma* B 8v ma* 0he change in ca!acitance is a linear function of tan' level. so. At this level. 0his resistance must be high when com!ared to the im!edance of the ca!acitors to ma'e its shunt !ath for current insignificant. 8 v is Cero and 8$ is ma*imum. =or use with conductive li3uids. W$. 0he dielectric constant of all gases is nearly unity. @hen the tan' is full. !igure #. the shunt resistance will be small. :f the fluid is conductive. efer again to !igure #'. 8onverting from a s!an given in !icofarads to one given in inches of tan' level re3uires 'nowing only the length of the !robe. As tan' level rises. 0he most commonly chosen insulating material is teflon. 0he actual value will vary with tan' level and the ty!e of nonBconductive material in the tan'. 0eflon has a dielectric constant of 2 and will for a ca!acitor with the :nstrument 0rainee 0as' 4odule 122/& %2 . Wv is smaller than W$. :n this regard. 0he total s!an of measured ca!acitance is the difference between 8 1 when the tan' is full and 8v. Fowever. ma'ing an accurate measurement of ca!acitance changes im!ossible. )are 8a!acitance 2robe dielectric constant. when the tan' is em!ty. 8 $ is Cero and 8v is ma*imum. "ome value of resistance is also !resent between ground and the !robe. Assuming the tan' is em!ty. 0he bare ca!acitance !robe can only be used on nonBconductive fluids. !robe and vessel acting as the !lates. 0he ca!acitor formed by the insulator is referred to as 8a above the li3uid and 8b below the li3uid surface. @hen the tan' is em!ty, the measured ca!acitance will be as shown in the sim!lified s'etch in !igure #(.a0. )oth 8v. and 8a are at their ma*imum values at this time. 8a!acitor 8v is small due to the low =igure 2+. :nsulated 8a!acitance 2robe =igure 2,. 8a!acitance $evel Detector 8ircuit dielectric constant of the va!or, therefore, the effective ca!acitance of the 8v and 8a in series is less than 8 v. As tan' level increases both 8 v and 8a decrease due to a decreasing !late area at a rate largely determined by -lements, 0randucers, and transmitters < 4odule 122/& %# 8v. Fowever, as shown in !igure #(./0+ the series combination of $ and 8 b is now in !arallel with 8v and 8a. 0he value of $ is small because the fluid is conductive. 0he value of $ can be made insignificant com!ared to Zcb , if the !ro!er fre3uency is chosen. 0he value of 8 b is increasing linearly with level at a rate much greater than 8 v is decreasing, so the net measured ca!acitance is increasing linearly. =inally, when the tan' is full, as shown in !igure #(.c0+ the measured ca!acitance, 8b as at its ma*imum value. 0he insulated !robe can be used on nonBconductive and slightly conductive fluids as well. Fowever, the resistance of the fluids has to be ta'en into account. Although this com!licates the measurement, it does not alter the result which is a measurable change in ca!acitance as a linear function of li3uid level. 0he level and measurement system ca!acitance, 81, will re!resent a static ca!acitance value unaffected by li3uid level. 0he value of 8 1 will be added to the ca!acitances calculated in !igure #( and must be subtracted by the measurement system. )are and insulated ca!acitive !robes can use the walls of the vessel as one !late of the ca!acitor, as we have seen. Fowever, this is not necessarily the only method. :f the vessel is of nonBuniform siCe, the second ca!acitive !late can be !art of the detector. 0o accom!lish this, the e*isting !robe is surrounded by another cylindrical !late. 0he outer !late is insulated from the inner !late and s!ace is allowed for !rocess li3uid and va!or to e*ist between them. 0he outer !late is then grounded to the vessel. 0hus a ca!acitor is formed between the !robe and outer !late with a dielectric that varies with !rocess li3uid level. 0he accuracy of ca!acitive level detection systems is affected by two significant errors. 0he first is that coating or wetting of the !robe by a conductive li3uid can change the detected surface level. 0his e*!lains the common use of the teflon since few materials will adhere to it. 0he second is that anything that can change the dielectric constant of the li3uid will also affect the measured ca!acitance. As can be seen form Ta/le "+ the dielectric constant of water varies greatly with tem!erature. 0his can be com!ensated for by the use of an 0D or thermistor as another in!ut to the measurement system. 8hemical and !hysical com!osition can also influence the value of the dielectric constant. =or these reasons, careful consideration should be given before selecting ca!acitive level measurement. 2.,./ A$0 A"O7:8 $-;-$ 4-A"A -4-70 0he determination of li3uid level by the use of ultrasonics im!lies the transmission and rece!tion of sound waves. Altrasonic sound waves are above the range of fre3uencies audible by the human ear 52/,/// FC6. "ound is actually the transmission of small magnitude !ressure variations through a medium. @ithout a vibrating transmitter, a medium of transfer and a receiver sensitive to small !ressure variations, there can be no sound. "ound, li'e any wave, is reflected whenever it encounters an interface of materials. An echo is an e*am!le of sound being reflected whenever it encounters an interface of materials. An echo is an e*am!le of sound being reflected by the interface of air and a solid material. 0he amount of sound reflected varies with the combination of materials at the interface. =inally, the velocity of sound in a given material is constant at a :nstrument 0rainee 0as' 4odule 122/& %% constant tem!erature. Fowever, the velocity will vary with chemical com!osition and tem!erature. =igure 2.. Altrasonic $evel 4easurement Altrasonic li3uid level measuring instruments normally utiliCe !ieCoelectric crystals identical to those used for ultrasonic flow measurement as discussed in 8ha!ter %. !igure #) shows a !ossible ultrasonic installation for the continuous measurement of li3uid level. An oscillator sends an A8 signal of short duration to the transmitter. 0he !ieCoelectric crystal converts the electrical signal to mechanical motion causing sound waves at an ultrasonic fre3uency to be !roduced. 0hese sound waves are directed at the li3uid surface. "ome of the waves are reflected from the surface bac' to a receiver. 0he receiver converts the sound waves to an electrical signal and determines the time interval from transmission to rece!tion of the sound. Once the time is 'nown, the distance to the li3uid surface can be determined using the e3uation below: Distance to "urface O ;elocity of sound P timeinterval 2 :f the distance to the surface is 'nown with res!ect to a reference !oint, normally the bottom of the tan', the height of the fluid can be found. !igure $* illustrates two other !ossible installations of ultrasonic level detectors. :n !igure $*./0+ the detectors are located at the base of the tan'. 0he sound is still reflected off the airBli3uid interface, but now the velocity of the sound wave in the li3uid is used in determining the distance traveled. :n !igure $*.a0 the detectors are located e*ternal to the tan'. 0he o!eration is identical to the circuit in !igure $*./0 e*ce!t that the time it ta'es for the sound to travel through the tan' wall is a constant value that is not related to the actual level. 0herefore, this time interval must be subtracted from the measured time before level can be accurately determined. -lements, 0randucers, and transmitters < 4odule 122/& %& / ):4-0A$$:8 "0 :2 0F..=igure #/. 2. atmos!heric conditions and the com!osition for the li3uid. "econdly. 0he coefficient of linear e*!ansion. "ubsurface Altrasonic $evel Detection 0he reliable determination of li3uid level using ultrasonics is limited by two factors. symboliCed [.. "ince the measurement is based on accurately 'nowing this velocity. ultrasonic level detection is suitable only for installations where these conditions are relatively constant. the velocity of sound varies with tem!erature. :t is defined as the change in length when it is heated. αO where: 2 B 1 1 ( 02 B 01 )  O a coefficient of linear e*!ansion l1 O length of the material at 01 l2 O length of the material at 02 01 O initial tem!erature 5M = or M 86 02 O new tem!erature 5M = or M 86 7ote: any unit of length may be used :nstrument 0rainee 0as' 4odule 122/& %( . 8are must be ta'en to ensure that the transmitted sound is reflected bac' to the receiver with sufficient intensity to reliably sto! the timer. and more significantly.4O4-0. 0he first is that the !lacement and aiming of the detectors is critical. is the 3uantity that describes how much a metal increases in length when it is heated. :t is defined as the change in length !er unit of original length !er degree change in tem!erature." All metals e*!and when heated9 some metals e*!and more than others. . when the stri! is heated. )imetallic 0em!erature Detector -lements. * 1/B( 1( * 1/B( 1% * 1/B( 12 * 1/B( /. @hen the room in which the stri! is located is cool.& * 1/B( + * 1/B( 1/ * 1/B( ( * 1/B( 1& * 1/B( =igure #1. :n !igure $" two !ieces of metal. they will always be at the same tem!erature. Ta/le # illustrates that the aluminum will e*!and twice as much as the iron when increased in tem!erature by the same amount. it will be forced to bend. 4any thermostats use this !rinci!le. 0randucers. the stri! is sufficiently straight to ma'e contact between !oints A and ). * 1/B( 11 * 1/B( #/ * 1/B( 0able 2. * 1/B( + * 1/B( /. and iron have been joined together. * 1/B( 1# * 1/B( 1. As the room is heated. 0he bimetallic stri! thermometer is a sim!le rugged device for measuring tem!erature. :t is made by bonding together stri!s of metal that have different coefficients of linear e*!ansion. 0he coefficients listed in Ta/le # are referenced at (. in order to accommodate the increased length of the aluminum. 8oefficient of $inear -*!ansion 1?Y= 12 * 1/B( 1/ * 1/B( 1/ * 1/B( . the device tends to bend toward the side whose metal has the lower coefficient of linear e*!ansion. 8oefficients of $inear -*!ansion 56 Aluminum )rass )ronCe 8o!!er Gold :ron :nvar 57iB=e6 7ic'el "ilver "teel 0in 1?Y8 22 * 1/B( 1. An e*am!le of a thermostat is shown in !igure #2.M =. the stri! bends until contact between A and ) is bro'en. * 1/B( . As the tem!erature of the bonded metal stri!s increases. as shown. aluminum. and transmitters < 4odule 122/& %+ . shutting off the furnace.0he coefficient of linear e*!ansion for metals is linear over only a small range of tem!eratures. "ince both stri!s of metal are joined. !erha!s turning on a furnace. 0hus. * 1/B( 1. 0he resultant motion moves the !ointer over a circular scale to indicate the tem!erature. 0he other end of the element is attached to a shaft. the deflection caused by heating the new element would have been much greater because aluminum e*!ands 2% times as much as :nvar when the tem!erature of the materials is changed by the same amount. the element e*!ands causing the shaft to rotate. 0he shaft and bimetallic element are su!!orted and centered in the case by guide bushings and bearings. Alloys of 7ic'elB:ronB8hromium or 7ic'elB:ronB4agnesium are used for the high e*!ansion metal more often than !ure metals because they have relatively higher and more linear coefficients of e*!ansion. if the aluminum stri! had been bonded with :nvar.=igure #2. . the alloy. Generally the range of bimetallic element is from B2//M = to 1///M =. the scale of bimetallic thermometer is no linear over its entire range. )ecause the deflection caused by a change in tem!erature is linear over only a small range of tem!eratures. @hen heat is transferred to the thermometer. 0he amount of deflection !roduced by a bimetallic element. is most fre3uently used for the low e*!ansion metal. as illustrated in !igure $$+ one end of a s!iral or helical element is fastened to the inside of the thermometer case. an alloy of nic'el and iron. becomes larger as the change in tem!erature becomes larger or as the difference between the coefficient of linear e*!ansion of the materials becomes larger. )imetallic "witch :n the e*am!le. :nvar. :nstrument 0rainee 0as' 4odule 122/& %. therefore. :n a bimetallic thermometer. A !ointer is fastened to the to! of the shaft. :n the manufacturing of bimetallic thermometers. and transmitters < 4odule 122/& %. 0he following e*am!le illustrates this !oint. .0he reason for winding the bimetallic element into a heli* or s!iral is to ma'e the length of the bimetal longer than the available containing s!ace. 0randucers. "ince a difference in metal length is what causes stri! deformation. :ndustrial )imetallic 0hermometer Asing Felical -lement 0he following e*am!le shows that the greater the initial length of a given bimetallic stri!. the greater the difference will be in final metal lengths following a given tem!erature change. we may conclude that a larger instrument deflection may be obtained by utiliCing a longer bimetal. -lements. :ncreasing the length of bimetal !roduces a greater movement. =igure ##. Fowever.1/. 0homson. the e*isting em!irical data allows very accurate and reliable tem!erature measurement with thermocou!les.4O8OA2$-" A thermocou!le is a device that can convert a difference in tem!erature directly to electrical energy./ 0F. Our understanding of the thermoelectric !henomenon is based on the observations of three men: 0homas "eebec'. :nstrument 0rainee 0as' 4odule 122/& &/ . 0he actual !hysical !rinci!les that underlie the thermoelectric effect are still not fully understood. \ean 2eltier and @.2. 0. "eebec'1s thermocou!le circuit is shown in !igure $%. a German scientist. the larger the observed current flow. if the direction of current flow is reversed.&%. )oth heating and cooling are !ossible from a single device by reversing the direction of current flow. and transmitters < 4odule 122/& &1 . 0homson in 1. =igure #%. the junction that was warm becomes cool and vice versa. 0his is 'nown as 2eltier heating and although small. and electric current began to flow in the circuit. 0homson theoriCed that an -4= is induced into a homogeneous material if a differential tem!erature e*ists across it. the tem!erature at the junction may be controlled. 2eltier heating should not be confused with \oulian 52 6 heating which e*ists in every electrical circuit. )y controlling the current flow to a ban' of thermocou!le junctions. when heat was a!!lied to one junction of the iron and co!!er wire. 0his minimiCes the actual current flow in the circuit. 0he -4= that causes this current flow is 'nown as the "eebec' -4= and the thermoelectric !henomenon is referred to as the "eebec' -ffect. :t is also one reason that most thermocou!le measuring devices are of the nullBbalance ty!e. "eebec'1s 0hermocou!le 8ircuit 0hirteen years after "eebec'1s discovery.0he first thermocou!le was formed by 0homas "eebec'. 0his effect may be reversed to e*!lain the current flow that results from an a!!lied differential tem!erature. 0his !henomenon is referred to as the 2eltier -ffect. 0he 2eltier -4= is not sufficient to e*!lain all of the -4= observed by "eebec'. 0. it is one source of error in thermocou!le circuits. 0hat is. in 1. 0homson was only able -lements. :t was later demonstrated that the "eebec' -ffect is actually the sum of two se!arate !henomenon9 the 0homson -ffect and the 2eltier -ffect. 0he 2eltier -ffect relates an a!!lied current to the resultant differential tem!erature. 0his -4= is generally small and is inca!able of sustaining a current flow.21. Another a!!lication of the 2eltier -ffect is the thermoelectric refrigeration or heating unit. 2eltier observed that by a!!lying an electric current he could cause one junction to become warmer and the other cooler than ambient tem!erature. 0he larger the difference in tem!erature. ecall that the tem!erature difference is a function of thermocou!le current flow. Fe observed a current flow that was directly related to the difference in tem!erature between the hot and cold junctions of two dissimilar metals. 0randucers. 0his discre!ancy was successfully e*!lained by @. \ean 2eltier modified the basic thermocou!le circuit with the addition of a battery. 8onversely. One conse3uence of the 2eltier -ffect is the heating or cooling of the measured junction in a thermocou!le circuit when current flows. its out!ut voltage !er M8 would be e*!ressed as 2(.. =or this reason.1 0hermoelectric 2ower 0o ma'e thermocou!le junctions useful./ ]v?Y8.1 B ]v?Y86 O %1. ]v?Y8 B 5B 1%. a method must be established to com!are the -4= resulting from one combination of metals with other !ossible combinations.%+ B#. 0hermoelectric !ower is designated by the symbol GDG and has units of microvolts?M8.1/.+( Asefol 0em!. 0he 0homson -4= e*actly accounts for the discre!ancy between the "eebec' and 2eltier -ffects. Assuming that a thermocou!le1s out!ut voltage has a !erfectly linear relationshi! with changes in tem!erature.1 2. 0hermocou!le 4aterials 0he magnitude of the voltage change !er degree 8elsius for a thermocou!le com!osed of any two materials is the difference of their thermoelectric !owers.to !rove his theory using inferred measurements and electrostatics.1 2(. 0he thermoelectric !ower of some common thermocou!le materials is listed in Ta/le $. the following formula may be used to determine the out!ut voltage for an a!!lied differential tem!erature. D2 O 0ermoelectric !owers f thermocou!le materials 0 O 0em!erature of measurement junction 0r O 0em!erature at reference junction :nstrument 0rainee 0as' 4odule 122/& &2 . it a major cause of inaccuracies in thermocou!le circuits.. -out O 5D1BD2650B0r6 @here: -out O Out!ut voltage D1. ange 5Y86 B2// B2// B1// B2// B2// 1// 1/// 11// 11// 2// 0able #. 0he thermoelectric !ower of a material is the -4= !roduced by a 1M8 tem!erature differential im!osed on a thermocou!le made of 2latinum and the material under test. the 1M 8 tem!erature difference is im!osed while one of the thermocou!le junctions is held at /M8. =or e*am!le. 2ure !latinum has been chosen because it has an e*tremely small and linear 0homson -4= associated with it. Although generally small. 0he fact that a thermocou!le circuit cannot sustain a current flow due solely to a differential tem!erature in often referred to as the $aw of Fomogeneous 8ircuits. 4aterial Aluminum 52ure6 8onstantan 5(/J 8u. 2. the 0homson -4= is distinctly nonB linear and may even reverse !olarity as the tem!erature difference varies. %/J 7i6 8hromel Alumel 8o!!er D 5micrivolts?Y8 at /Y8 B/. if a 8hromelAlumel thermocou!le is constructed.. B1%. 0his junction is referred to as the reference junction. 0o ensure the resulting -4=s will be com!arable. 0he use of thermocou!le charts is e*!lained later in this cha!ter. by welding. :f this were to occur. :f the value of D for each wire is constant with tem!erature. 8hoosing metals that !roduce a large change in out!ut voltage !er degree change in tem!erature is only one consideration in the choice of thermocou!le metals. 0hey must not corrode. the values of D should change as little as !ossible over the tem!erature range to be measured by the thermocou!le. for e*am!le. thermocou!le charts are used to determine the out!ut -4= of a thermocou!le for a given tem!erature differential. then the thermally induced -4= will increase linearly with tem!erature. is not the case.0he terms measurement and reference junction are terms that are used when a thermocou!le is used to measure an un'nown tem!erature. the value of D is rarely a constant over an a!!reciable range of tem!eratures. -lements. can the out!ut -4= be !ro!ortional to the tem!erature at the measurement junction. 0he formula is seldom used to determine actual thermocou!le circuit out!ut voltage because it assumes that the out!ut voltage is linearly related to the differential tem!erature. =or this reason. All of these restrictions considerably limit the metals which can be used in thermocou!les. 0randucers. =or this reason. or is com!ensated for these changes. o*idiCe or otherwise be contaminated by their environment. :n !ractice. in !rinci!le. 0his. 2. the useful range of the thermocou!le is limited to that tem!erature range over which the values of D of the two metals do not differ greatly from some constant value. 0he measurement junction is located in the un'nown tem!erature environment and the reference junction is located in an area of stable tem!erature. 0he !air of metals selected must be easily attached to each other. Only if the tem!erature at the reference junction does not change. the values of D might be a!!reciably altered. 0he thermocou!le wires must be easily and economically manufactured into uniform. homogenous wire. ecall that the -4= !roduced by a thermocou!le circuit is !ro!ortional to a differential tem!erature. 0he wires must be able to withstand the environment in which they are to be !laced. and transmitters < 4odule 122/& &# . %. however. :n order for a thermocou!le to be of !ractical use the metals must meet several re3uirements: 1. be used for a thermocou!le. 0he melting tem!erature of the metals must be greater than the highest tem!erature to be measured by the thermocou!le. of the two metals must be large enough so that the -4= !roduced can be measured accurately. D. #. ecall that the 0homson -4= is distinctly nonBlinear. Any two dissimilar metals can. &. :n addition. re!roducible. 0he difference in the thermoelectric !owers. :nstrument 0rainee 0as' 4odule 122/& &% . 8hromel vs alumel 50y!e W6 is used over the tem!erature range of B2// to 2#//M= and is more resistant to o*idation than other baseBmetal combinations. 2latinum B 1/J rhodium vs. 0hey are 'nown as the $aw of :ntermediate 0em!eratures and the $aw of :ntermediate 4etals.4-0A$" 0hermocou!le materials are available for use within the a!!ro*imate limits ofB#// to I#2//M = 5B1. 0he constantan is an alloy of a!!ro*imately &&J co!!er and %&J nic'el. )ase 4etals 1.%M =6. 8o!!er vs constantan 50y!e 06 is used over the tem!erature range of B#// to I+//M = 5B1./J nic'el and 1/J chromium. 2latinum B #/J rhodium vs !latinum B (J rhodium 50y!e )6 is similar to 0y!es " and . #. :ron vs constantan 50y!e \6 is used over the tem!erature range of B2// to I1%//M= 51#/ to I+(/M86 and e*hibits good stability at 1%//M= 5+(/M86 in nono*idiCing atmos!heres.4O8OA2$. 2.&M 8 511((M =6 to 1/(#M 8 51. 2J aluminum. 8hromel vs constantan 50y!e -6 !roduces the highest thermoelectric out!ut of any conventional thermocou!les. #. but each !ossesses characteristics desirable for selected a!!lications. 0his combination must be !rotected against reducing atmos!heres. 2rimary uses are found in the measurement of subBCero tem!eratures and has su!erior corrosion resistance in moist atmos!heres. !latinum 50y!e "6 is used for defining the :nternational 0em!erature "cale from (#/.1/. :t is used u! to 1%//M= 5+(/M86 and e*hibits a high degree of calibration stability at tem!eratures not e*ceeding 1///M= 5&#. 2.%&. 2.2 0F.% to I#+/M86.%J nic'el.1/. :t is chemically inert and stable at high tem!eratures in o*idiCing atmos!heres./&J of the measured -4=. #J manganese. Alternate cycling between o*idiCing and reducing atmos!heres is !articularly destructive.2.# 0hermocou!le $aws 0here are two basic laws of thermoelectricity which govern thermocou!le theory and !ractice.M86. :t is widely used as a standard for calibration of baseBmetal thermocou!les. and 1 J silicon. 8hromel is an alloy of a!!ro*imately . %. 2latinum B 1#J rhodium vs !latinum 50y!e 6 is similar to 0y!e " and !roduces a slightly greater -4= for a given tem!erature.& to I1+(/M86. 0his thermocou!le will match the standard reference table to L. but yields somewhat greater !hysical strength and stability and can withstand somewhat higher tem!eratures. 0hermocou!les are classified into two grou!s identified as noble metals and base metals: Noble Metals 1. 7o single thermocou!le meets all a!!lication re3uirements. Alumel is a!!ro*imately . 0randucers. as long as the junctions of the two thermocou!le metals with the third metal occur at the same tem!erature. "im!ly stated. 0his relationshi! is shown gra!hically in !igure $&.0he $aw of :ntermediate 0em!eratures !rovides a means for relating the -4= generated by a thermocou!le under ordinary circumstances to the -4= generated by the thermocou!le with a standardiCed reference tem!erature 5normally /M 86. by observing the $aw of :ntermediate 4etals. the law means that if the tem!erature to -4= relationshi! is 'now for some reference tem!erature. :n effect. if and and then : : : : A ) 8 8 voltas volts volts volts O O O O out!ut of thermocou!le A with a ^0 of 02 _ 01 out!ut of thermocou!le ) with a ^0 of 0# _ 02 out!ut of thermocou!le 8 with ^0 of 0# _ 01 5A I )6 volts =igure #&. and transmitters < 4odule 122/& && . this can be avoided. 0his law is illustrated in !igure #(. then the -4= generate at any other reference tem!erature is !redictable. the law states that the sum of the -4=s generated by two thermocou!les9 one with its reference junction at /M 8 and its measurement junction at some intermediate reference tem!erature. 0he $aw of :ntermediate 4etals states that the introduction of a third metal into a thermocou!le will have no effect u!on the -4= generated. Fowever. =igure #(. the other with its reference junction at the same intermediate tem!erature and its measurement junction at the tem!erature to be measured9 is e3uivalent to the -4= !roduced by a single thermocou!le with its reference junction at / M 8 and its measurement junction at the tem!erature to be measured. $aw of :ntermediate 4etals -lements. $aw of :ntermediate 0em!eratures :t would seem that the introduction of a third metal into a thermocou!le circuit would modify the -4= develo!ed by the thermocou!le and affect its calibration. :t is essential that when the thermocou!le e*tension wires are connected. the correct wires are joined together.1/. in a more !ractical situation. Ta/les & and ' !resent the data for si* common thermocou!le ty!es in degrees =ahrenheit and 8elsius. :n this case. Fowever. 0he actual tem!erature./2+ mv. new thermocou!les could be formed and incorrect tem!erature indications will result. Fowever. according to the $aw of :ntermediate 4etals. :f the tem!erature of the junctions of the two thermocou!le wires and the e*tension wires is the same. the e*tension wire must be of the same material as the thermocou!le wire. res!ectively. :f the reference junction is at #2M= and the -4= is measured. assume a 0y!e 0 thermocou!le with a reference junction tem!erature of #2M= !roduces 11. 0his ma'es it !ossible to use welded or soldered thermocou!les and to use measuring instruments 5!robably co!!er6 with various connections in thermocou!le circuits. :f !ro!er connections are not made. or it must have the same tem!eratureB-4= relationshi! as the thermocou!le wire over the range of the ambient tem!eratures to which the e*tension wire is e*!ected to be e*!osed.% 0hermocou!le 0ables 0hermocou!le data is usually !resented in tables of -4= 5mv6 vs 0em!erature 5M8 or M=6 with /M8 or #2M= used as the reference junction tem!erature. from the table is %&/M=. 0hermocou!le 0y!e ) \ W or " 0 -*tension wire )Z -Z \Z WZ "Z 0Z -*tension @ire -lements )2Z 8o!!er )7Z 8o!!er -2Z 7ic'el 8hromium -7Z 8o!!er 7ic'el \2Z :ron \7Z 8o!!er 7ic'el W2Z 7ic'el 8hromium 27Z 7ic'el Aluminum "2Z 8o!!er "7Z 8o!!er 7ic'el Alloy 02Z co!!er 07Z 8o!!er 7ic'el 0able %. then co!!er could be used as the e*tension wire. 0he $aw of :ntermediate 4etals also ma'es it !ossible to use e*tension wires of metals less e*!ensive than the thermocou!le wire to connect the thermocou!le to the measuring instrument. 0ables are used because the relationshi! of -4= generated with res!ect to tem!erature is not !erfectly linear. !roviding all the junctions are at the same tem!erature. 0he use of these tables is e*!lained on the following !age. Ta/le % lists the ty!es of e*tension wire used in industry along with the ty!e of thermocou!le for which it is used. the reference junction will not be at #2M=. if this is not the case. -*tension @ire?0hermocou!le used in industry. =or e*am!le. the measurement junction tem!erature can be determined directly from the table. 2. the $aw of :nstrument 0rainee 0as' 4odule 122/& &( .Any number of different metals may be introduced. 1/..&/ .%2 2/. 1+.+/..((( #. and transmitters < 4odule 122/& &+ .&2/ 1./..%.(2 2..+1+ 1.1 1// 1..(#+ 2. we find the out!ut of a thermocou!le with a 5&/B#26M= differential tem!erature !lus the out!ut of a thermocou!le with a #2M= to actual measured tem!erature differential a!!lied..%2 1..%(& 2./#2 1. +.12& +.2+./1 # 1&.( 1....(+% &+..2%& 1..// /.1..% &...%#% 2(..&../&& my 5-4= associated with reference junction tem!erature6 !lus 2.+/ # 1.// . mv.+/ 2 12.#21 (.&.. 1&.+(/ ../ %.#.. 0he actual measured tem!erature is determined as follows.&.2 +.2..+/+ #%.(.%&# &..((.2.&( / 11.:ntermediate 0em!eratures must be a!!lied before the charts may be used to determine the actual tem!erature.&. 2.##...#% ./&. my 5the actual thermocou!le out!ut6 which e3uals 2. #.&1& #1.(%.%/2 /.&(( 1%. )y a!!lication of the $aw of :ntermediate 0em!eratures..1 1&/ 2.%(( (.( 2+.1(.2.&% #1. /.&# 2(. #&.&1( #.%11 2.2 ##.1 /.+2# my is (&/Y.#1+ 2%. 0herefore. &.(.. Assume a 0y!e " thermocou!le with a reference junction tem!erature of &/M= generates and -4= of 2./1 &.#/# %%./// /.. #. 2%.+/( 1#..+. 0randucers.(1( 2latinum vs 2latinum I 1/J rhodium 50y!e 6 /./ %./2/ &1../2# 12.+21 2/.1&% #.&/+ &../// /.(%+ . # 21..+%/ (.+ /.(/.211 2. /.// .% +.+# 2latinum vs 2latinum I 1/J rhodium 50y!e "6 /.2 2.../( ( 22.. 1. 1+.+2# mv..( #./( (.. 1.11/ #. %./. 1&.1 (.+.2/& . %.#.2. the corrected thermocou!le out!ut referenced to #2M= is /.2.2& %. %../&% /.%#/ 1/..2&# 21. the tem!erature interval is smaller than the &/Y= 5or &/Y86 intervals used here.+&.+%.&%+ %(.&2# 11. 1../&( %2. 1+. 1+..(. 2.221 /. %. (..### %. #./2+ 12.1.+11 %.&1.( %.& #.2%..%/+ %1.( #.&(2 #+.# 11.#.%(/ -lements.1+ .( %.&2 # 1./2/ 1.#1.2(& 1.&&2 2. :n the charts usually su!!lied with thermocou!le devices.2%( &#.&+2 1%.(% (. 2.1%# (././1+ 2./// /.%..1&# 1&.1.& 2#./(2 ..11/ #2.& % 1%..%(& &&. #&..%// /./%1 2// 2&/ #// #&/ %// %&/ &// &&/ (// (&/ +// +&/ .((. 2. %..& 2/.%+. =rom the table.&/ 1// / 1/& / 11/ / 11& / 12/ / #.2%.#./.& &.+(. 11.&/+ /.%+ .+.(+ &.%1( 1./&& /.&/. 5I6 8hromel :ron 8o!!er 8hromel 5B6 vs vs vs vs Y= Alumel 8onstanta 8onstanta 8onstanta 50y!e W6 n n n 50y!e \6 50y!e 06 50y!e -6 #2 /.+2# 2./2.+ /. 1(./.#1% . #+.# 22. 0his reduces the labor of inter!olation and allows more accurate results./# 1../// /.+.../..21.%2/ +. %/./// &/ /.2# (. 21.1+/ 11.2#% %../%1 1.// .#1 & #/....%...2++ #&.. ..+./(# (2./// /. #/./.#1% #+. 2&.#1/ %1.(%& 1.. .&1% +..2# 2.%1.1 # (/.+ 1#.2% (.. 5I6 8hromel :ron vs 8o!!er 8hromel vs 2latinum vs 2latinum vs 5B6 vs 8onstanta vs 8onstantan 2latinum I 2latinum I Y8 Alumel n 8onstanta 50y!e -6 1/J rhodium 1/J rhodium 50y!e W6 50y!e \6 n 50y!e "6 50y!e 6 50y!e 06 / &/ 1// 1&/ 2// 2&/ #// #&/ %// %&/ &// &&/ (// (&/ +// +&/ .%(. .+..&.1# +.&.+%1 +./11 1%./// 2.2#+ &.1#/ %2.. %1.2.( /.2.&/# 11.../21 &.#./.( #.%# #2./// 2.22.// .& 1/.%# /..%+ & #1.(1 2. %#.%# & 2%.#1+ .%+1 &. 1. 1%.+++ 1#.(%+ 1. 1/.. #2. 0em!erature 5M=6 for 0hermocou!les ef .#2& #.+.11/ &+./22 (%. 1/...12.(1 ..(/ #(.&1# 2/.#2& 1./## 2%.//.#%& +./22 2..++ & ##../.&# %..1#+ .&/ 1// / 1/& /.(2 . .21% ##.1( 2/.%( :nstrument 0rainee 0as' 4odule 122/& &.%// 2.+# /.+.%/+ #..2 (.+# 2. 1.##./22 %.2/.& &.1(& 1/.+ & 2.2/2 %&.# %&...2./// #./#& %..2%/ (.2 ... &#.&+/ .2(. /.+/2 ..& (.../12 .1/.12& / 1#/ / 1#& / 1%/ / 1%& / 1&/ / 1 2#. .%( 2%./..#&..212 +. 1+.(/ 1+.+#2 &.& %.+&1 (.+.1% .( #(.# (#./2.. +.1&1 12...( #..( 12.## %. 2(..++2 2%.#. 21.2(.2(/ #./& +. #1.//( #.1+../// 2.# +(.1&& (.1/.2.%%./// /.#2# 2.&&# 1(./%& %&. -lectromotive =orce vs.# +2.%%/ 1../.1%1 0able &..1#+ 1/. /.&./%+ (..(%/ 22...&/ .2++ (.&.2/# .+&% /.....& 1. /.2 1(.2+% (.. . 2.(/+ 2+..%./2 2+.21/ ##.+%# %. . 0rainees should be familiar with both the letter designations and the color codes used to identify thermocou!le leads and thermocou!le e*tension wires.2 1%.1+( 12. 0hey are designated by the letter Z.. 48.%%& 1.+(1 1%.#. 0he !receding !age lists the ty!e and the materials used to define the tem!eratureB-4= relationshi!s 2olarity is also indicated by the thermocou!le letter designations./ 11/ / 11& / 12/ / 12& / 1#/ / 1#& / 1%/ / 1%& / 1&/ / 1&& / %(. and transmitters < 4odule 122/& &.+# 1&. :n the letter designation. Although the letter designation is often associated with a !articular thermocou!le material.. lists the lead designations for thermocou!les.%+ 12. 0o ma'e this tas' easier. -lectromotive =orce 5m. -lements.& Designations for 0hermocou!le @ire 0he trainee must be able to recogniCe the ty!e of thermocou!le and the !olarity of thermocou!le wires in order to ensure that thermocou!le circuits are !ro!erly installed. the letter designation a!!lies only to the tem!eratureB-4= relationshi! and not to the material. &/.(2% 1&.22% 1#.& %.#(.6 vs.. for the !ur!ose of establishing uniformity in the designation of thermocou!le wire. $etter designations have been used for many years to identify thermocou!le ty!es. -*tension wires are fre3uently used the thermocou!le circuits.#2.1&& 1#. 0randucers.&&/ 1#..#%..12& 11... the :nstrument "ociety of America 5:"A6 has develo!ed a standard. 1%./#& 1(.1%% 0able (. 11. 0em!erature 5M86 for 0hermocou!les ef. Other materials having the same tem!eratureB-4= relationshi! and better !hysical !ro!erties for a !articular a!!lication may be given the same letter designation.2. 2 re!resents the !ositive wire and 7 re!resents the negative wire.. Ta/le .1/. 1(.(.(## &2.+%1 1+.. \unction at /M8 2.&+( 1(. .1 0em!erature 4easurement 0hermocou!les.&22 1#. &%. in the :ron vs. 0his redBnegative designation is endorsed by the :"A for all thermocou!le ty!es. Ta/le ( summariCes the !ractice of color coding leads of thermocou!les and thermocou!le e*tension leads. 0he first wire named is !ositive and the second is negative. the :ron wire is !ositive and the 8onstantan wire is negative. 0he indicated !olarity of the thermocou!le leads a!!ly for conditions where the measuring junction is at a higher tem!erature than the reference junction. the color red designates the negative wire. =or thermocou!les and thermocou!le e*tension wires. Another common method of designating the !ositive and negative !olarity of thermocou!le wire is by the order in which the material of the leads is written or s!o'en. =or e*am!le.0hermocou!le @ire 0y!e ) \ W " 0 2ositive )2 -2 \2 W2 2 "2 02 7egative )7 -7 \7 W7 7 "7 07 0able +. 8onstantan thermocou!le 50y!e \6. 8olor 8oding of 0hermocou!le @ire :nstrument 0rainee 0as' 4odule 122/& (/ . Du!le* :nsulated 0hermocou!le @ire 0y!e \ W 0 0hermocou!le 2ositive -2 \2 W2 02 7egative -7 \7 W7 07 OverallQ )rown )rown )rown )rown 8olor of :nsulation 2ositiveQ 7egative 2ur!le ed @hite ed Eellow ed )lue ed "ingle 8onductor :nsulator -*tension @ire -*tension @ire 0y!e 0y!e 2ositive 7egative ) )2Z )7Z -2Z -7Z \ \2Z \7Z W W2Z W7Z or " "2Z "7Z 0 02Z 07Z 8olor of :nsulation 2ositive 7egativeQ Gray edBGray 0race 2ur!le edB2ur!le 0race @hite edB@hite 0race Eellow edBEellow 0race )lac' edB)lac' 0race )lue edB)lue 0race 0able . "ymbols for :ndicating 2ositive and 7egative 0he use of color codes in electronics for indicating the value and !olarity of com!onents has been an acce!ted !ractice for many years. 0he manufacturers of thermocou!les also use color coding to designate both the ty!e and the !olarity of thermocou!les and thermocou!le e*tension wires.. 0randucers. and " thermocou!les as well as all ty!es !reviously discussed. and transmitters < 4odule 122/& (1 . -lements. gauge wire.Du!le* :nsualted -*tension @ire -*tension @ire 0y!e 0y!e ) \ W or " 0 2ositive )2Z -2Z \2Z W2Z "2Z 02Z 7egative )7Z -7Z \7Z W7Z "7Z 07Z Overall Gray 2ur!le )lac' Eellow Green )lue 8olor of :nsulation 2ositive Gray 2ur!le @hite Eellow )lac' )lue 7egativeQ ed ed ed ed ed ed 0able . the thermal and chemical environment to which the thermocou!le is to be subjected. 0he factors that are considered in determining the correct fabrication !rocess include the ty!es of material used. without damaging the metallurgical !ro!erties of the wires. .1/. 1% gauge wire. .( 0hermocou!le 8onstruction 0here are various fabrication methods used in the manufacture of thermocou!les. 0he !rimary re3uirement when joining thermocou!le wires is to maintain good thermal and electrical conductivity. 4ethod 5a6 is em!loyed in the manufacture of 0y!e \ and W thermocou!les using 7o. method 5d6 illustrates the G. "everal joining methods are illustrated in !igure $. and is utiliCed to join 0y!es ). and W thermocou!les. through 7o. 8olor 8oding of 0hermocou!le @ire 2. \. W and 0 thermocou!les using 7o. 0he twisted wires shown in method 5c6 of the figure are either arc welded or gas welded. 0his manufacturing method can be used in the fabrication of 0y!es -. Analysis of these factors determines how to join the dissimilar metals.. and what ty!e of insulation and !rotection tube to use. gas or 0:G welded. and is used to join 7o. 2. and utiliCes the resistance welding techni3ue. .G joint which may be arc. and the electrical insulation and mechanical strength re3uirements. through 7o. 58ontinued6. 2/ gauge wires in 0y!e -. =inally. 0his joint is also made with resistance welding. and 7o. . \. 4ethod 5b6 is 'nown as a butt weld. "ome materials such as teflon and !olyvinyl chloride are used because of their natural resistance to absorbing moisture. A sheathed thermocou!le is shown in !igure $(. 0he metal is then reduced in diameter by some mechanical !rocess which thereby tightly com!acts the insulation around the thermocou!le wires. 0he tube also !rovides a layer of se!aration between the element and any !otentially harmful environmental conditions. a !rotective tube is used to !revent mechanical damage to the sensing element. Another ty!e of construction that is becoming increasingly more !o!ular is the sheathed thermocou!le. 0his device consists of a matched !air of thermocou!le wires surrounded by a metal case. and resistance to corrosion. 8eramic or 8ermet tubes are used at tem!eratures e*ceeding a!!ro*imately 21//M=. 0his ty!e of construction !ossesses the most desirable characteristics of the !reviously discussed ty!es and in addition. "heathed 0hermocou!le #. resin or similar substance.arious 0hermocou!le \oints 0here are several insulating materials that can be used to cover the sensing wires in order to !revent electrical shorts and to !rovide abrasion !rotection. such as carbon steel.=igure #+. it is easier and less costly to fabricate. austenitic stainless steel. 8overing the insulator./ "-8O7DA E -$-4-70"B :nstrument 0rainee 0as' 4odule 122/& (2 . leaving the thermocou!le wire un!rotected. =igure #./. 8ertain fiber ty!e materials may also be used when im!regnated with a wa*. and high nic'el alloys9 ceramics li'e mullite9 and combinations of metals and ceramics.. 0hese im!regnated fibers should not be used in a!!lications where tem!erature e*ceeds %//M=. highBdensity. they are sometimes unsuitable due to their susce!tibility to corrosion. . materials li'e fibrous silica. 8eramic insulators can o!erate in environments e*ceeding 2&//M=. :f it is desired to measure tem!eratures greater than 12//M=. and asbestos are among the best of the nonBceramic ty!es. 0hese insulators are ca!able of withstanding o!erating tem!eratures u! to 12//M=. 0he inside of the case is filled with a noncom!acted ceramic insulating material. because of their highB strength. =or high tem!erature a!!lications. @hile metals !rovide ade3uate !rotection against mechanical damage. called cermets. as this would va!oriCe the wa* or resin. and because of their tendency to become !orous at elevated tem!eratures. fiberglass. 4aterials utiliCed for tube construction are various metals. then a ceramic ty!e such as mullite or stellite must be used. 0randucers. is the bourdon tube. =igure #. s!iral ty!e. !igure $) shows a 8Bty!e bourdon tube. is e*!ressed in various forms of the following derived e3uation: ∆α O W α2 - ` 5A. 0he orifice !late is the !rimary element. 0he basic observation is that a bent tube with a crossBsection that is not a !erfect circle. then return to its original sha!e when !ressure is removed. with some !o!ular ones now discussed./ )OA DO7 0A)Another device that uses dis!lacement distance to measure !ressure. 8B0y!e )ourdon 0ube A 8Bty!e bourdon tube is fabricated by flattening the side of a hollow tube. 0he material of the tube is selected for having elastic !ro!erties. or the angular deflection of the ti!. 0here are many ty!es of secondary elements to choose from. however. and helical ty!e.1. 2resent designs are based on data collected.t.A secondary element is often used with a !rimary element in order to obtain a usable out!ut from a detector. One end of the tube is sealed. 0he device which measures the resulting !ressure dro! is called the secondary element. and transmitters < 4odule 122/& (# . which can be measured to calculate flow.). )ourdon tubes come in three basic ty!es: 8Bty!e.. #. 6 0he terms of this e3uation are shown in !igure %*. straightens out as !ressure is a!!lied. it is the device itself that dis!laces. so that it can deform under !ressure. :n this case. and a subse3uent e3uation derived from !ractical observation. an orifice !late causes a !ressure dro!. then bending the tube into the sha!e of a G8G. 0he amount that the tube straightens out. and the other o!en end is fi*ed to a su!!ort base. -lements. 0he !rinci!le under which the 8Bty!e bourdon tube wor's is not com!letely definable. since it causes the !ressure dro!. =or e*am!le. And ) change for a given T W tubes  2 tube A. of the ti! O 8onstant. generally about 1?% in. of the curvature =igure %/. terms . of the tube radius. 0he actual amount of ti! dis!lacement is relatively small. of the )ourdon element O differential 2ressure. 0he small and nonBlinear ti! motion characteristics are com!ensated for mechanically. :t is !rovided for a better understanding of the factors affecting bourdon tube o!eration. eferring to the e3uation. of the tube 0hic'ness.) t O Angular Deflection. determined by test on a number of )ourdon O 0otal Arc. 0he result is that the angular dis!lacement does not change in a linear manner. 8B0y!e )ourdon 0ube 2arameters change in 2. between inside and outside of the O O O O -lsticity. :nstrument 0rainee 0as' 4odule 122/& (% . note that as the tube straightens out.0he e3uation is not given in this cha!ter for !ur!oses of a!!lication. of the tube material 8rossectional $ength and @idth. as shown with the direct indicating !ressure gauge in !igure %". A. 0his angle is called the traveling angle because the angle changes the bourdon tube moves. @hen the effective tail section length is reduced.a0+ the bourdon tube ti! is !ositioned so such that the connecting lin' and the movement sector tail form a . 2ressure Gauge 0he !inion and movement sector !rovides mechanical am!lification. and the effective tail section length is decreased./0+ the bourdon tube ti! is !ositioned so such that the traveling angle is less than . A 1?%B inch ti! movement e3uates to an arc dis!lacement of 2/ M. smaller ti! movement in 5b6 causes an e3ual !inion movement com!ared to a larger ti! movement in 5a6./M.& to 1 results in a gauge needle dis!lacement of 2+/M. 0randucers. :n !igure %#./M traveling angle. A movement sector to !inion gear ratio of 1#.=igure %1. and transmitters < 4odule 122/& (& . $inearity com!ensation is !rovided by the angle formed between the connecting lin' and the tail of the movement sector. -lements. :n !igure %#. 0his results in an effective tail section length e3ual to the actual length between the !ivot !oint and the connection !oint. as shown in !igure %#. the traveling angle is designed to increase for an increasing !ressure. 0raveling Angle of a 2ressure Gauge As a!!lied !ressure to the bourdon tube increases. 8oeff./// 2sig = G 2 2 G 0able ./// 2sig 12.=igure %2. 0ube 4aterial 2hos!horus )ronCe )erryllium 8o!!er #1( "tainless "teel %/# "tainless "teel 7iB"!an 8Q 8orrosion esistanc e 2 2 G G G "!ring ate = G 2 2 G 0em!.. Another disadvantage is that using the gauge to monitor li3uid !ressure might yield a different indication than monitoring an e3ual gas !ressure. 8Bty!e )ourdon tubes have some disadvantages. 0he first is the large overhang of the tube. 0em!erature coefficient B Andesired ti! angular dis!lacement due to change in tem!erature of the tube Fysteresis B Difference in ti! angular dis!lacement between an increasing and a decreasing !ressure. 2ressure . the change in ti! motion decreases9 therefore. which ma'es it susce!tible to shoc' or vibration. G=G for fair and GGG for good./// 2sig 2/. lists different ty!es of materials used to ma'e bourdon tubes. 0heir characteristics are stated as G2G for !oor. Ta/le . 4a*imum !ressure B Fighest !ressure that can be safely a!!lied to the tube and still give satisfactory dis!lacement characteristics.// 2sig &// 2sig 1/. 2 2 2 2 G Fystersis 4a*. )ourdon 0ube 4aterials 8orrosion resistance B Ability to avoid chemical interaction with the li3uid or gas a!!lied to the tube "!ring rate B Amount of ti! angular dis!lacement for a given amount of a!!lied !ressure. :nstrument 0rainee 0as' 4odule 122/& (( . s!irals are available that can measure !ressure u! to 1//. !igure %% shows a Felical 0y!e )ourdon 0ube which !rovide even more ti! movement. or gears. A third disadvantage is that the internal gears and lin's can become dirty or worn and cause severe loss of accuracy. "!iral ty!es are normally used in the range of / to %. however. 0randucers.due to the weight of the li3uid in the tube. some unflattened. )ecause of the increased ti! movement. 0his results in an increase in sensitivity and accuracy because there is no lost motion from loose or stic'ing lin's./// !sig. As described earlier./// !sig. if not mechanically com!ensated for. levers. !igure %$ shows a s!iral ty!e bourdon tube that !rovides more ti! movement. mechanical am!lification is not normally needed. 0he same materials that are used for 8Bty!e are also used for s!iral ty!e bourdon tubes. one limitation of the 8Bty!e bourdon tube is the relatively small amount of ti! movement. heavy wall. and transmitters < 4odule 122/& (+ . "ince the s!iral length is #?% times longer than 8Bty!e length. can be minimiCed by o!erating the s!iral ty!e over only a !ortion of its range. -lements. =igure %#. "!iral 0y!e )ourdon 0ube 0he s!iral ty!e wor's under the same observed !rinci!les as the 8Bty!e: as the a!!lied !ressure increases. the ti! movement is #?% times greater. the s!iral uncoils. 0he effects of nonBlinear ti! motion. Figh !ressure helical ty!es might have as many as twenty coils.2. #.D-. s!iral ty!e. starting at the origin and a!!roaching astm!totically the !oint of bursting !ressure. 8Bty!e.e./// !sig. the deflection of a flat metallic dia!hragm is !ro!ortional to !ressure in a linear fashion only for a small range of low !ressures and low vacuums./. Felical 0y!e )ourdon 0ube 0he observed !rinci!les of o!erations are again the same as for the 8B ty!e.&J of the bursting load.=igure %%. )asic Dia!hragm Gauge 0he distortion of the dia!hragm under !ressure is transmitted to the gauge dial by a lin'age connected to the center of the dia!hragm. ecall that the change in ti! motion decreases as the a!!lied !ressure becomes larger. or helical ty!e6 to use is generally made by the manufacturer. u! to a ma*imum range of . while low !ressure helical ty!es might have only two or three coils. =igure %&. . 0herefore. Adding more coils com!ensates for this motion decrease. :nstrument 0rainee 0as' 4odule 122/& (. 0he !ro!ortional limits occurs at a!!ro*imately /. the decision over what ty!e of bourdon tube 5i. Ander !ressure./ D:A2F AG4 2 -""A .:8-" 0he metallic dia!hragm gauge consists of a metal disc built into a housing with one side of the disc e*!osed to the !ressure to be measured and the other side e*!osed to atmos!heric !ressure. =or a given !ressure gauge a!!lication. A basic dia!hragm gauge is shown in !igure %&. Felical ty!es are generally used for !ressure ranges above %/// !sig. a circular metallic dia!hragm will e*hibit a deflection curve sha!ed roughly li'e the letter G"G. and transmitters < 4odule 122/& (. teflon. im!regnated sil'. #./ 2 -""A . 2. be corrected in some way for tem!erature. 0he dia!hragm must be large enough to !roduce a satisfactory deflection under !ressure. -lements. 0his limits the !ressure range to low ranges. A corrugated dia!hragm !roduces a!!ro*imately four times the deflection of a flat dia!hragm subjected to the same !ressure. 0he single metallic dia!hragm is not generally used as a !ressure gauge element because of the following disadvantages: 1.0he deflection characteristics of a metal dia!hragm can be somewhat im!roved by corrugating the surface of the disc. !igure %' shows a corrugated dia!hragm. 0he dial scale must be hand calibrated to account for the !eculiarities of each dia!hragm. -ven a small !ressure overrange of the gauge can cause a !ermanent set. 8orrugated Dia!hragm 0he actual deflection de!ends on the diameter of the dia!hragm.. as shown in !igure %. %. therefore. :n addition. causing inaccuracies in subse3uent readings. 7onBmetallic dia!hragms are used to measure e*tremely low !ressures or vacuums. the thic'ness of the dia!hragm. the deflection curve is slightly more linear in character. 0he !ressure readout must. =igure %(. #. 0he elastic modulus coefficient of the metal is sensitive to tem!erature changes. and neo!rene are ty!ical materials used in these gauges.#. the sha!e of the corrugations. . the number of corrugations. 0randucers. 0his is not an unusual !ractice 5!recision bourdon tubes re3uire it6 but it does add to the cost of the instrument. 0hese ca!sules can be used singularly or stac'ed. the elasticity of the dia!hragm and the magnitude of !ressure a!!lied. two circular dia!hragms are soldered or welded together to form a !ressure ca!sule. $eather.8A2"A$-" =re3uently. 0he moving end of the bellows is usually connected with a sim!le lin'age to an indicator !ointer. :t is necessary to arrange that all travel of the bellows be made on the com!ressive side of the !oint of !ressure e3uilibrium.D-. colla!sible. #. Fowever. the s!ring in !igure :nstrument 0rainee 0as' 4odule 122/& +/ . and 7iB"!an 8Q 5a nic'el alloy6. 0he fle*ibility of a metallic bellows is similar to that of a helical. )asic )ellows 0he bellows ty!e !ressure gauge is usually built as a oneB!iece./ )-$$O@" 2 -""A .%. 0he use of metallic bellows has been most successful on !ressures ranging from /. 0he advantage of the 7iB "!an 8 ca!sule is that it is virtually unaffected by tem!erature changes. seamless metallic unit with dee! folds formed from very thinB walled tubing. !igure %( illustrates a basic bellows sensing element. 0he relationshi! between increments of load and deflection is linear u! to the elastic limit. this linear relationshi! e*ists only when the travel of the bellows occurs under the influence of a minium com!ressive force.& to +& !sig. coiled com!ression s!ring.. 0heir range is a com!romise: /B#/ !si.=igure %+. =or this reason. "tac'ed 2ressure 8a!sule Gauge 0he !ressure ca!sules are commonly made of !hos!hor bronCe. 0he !hos!hor bronCe ca!sules are suitable for most a!!lications from / to #/ !si. stainless steel.:8-" 0he need for a !ressureBsensing element more sensitive than the )ourdon tube or the basic metallic dia!hragm to low !ressures and !roviding greater !ower for actuating recording and indicating mechanisms resulted in the develo!ment of the metallic bellows. =igure %. a change in a!!lied !ressure would cause !ro!ortional change in ca!acitance. 0randucers. is e*erting on the bellows. shows a dia!hragm assembly arranged as a 8a!acitance 2ressure "ensor. the bellows is always o!!osed by a s!ring. it moves away from the stationary metal housing. :f the dia!hragm and housing were viewed as the two !lates of a ca!acitor. As the dia!hragm deflects.%.0E2. and transmitters < 4odule 122/& +1 . and the deflection characteristics of the unit is the net result of the s!ring and the bellows."-7"O As described earlier. or: d O2 where: d 2 A Wb Ws O O O O O Deflection of the bellows 5in6 A!!lied !ressure 5!si6 -ffective area of the bellows 5in26 "!ring rate of the bellows 5lb?in6 "!ring rate of the restraining s!ring 5lb?in6 A W b IW s :f the bellows must o!erate an electric switch or some other mechanism: 2O where: 2 = D = I d 5W b IW s A O A!!lied !ressure value when the switch is o!erated 5!si6 O =orce re3uired to actuate the switch 5:b6 O Deflection re3uired to o!erate the switch #. :n !ractice. a com!ressive force that the movement or measuring action of the bellows must overcome. !ressure a!!lied to a metallic dia!hragm causes the dia!hragm to deflect.2 -""A . -lements. !igure %.&./ 8A2A8:0A78. b6 vacuum for !sia readings. :solating Dia!hragm ) can be referenced to a6 atmos!here for !sig readings.=igure %.. the sensing dia!hragm will deflect to the right. and also as the ca!acitor dielectric. 0he sensing dia!hragm deflects by an amount and in a direction as determined by the net a!!lied !ressure. 0he a!!lied !ressure is transmitted by the isolating dia!hragm to the silicone oil. or c6 another !ressure for differential !ressure readings. A and ). :nstrument 0rainee 0as' 4odule 122/& +2 . 0hese two dia!hragms are called isolating because they !revent the fluid being measured from coming into contact with the sensing dia!hragm. 0his oil serves as a nonBcorrosive !ressure transmitting fluid. :f the !ressure that is a!!lied to :solating Dia!hragm A is larger than that a!!lied to :solating Dia!hragm ). !igure &* shows the sensing dia!hragm and ca!acitor !lates as actually two ca!acitors. 8a!acitance 2ressure "ensor 0he !ressure to be measured is a!!lied to :solating Dia!hragm A. As the sensing dia!hragm deflects to the right in !igure &*+ the ca!acitance of ) increase and the ca!acitance of A decreases. and transmitters < 4odule 122/& +# . 8a!acitance 2ressure "ensor as 0wo 8a!acitors 0he device is connected to a ca!acitance measuring instrument. =rom basic electricity: :O andZcO : 2πf8 therefore: :O or: or : O Zc 1 2πf8 : O. with the resultant change in ca!acitance being directly !ro!ortional to a change in !ressure.=igure &/.f 2 8a P 8b 8a I 8b O Out!ut current O 8onstant voltage and fre3uency su!!lied to the sensor from a reference source O 4athematical constant -lements. 0randucers. 0he ca!acitor current then is !ro!ortional to the change in ca!acitance. 0he ca!acitance measuring device !rovides a stable A8 voltage.P 2πf where: : -.P 2 π f8 )ecause the change in out!ut current is de!endent on the change in ca!acitance of the two ca!acitors A and ): 8O therefore: 8a P 8b 8a I 8b : O . which is very im!ortant.(. =igure &1. !igure &" shows a dia!hragm seal used with a !ressure gauge to !revent the corrosive li3uid of the !rocess from coming into contact with the bourdon tube. One design consists of a sintered stainless steel disc or cylinder held in a stainless steel body9 another is a ca!tive stainless steel !in in a orifice o!ening./ D:A2F AG4 "-A$" "tainless steel dia!hragm seals can be used when a sensor is not corrosion resistant or is subject to !ossible contamination. :nstrument 0rainee 0as' 4odule 122/& +% . the !ressure can vary ra!idly or !ulsate. 0hese !ulsations can cause e*cessive wear of the internal moving !arts of a !ressure sensor. Dam!eners that act to bloc' the !ulsations and !ass the steadyBstate !ressure can be included in sensing lines. #. Dia!hragm seals are commonly used on sensing instrument lines where the movement of the sensor is minimal. 0he seals are usually assembled in the factory to ensure com!lete filling. -*cessive dis!lacement may involve an error arising from the straining of the dia!hragm seal.8a8b O 8a!acitanc e of the"ensor=or an A!!lied 2ressure 8a I8b Asing different ty!es of material to ma'e the sensing dia!hragm results in ca!acitance ty!e !ressure sensors of different !ressure ranges. Dia!hragm "eal 2ulsation Dam!eners :n some !rocess systems. 0he s!ace between the sealing dia!hragm and the sensor is filled with a suitable li3uid whose !ressure du!licates that of the !rocess side of the dia!hragm. ? : 2F ? : INPUT SI(NAL 8urrent 2neumatic 8urrent esistance 4illivolts !F &UTPUT SI(NAL 2neumatic 8urrent esistance 8urrent 8urrent 8urrent -lements./ 0 A7"DA8- 0E2-" 4any ty!es of transducers are available for different a!!lications. :n most cases. 0able 1/ lists common transducer ty!es. 0he res!onse of most !ressure sensors to a full scale !ressure change is e*tremely ra!id. !eriodic cleaning of !ulsation dam!eners may be re3uired. %. transducers are re3uired. T'PE :?2 2?: :? ?: 4. %. =or e*am!le. :t will. and transmitters < 4odule 122/& +& . %.2. however. !rocess heat. 2ressure "ensor 2ositioning 0he !ressure detector should be !ositioned on a stable. or system vibration to the sensor mechanism. 0ransducers convert in!ut signals of one form into out!ut signals of another form. be affected by the length and diameter of the !i!e or tubing used to connect the sensor to the !rocess ta'eBoff !oint. =or low !ressure measurement. shoc' mounted housing adjacent to the !rocess line in such a way as to reduce the transmission of !i!ing or vessel e*!ansion strains. an instrument system that has an electronic controller and a !neumatic final element re3uires a transducer that will convert an electrical or current signal into a !ro!ortional !neumatic signal./ 0 A7"DA8- =A780:O7" An industrial !lant contains many ty!es of instrument systems. 0he longer the sensing line./. the final element is a !neumatically o!erated control valve.)ecause !lugging may !resent a !roblem. 0he transducers used in these instrument systems must acce!t a signal generated by a controller or transmitter and convert it into a usable signal for another com!onent in the loo!. the slower the res!onse. :n order for an instrument system to use both electronic and !neumatic com!onents together. 0he following sections e*!lain the two most common ty!es of transducers: :?2 and 2?: transducers./ 0 A7"DA8.1. the length of the sensing line should be short and the diameter small. -lectronics and !neumatics are used most often for signal transmission."B 0ransducers are used in instrument systems to ensure !ro!er signal transmission to the system1s final element or controller. 0randucers. 2?: transducers are usually mounted in a cabinet on a wall just outside these contaminated areas.#./ 2?: 0 A7"DA8. sluggish. -lectrical measuring instruments are rarely used in these areas because of high corrosion." 4ost :?2. :n many cases. :nstrument 0rainee 0as' 4odule 122/& +( . these signals must be converted to a !ro!ortional mA signal. 8ommon 0ransducer 0y!es %. !neumatic signal transmission. :n order for the !neumatic signals to reach their controller with minimal lag time. =igure &2. A transducer is usually mounted close to its receiving instrument to avoid long. or currentBtoB!neumatic transducers are fieldBmounted instruments.%. !igure &# shows a ty!ical :?2 transducer mounting. 0y!ical :?2 0ransducer 4ounting %." 2?:./ :?2 0 A7"DA8.0able 1/. !igure &$ shows a ty!ical 2?: transducer mounting. or !neumaticBtoBcurrent transducers are generally used in a chemical or humid area where !neumatic measuring instruments are used. a transducer is mounted directly onto a receiving instrument such as a control valve actuator. 0he following sections e*!lain the o!eration of 2?: and :?2 transducers. 4ost :?2 transducers re3uire either a % to 2/ mA or a 1/ to &/ mA signal for !ro!ortionate signal conversion.A0:O7 )oth 2?: and :?2 transducers o!erate li'e a ty!ical electro!neumatic instrument.&. 0randucers. 4ost 2?: transducers receive a # to 1& !si signal from a controller or transmitter.=igure &#./ 0 A7"DA8- O2. 0he transducer circuitry relies on electronic and !neumatic com!onents. and transmitters < 4odule 122/& ++ ./ A2 0 A7"DA8- O2. !igure &% shows the relationshi! between coil movement and fla!!er movement.A0:O7 An :?2 transducer receives a D8 milliam!ere in!ut signal in the range of % to 2/ mA or 1/ to &/ mA. %. "ignals received out of this range cause an incorrect out!ut signal or no out!ut signal at all. 0y!ical 2?: 0ransducer 4ounting %. 0he coil reacts to the mA signal by !roducing a thrust !arallel to the sha!e of the magnet. 0his thrust varies the ga! between a fla!!er noCCle and fla!!er. 0his signal is received by a coil !ositioned in the electrical field of a !ermanent magnet.(. -lements. elationshi! )etween 8oil 4ovement and =la!!er 4ovement 0he change in distance between the fla!!er and fla!!er noCCle changes the out!ut !ressure of the relay. 0his ty!e of transducer usually consists of a !neumatic receiver and solidB state circuitry. .+. O!eration of :?2 0ransducer %./ 2?: 0 A7"DA8- O2. 0his relay out!ut !ressure. also called the transducer out!ut !ressure.A0:O7 2?: transducers convert a !neumatic signal to a !ro!ortional mA out!ut. 0he solidBstate circuitry includes the following com!onents: • • 4agnetic coils B create or induce an electromotive force 5-4=6.=igure &%. a throttling relationshi! is established between the fla!!er and fla!!er noCCle. 0he bellows !ivots on an adjusting fulcrum and moves the fla!!er noCCle. !igure && shows the o!eration of an :?2 transducer. =igure &&. As the fla!!er noCCle moves. Oscillator B generates a constantly changing electrical signal :nstrument 0rainee 0as' 4odule 122/& +. is fed to a feedbac' bellows. =igure &(. !si and its out!ut is 12 mA. 0his voltage dro! is fed to an am!lifier that drives the out!ut transistor to !roduce an out!ut current. efer to !igure &' for a schematic of a ty!ical 2?: transducer. A detector receives the am!litude change and !roduces a !ro!ortional voltage dro! across a resistor. Arrangement of 0y!ical 2?: 0ransducer @hen a 2?: transducer receives an in!ut signal. As the bellows e*!ands and contracts.• • • Detector B receives a constantly changing electrical signal Am!lifier B receives a constantly changing signal from an oscillator and res!onds to a voltage dro! across a resistor. 0randucers. Driver B energiCes the out!ut transistor. a connecting lin'age changes the inductance of the coils. !igure &. . shows the relationshi! of the bellows and coils. 0he oscillator am!lifier -lements. it a!!lies it to a bellows. and transmitters < 4odule 122/& +. the transducer is in a balanced mode. elationshi! of )ellows and 8oils @hen a transducer receives an in!ut signal of . =igure &+. 0he change in inductance alters the am!litude of an oscillator inside the transducer. !igure &' shows the arrangement of a ty!ical 2?: transducer. =igure &. %. as seen in !igure &)+ is an electric conductor of fine wire loo!ed bac' and forth on a fle*ible mounting !late. :t can be utiliCed as a very versatile detector for measuring weight. a bonded strain gauge. 0his creates an e3ual balance between the total number of milliam! currents sent to the coils and the reference current !roduced by the regulator. mechanical force or dis!lacement. !ressure. !si in!ut signal disru!ts the transducer1s balanced mode.. 0his causes a different current to flow through the coils and decreases the oscillator am!litude./ 4-0A$$:8 "0 A:7 GAAG0he strain gauge is a transducer em!loying electrical resistance variation to sense the strain or other results of force. A !ositive change in current flow in a 2?: transducer !roduces an increase in out!ut current... A negative change in current flow results in a decrease in out!ut current. $ocation of egulator in a 2?: 0ransducer A change in the . A!!lication of a "train Gauge :nstrument 0rainee 0as' 4odule 122/& . :n basic construction.regulates the oscillator am!litude./ . which is usually bonded or cemented to the member of test !iece undergoing =igure &. !igure &( shows the location of the regulator in a 2?1 transducer.. varies directly with length $ and inversely with crossBsectional area A. 0he e*tension in length of the hair!in loo!s increases the effect of a stress a!!lied in the direction of length. =igure (/. an oscillosco!e is used to measure the degree of unbalance.1 ./// ohms. which has the same current flowing through it. 0he result would be an increase in its resistance. whereas in industrial a!!lications a !otentiometer is sufficient.stress. :t is as stable as the metallic ty!e and has a higher out!ut level. One way to correct for the heating effect is to use a dummy gauge.. "emiconductor strain gauges -lements. a tensile stress would elongate the wire and thereby increase its length and decrease its crossB sectional area. as used in research laboratories.//1 in. the diameter of the conductor is about /. 0hey are calibrated to determine strain due to tensile and com!ressive loads. "train gauges are available in length from 1?1( to 1B1?2 in.. Generally.1 "emiconductor "train Gauges "trainBgauge sensitivity has been im!roved by using semiconductors. either tensile or com!ressive strains can be measured. @ith a good bond between the strain gauge and the test !iece.. 0he fle*ible silicon strain gauge is a very !ractical device. at a constant tem!erature. or to measure its unbalance. 0he schematic arrangement is shown in !igure '*. O '$ A where ' is a constant de!ending u!on the wire. 0randucers. in the o!!osite arm of the bridge circuit. =or highBs!eed analysis. and the resistance of the conductor is about 1// ohms. the current !assed through the gauge is about 2& mA. "ince a current flow through the strainBgauge element will have a heating effect which is !ro!ortional to the s3uare of the current. de!ends u!on the re3uired s!eed of res!onse. any resulting change in resistance will have to be a!!lied as a correction. because the resistance of a metallic conductor. with a resistance range of (/ to (. :n short. %. "ymbolically. and transmitters < 4odule 122/& . A esistance )ridge 8ircuit for 4easuring "train 0he ty!e of measuring device used to balance the bridge circuit. A0:7G 0 A7"DA8. creates electrical !ressure !ro!ortional to :nstrument 0rainee 0as' 4odule 122/& .1/. -ach gauge will be in either com!ression or tension de!ending on the vertical motion of the bellows. 0he moving arm of the voltage divider !roduces a voltage out!ut !ro!ortional to the !ressure variable... the motion of the bellows de!ends on the !ressure. such as 3uartC.can detect microBinches of change in length !er unit inch of length."B %. tourmaline.O$0AG-BG-7.1 ." 0he deformation of various crystals.1 2:-aO-$-80 :8 0 A7"DA8. =igure (2.oltageBdivider 2ressure 0ransducer A sim!le method of using the motion of a !ressure Bsensitive element such as a bellows or dia!hragm is to actuate the arm of a !otentiometer voltage divider."0 A:7 GAAG-" A means of detecting very small variations in !ressure. A semiconductor element used in microstrain systems is shown in !igure (1. 0he bellowscantilever beam ty!e of strain gauge device is suitable for measuring !ressures from / to &/. :n turn. A "emiconductor Ased in 4icrostrain "ystems %. 0he voltageBdivider !otentiometer is suited to a D8 indicating and recording system which may not re3uire am!lification. as seen in !igure '#+ incor!orates strain gauges cemented or bonded on both sides of a fle*ible cantilever. %.. instead of a sim!le resistance variation./// !si./ 2 -""A . ochelle salt./ .. =igure (1.2 . and barium titanate. Any slight !ressure variation can be detected by the strain gauge bridge circuit and read on a suitable meter. A!!lication of the "train Gauge for 2ressure 4easurement %.1/. 0he main advantages of !ieCoelectric !ower. am!litude. 0he transducer is generally referred to as the linearBvariable differential transformer 5$. as is true of most highBim!edance devices. 2ieCoelectric crystals are found in such devices as !honogra!h !ic'u! cartridges and ceramic micro!hones. the !ieCoelectric crystal is inherently a dynamic res!onding sensor and is not really suitable for steadyBstate conditions. and fre3uency res!onse. 0randucers. would in o!!osite directions.D0 is an electromechanical transducer of the variableBinductance ty!e. which induces a voltage in each of two secondary coils./ $:7-A B.# . =igure (#. As mentioned. sensitivity with long leads. and transmitters < 4odule 122/& . is as follows: 0hree coils are wound on a cylindrical coil form or tube.the a!!lied force or !ressure. because of the high out!ut voltage. careful shielding is re3uired.11. 0his action of generating a voltage by the a!!lication of a force or !ressure is the !rinci!le used in the !ieCoelectric crystal transducer.A :A)$. small siCe. the !ieCoelectric crystal transducer is used in highB fre3uency accelerometers.oltageBgenerating 52ieCoelectric6 0ransducer in a 2ressureB4easuring "ystem :n instrument !ractice. 0he out!ut necessitating electronicB to tem!erature variations.D:==. the generation of noise. -lements. dynamic res!onse. shown in the schematic diagram in !igure (%. 0he $. disadvantages are the highBim!edance signalBconditioning systems. 0he center coil is the !rimary. %. and. 0he basic !rinci!le. A sectional view of a voltageBgenerating 5!ieCoelectric6 !ressure transducer is shown in !igure (#. .D06. 0his ty!e of transducer is used with an aBc am!lifier to increase the crystal out!ut signal for readout !ur!oses. on either side of it. transducers are selfBgenerating and rugged construction. 0he crystal sensing element has a high out!ut im!edance and low current out!ut.-70:A$ 0 A7"=O 4- 0he differential transformer may be designed to !rovide an electrical out!ut which is linearly !ro!ortional to a mechanical dis!lacement. 0he magnetic core is free to move a*ially inside the assembly as a result of a dis!lacement. )ecause it is a highBim!edance measuring system. %. a high out!ut voltage is re3uired. the two voltages in the secondary circuit are o!!osite in !hase. because there is a builtBin converter for changing the D8 to A8 e*citation and there is a demodulator for su!!lying a D8 out!ut.=igure (%. assemblies.O4-0- :n accordance with 7ewton1s basic law of motion. 4any differential transformers are actuated by direct current. 0o select one. :f the core is now moved from the null !osition. or acceleration. owing to dis!lacement variation. the secondary coil is wound with many turns to !roduce a large o!enBcircuit voltage. !osition the movable magnetic core is at a central !oint. weight. 0he resulting out!ut of the transformer circuit is thus the difference between the two voltages. =or this reason.% . $inearBvariable Differential 0ransformer "chematic and $inear =unction @hen the !rimary coil is energiCed by alternating current. %. :n measuring systems.D0. and techni3ues are available to meet the demands of research and industry. @hen the $. vacuumB tube voltmeter. instead of being !roduced directly by an e*ternal force. =orce5=6 O mass 5m6 * acceleration 5a6 the elastic deformation of the sensitive measuring element.12. "ince these coils are connected in series o!!osition. such as !ressure.1 A!!lications 0he linearBvariable differential transformer can be used as a transducer to measure other variables having the characteristic of force. or other highBim!edance load.D0 out!ut is connected to an am!lifier grid in!ut.11. transformer is designed to !roduce a differentialBvoltage out!ut which varies linearly as seen in !igure '%. or balance. may be a function of :nstrument 0rainee 0as' 4odule 122/& ./ A88-$. 0he entire device is com!act occu!ying only about half a cubic inch. the accelerometer is a good e*am!le which em!loys the $. A variety of designs. voltages are induced in the two secondary coils. the voltage induced in the secondary coil will either increase or decrease according to the direction of the dis!lacement change. :n the null. $inearBvariable Differential 0ransformer Ased to 4easure Acceleration 0he secondaryBcircuit out!ut signal. =igure (&. "chaevitC -ngineering manufactures the accelerometer shown in !igure '&. &.& . record. &.1."B 0ransmitters usually consist of a sensor and an out!ut device. 0ransmitters can be !neumatic or electronic.1. -lements./ 27-A4A0:8 0 A7"4:00.acceleration.1 =orce )alance Differential 2ressure 2neumatic 0ransmitters 0he most common ty!e of !neumatic transmitter is the force balance differential !ressure transmitter. o!erate on a similar !rinci!le./. which can be !ic'ed u! by any of the conventional measuring circuits described in this and !receding sections. &. or control the !rocess variable. 0he out!ut signal of the D2 is usually #B1& !si. 0he out!ut device converts this initial signal to a transmittable signal that is !ro!ortional to the measurement. 0randucers. 0he s'illed craftwor'er should understand transmitters and how they function. 0he sensor detects a signal from a !rimary element that is measuring a !rocess variable. 0his signal can then be used to indicate./ 0 A7"4:00. 0hese units differ in their basic o!erating !rinci!les. 0he D2 cell usually o!erates on a regulated 2/ !si air su!!ly. 0his ty!e transmitter is also called a D2 cell. is a function of the dis!lacement of the moving magnetic core caused by acceleration. ca!able of withstanding the e*treme conditions of missile testing." 0he two main o!erating ty!es of !neumatic transmitters are force balance transmitters and motion balance transmitters. and transmitters < 4odule 122/& . Accelerometers of the baseBmounting ty!e. =igure ((. 0he dia!hragm ca!sule s!lits the transmitter body into a high !ressure chamber and a low !ressure chamber.1.( . :nstrument 0rainee 0as' 4odule 122/& . 2neumatic D2 8ell &. shows a transmitter measuring section. !igure '.2 2rocess 4easuring "ection 0he !rocess measuring section of a D2 cell consists of a transmitter body and a dia!hragm ca!sule.A !neumatic D2 cell can be divided into four basic sections: • • • • 2rocess measuring section =orce bar section )alancing section 2neumatic in!ut?out!ut section !igure (( shows a !neumatic D2 cell. # =orce )ar "ection 0he bottom of a D2 cell force bar section is connected to the dia!hragm lin'. A force bar seal !revents !rocess fluid from lea'ing out of the transmitter body. shows a force bar section..=igure (+.+ . and transmitters < 4odule 122/& . 2ressure acts on the sealed metallic dia!hragm and causes a small movement of the dia!hragm lin'. =igure (. 0ransmitter 4easuring "ection 2rocess fluid is !i!ed to the high and low !ressure sides of the transmitter body. 0randucers. =orce )ar "ection -lements.1. &. !igure (. 0he to! of the force bar is connected to the !neumatic fla!!erBnoCCle. An increase in the high !ressure side causes the base of the force bar to move toward the low !ressure side. the total measuring range of the instrument can be changed. !igure .. usually # !si. 4ovement of the fla!!erBnoCCle by the force bar causes changes in the amount of air su!!lied to the feedbac' bellows. 0he Cero adjustment establishes Cero. force balance.% )alancing "ection 0he balancing section of a D2 cell consists of a feedbac' bellows. range rod. 0his movement is transferred to the force bar at the lin' connection.Any difference in !ressure in the transmitter body causes a movement of the dia!hragm lin'. 0he range wheel is the balancing !oint for the motion of the range rod. comes from these counterbalancing actions. 0his action draws the fla!!er closer to the noCCle.. 0he term. shows the effect of dia!hragm movement on fla!!erBnoCCle !osition. :nstrument 0rainee 0as' 4odule 122/& . )y adjusting the range wheel u! or down on the range rod.* shows this balancing section. !igure (. range wheel. -ffect of Dia!hragm 4ovement on =la!!erB7oCCle 2osition &.1. 0he bellows air su!!ly is changed until the bellows e*erts enough !ressure on the range rod to balance the original movement of the force bar. =igure (. and Cero adjustment. . by allowing a manual setting of the fla!!erBnoCCle !osition. An increase in !ressure on the low !ressure side moves the fla!!er away from the noCCle. -lements. 0randucers.1. a noCCle.# shows a !neumatic relay. and an out!ut connection. 0he !neumatic relay o!erates from a 2/ !si air su!!ly. )alancing "ection &. A fi*ed restrictor within the relay body !rovides a constant su!!ly of air to the noCCle." shows a !neumatic in!ut? out!ut section. and transmitters < 4odule 122/& .& 2neumatic :n!ut?Out!ut "ectionB 0he !neumatic in!ut?out!ut section of a D2 cell consists of a regulated 2/ !si air su!!ly. .=igure +/. 0he noCCle !ressure acts on a dia!hragm inside the relay.. a !neumatic relay. 0he dia!hragm acts on a ball chec' valve that su!!lies air to the feedbac' bellows and out!ut signal. !igure . !igure . 2neumatic elay An increase in !rocess !ressure on the high !ressure side of the ca!sule causes the fla!!er to cover the noCCle. 0he downward movement of the dia!hragm causes the ball chec' valve to o!en and increase the air !ressure to the feedbac' bellows and out!ut signal. 2neumatic :n!ut?Out!ut "ection estrictor =igure +2.=igure +1. 0his causes air !ressure to build u! on the noCCle su!!ly side of the dia!hragm./ . 0his continues until :nstrument 0rainee 0as' 4odule 122/& . a D2 cell is most commonly used with an orifice !late.1. !igure . -lements. the dia!hragm within the relay allows less air !ressure to the feedbac' bellows and out!ut signal.2.the !ressure of the bellows against the range rod counterbalances the original movement of the force bar./ D2 8-$$ =$O@ 4-A"A -4-70 =or flow measurement. 0randucers. and transmitters < 4odule 122/& . 0his action occurs until the unit is again in balance. &. D2 8ell =low :nstallation 0he D2 cell can be mounted on the !rocess !i!ing or a location as close as !ossible to the orifice !late. :f the fla!!er moves away from the noCCle. 8ommon !rocess measurements made with !neumatic force balance transmitters are: • • • • =low measurement $i3uid level measurement 2ressure measurement 0em!erature measurement &. 0he valves located near the orifice !late can be closed to ma'e re!air or removal !ossible without shutting off the !rocess flow. =igure +#.( 2neumatic =orce )alance 0ransmitters A!!lications 2neumatic force balance transmitters can be used in most !rocess measurement a!!lications where differential !ressure measurement devices are used.$ shows a D2 cell flow installation.1 . a D2 cell can be used to measure the li3uid level in both o!enBnon!ressuriCed tan's and closedB !ressuriCed tan's. Due to some !rocess conditions. the low leg can fill u! with the !rocess fluid and cause measurement errors.% is also a closedB!ressuriCed vessel. it !ermits e3ual !ressure to be a!!lied to both sides of the dia!hragm ca!sule.1 D2 8ell $i3uid $evel 4easurement @ith a slight modification in tubing connections. Figh !ressure a!!lied to only one side of the dia!hragm ca!sule can cause damage. 0he only change in !ressure that is not sensed by the low side is an increase or decrease in !ressure cause by a rise or fall of the li3uid level. =igure +%. 0an' 8 in !igure . :ncreasing the level in the tan' causes more !ressure to be a!!lied to the high side of the ca!sule. 0he high side of the D2 is connected near the bottom and the low side at the to!. &. if steam from a container of hot water fills the low !ressure !i!ing leg. 8hanges in the tan' !ressure not related to li3uid level are e3ually sensed by both sides of the transmitter. :ncreasing or decreasing li3uid !ressure on the high side are the only !ressure changes that affect the signal from the D2 cell.2 .% is an o!enBnon!ressuriCed vessel. this steam will condense into a li3uid as it cools off. @hen the valve is o!en. :nstrument 0rainee 0as' 4odule 122/& . !igure . 0an' ) in !igure +% is a closedB!ressuriCed vessel.% shows D2 cell li3uid level a!!lications.2. =or e*am!le. 0he low side of the D2 cell is o!en to the atmos!here. 0his causes an increase in the out!ut signal. D2 8ell $i3uid $evel A!!lications 0an' A in !igure . 0he high !ressure side of the D2 cell is connected near the bottom of the tan'. 0he low !ressure side is vented to atmos!heric !ressure. 0his will create a false li3uid level !ressure on the low side and cause errors in measurement.0he e3ualiCing valve on the !rocess tubing is used in re!lacement or removal !rocedures. & shows a D2 cell used for !ressure measurement. 0he D2 cell can then be mechanically adjusted to balance out the li3uid !ressure on the low leg. =igure +&. the low leg is intentionally filled with li3uid.A0A ./ 27-A4A0:8 =O 8.#. 0randucers. 0he low side is usually a metal blan' with a small vent for atmos!heric !ressure. and transmitters < 4odule 122/& . &.2. the D2 cell is constructed with only a high side connection.2 D2 8ell 2ressure 4easurement =or !ressure measurement. D2 8ell Ased for 2ressure 4easurement 0he D2 cell can be mounted directly on a vessel or !i!eline to measure !ressure.4-A"A -4-70 0he body of a !neumatic force balance tem!erature transmitter is different from a normal D2 cell. 0he valve !ermits removal or re!lacement of the D2 cell without !rocess shutdown.' shows a force balance tem!erature transmitter.0-42. &. "!ecial construction of the dia!hragm allows e*!osure to high !ressure on one side of the unit without damage. !igure .0o eliminate the !roblem of li3uid collecting in the low leg.# . -lements.)A$A78. !igure . =igure +(. =orce )alance 0em!erature 0ransmitter A !neumatic force balance tem!erature transmitter has a thermal element that senses tem!erature changes. 0he bellows on the end of this thermal element res!onds to tem!erature changes by e*!anding and contracting. 0his force is transferred to the force bar causing fla!!erBnoCCle !osition changes. 0he change in the fla!!erBnoCCle !osition is sensed by the relay and balancing !ressures are sent to the feedbac' bellows. A com!ensation bellows unit on the force bar eliminates the effects of ambient tem!erature. &.%./ 4O0:O7 )A$A78- 27-A4A0:8 0 A7"4:00- " 4otion balance transmitters convert the in!ut motion of basic measuring elements into a !neumatic out!ut that is directly !ro!ortional to the measured variable. )asic measuring elements such as bourdon tubes, bellows, and dia!hragm assemblies can move distances of u! to 1?2 inch. 2neumatic motion balance transmitters convert this in!ut motion into an out!ut !ressure. !igure ++ shows a motion balance transmitter. 4otion balance transmitters receive an air su!!ly in!ut of 2/ !si. :ts out!ut of # to 1& !si is !ro!ortional to the measured value. 0he three sections of a motion balance transmitter are: • • • 4easuring section $in' an fla!!erBnoCCle section )ellows and relay section .( :nstrument 0rainee 0as' 4odule 122/& .% =igure ++. 4otion )alance 0ransmitter &.%.1 4easuring "ection 0he measuring section of a motion balance transmitter is the basic measuring element. :n most motion balance transmitters, this basic element is either a bourdon tube, a dia!hragm assembly, or a bellows unit. 0hese measuring elements rely on a lin' to transfer the measurement motion. !igure +. shows some common measuring elements used in motion balance transmitters. 0he measuring element section !rovides the initial motion to the transmitters. -lements, 0randucers, and transmitters < 4odule 122/& .& =igure +,. 8ommon 4otion )alance 4easuring -lements &.%.2 $in' and =la!!erB7oCCle "ection 8hanges in the measured variable move the connecting lin' on the measuring element. 0his motion is transferred to the lin' assembly. 0he lin' assembly moves u! or down on its !ivot !oint. 0his movement causes the fla!!erBnoCCle relationshi! to change. !igure +. shows the lin' and fla!!erBnoCCle assembly. =igure +.. $in' and =la!!erB7oCCle Assembly &.%.# )ellows and elay "ection 8hanges in the fla!!erBnoCCle !osition affect the out!ut of the !neumatic relay. At this !oint the action of the motion balance transmitter is the same as the action of a force balance transmitter. :nstrument 0rainee 0as' 4odule 122/& .( 0he !ressure change continues until the unit is in balance. 0his motion activates the transmitter. 0he !ressure change continues until the unit is in balance./.%.As the noCCle is covered or uncovered. the thermal fluid e*!ands and contracts within the bourdon tube causing a motion at its connecting lin'. As in a force balance transmitter.+ . =igure . &. !igure (" shows a motion balance tem!erature measurement installation. )ellows and elay Assembly &. !igure (* shows the bellows and relay assembly.%. 0he filled thermal !i!ing is connected to a bourdon tube inside the transmitter housing. As the tem!erature of the !rocess rises and falls. -lements. 0randucers.% A!!lications of 4otion )alance 0ransmitters 4otion balance transmitters are most commonly used in tem!erature and !ressure measurement. the feedbac' !ressure is the out!ut signal of the transmitter. and transmitters < 4odule 122/& . A Cero adjustment is !rovided for an initial # !si out!ut. Any movement of the fla!!er toward or away from the noCCle causes a !ressure change in the bellows. the bac' !ressure changes in the !neumatic relay assembly. 0hese changes are fed bac' to the bellows.& 2neumatic 4otion )alance 0em!erature 4easurement A thermal e*!ansion system is used as the basic measuring element in !neumatic motion balance tem!erature measurement. 4otion )alance 2ressure 4easurement :nstallation (.( 2neumatic 4otion )alance 2ressure 4easurement :n a !neumatic motion balance !ressure measurement system.2. =igure .=igure . :nstrument 0rainee 0as' 4odule 122/& ./. 0his motion activates the transmitter./ -$-80 O7:8 0 A7"4:00."B -lectronic transmitters offer higher degree of accuracy than !neumatic transmitters. A bourdon or bellows assembly is located in the transmitter housing and is used as the basic measuring element. !igure (# shows a motion balance !ressure measurement installation. 0wo common electronic transmitter systems are the force balance system and variable ca!acitance cell. 0heir o!eration varies with the manufacturer..1. 4otion )alance 0em!erature 4easurement :nstallation &. A change is !rocess !ressure moves the connecting lin' of the measuring element. . 0hey also have a faster and longer range transmission system. A change in !rocess !ressure moves the connecting lin' of the measuring element. the transmitter is connected directly into the !rocess !i!ing or vessel.%. 0his lin' movement is transferred to a force bar that transmits it to the range rod. !igure ($ shows an electronic force balance D2 transmitter. 2ressure on this dia!hragm causes a small movement of the dia!hragm lin'. 7ew systems are commonly % to 2/ mA.#. 0he range rod transmits the force to the armature. 0randucers. .(.dc..& . causing it to move.1.1 =orce )alance Differential 2ressure -lectronic 0ransmitters An electronic force balance transmitter has a su!!ly voltage of u! to . -lectronic =orce )alance D2 0ransmitter (. 0he major difference is the out!ut device.1. and transmitters < 4odule 122/& . -lements. =igure . . !igure (% shows a sensor assembly.2 "ensor Assembly A sensor assembly is a sealed metallic dia!hragm. 0he sensor system of an electronic force balance transmitter is similar to a !neumatic D2 cell. :ts out!ut can be 1/ to &/ mA or % to 2/ mA. "ensor Assembly (. =or e*am!le. if the armature is moved toward the secondary winding. the voltage at that winding will increase.=igure . :nstrument 0rainee 0as' 4odule 122/& 1// . !igure (& shows an out!ut section.1.%.# Out!ut "ection 0he armature !ortion of a range rod can move bac' and forth within the transformer. the out!ut signal decreases. +. 8hanges in ca!acitance caused by !ressure changes within the dia!hragm cause the current to change. -lectrons injected across a forward biased 27 junction combine with holes to !roduce !hotons of light which esca!e from -lements. low signal loss.% Out!ut Device 0he out!ut device of a variable ca!acitance cell is an electronic ca!acitance bridge circuit. and clarity of signal. if the armature moves in the other direction. Fowever.&./ =:).1. fibero!tics are used more and more with transmitters. and transmitters < 4odule 122/& 1/1 . 0hese changes are converted by the bridge circuit to a % to 2/ mA signal that is a linear out!ut directly !ro!ortional to the !ressure changes. 0hese are solid state devices that !roduce an increased light out!ut with an increasing forward bias current. (.=igure . 0randucers. Due to the e*cellent noise !ro!erties. +. fiberBo!tics have started to !lay a larger role in transmitter technology." 0he two ty!es of o!tical sources which have been used widely are the light emitting diode 5$-D6 and the injection laser diode 5:$D6.1./. "mall lead wires from the circuit assembly area attached to the ca!acitor !lates and sealed in the dia!hragm. A constant electric current is !assed through these wires to the ca!acitor !lates.BO20:8"B During the !ast ten years./ O20:8A$ 0 A7"4:00. Out!ut "ection 0his increase in voltage causes an increase in the out!ut signal of an oscillatorBam!lifier. =or e*am!le. $-D or :$D. 0he choice of $-D or :$D o!tical source rests on understanding the differences between the devices. fiber ty!e. which is im!ortant for long distance transmission. detector. the :$D has a shorter rise time than the $-D. +.1. @hich device ty!e. Fowever the laser diode is strongly tem!erature de!endent and has a significantly higher cost. arsenic. indium and !hos!horous are used to !roduce devices which have a !ea' light emission at wavelengths from (// nanometer 5nm6 in the visible range to 1/// nor in the infrared range. 0hey are the reverse biased 27 junction diode and the !hoto transistor.1 2hotodetectors 0he o!tical receiver must detect the received o!tical !ower and am!lify the resulting electrical signal with minimal distortion. which ma'es the :$D more suitable for wide bandwidth and high data rate a!!lications. 8ommon reverse biased diodes are the 2:7 diode and the avalanche !hotodiode 5A2D6. data rate.arious semiconductor com!ounds of gallium. :n a !hoto transistor the light detection diode becomes the baseBcollector junction of the transistor. :nstrument 0rainee 0as' 4odule 122/& 1/2 . :ncluded also are other gas and solid state lasers used in o!tical communications. :t consists of a !hotodetector and a !reBam!lifier.the structure. and what carrier wavelength to select de!ends on transmission range. and total system cost. . :n addition more o!tical !ower can be cou!led into a fiber with an :$D. 0wo general classes of !hotodetectors e*ist. 4cGrawBFill )oo' 8om!any. Instrumentation+ American 0echnical "ociety. -lements. etc. flow. and transmitters < 4odule 122/& 1/# . as well as the theory of o!eration of each ty!e of transmitter. Also various trade ti!s were given to hel! the trainee better utiliCe the information discussed on the job. 0randucers. 1asic Instrumentation+ 2atric' \.(.((.++. 0he ty!ical range which transmitters o!erate in were given. 0ransducers were discussed and the role they !lay in a !rocess. the following wor's are suggested: Process Measurement !undamentals+ G2". 0he !rinci!le of o!eration as well as ty!ical a!!lication were discussed. eferences =or advanced study of to!ics covered in this 0as' 4odule. and level were covered. 4ajor com!onents involved in measuring !ressure."A44A E 0his module has covered the basics of elements. sensitivity.. along with a!!lications and e*!lanations of their o!eration. 1. 0he interBrelationshi! of all these different devices was stressed. 0he various items regarding accuracy. 1. transducers. range. 1. 0ransmitters were shown to be found in both electrical as well as !neumatic ty!es. =ran'lin Wir'. Figgins. tem!erature. were covered as a!!lies to measuring devices. and transmitters. 0he various ty!es of transducers were shown. of !rocess flow 2. 0he re!resentative value. d. A bourdon tube is a basicbbbbbbbbbbbbmeasuring device. A change in tem!erature causes a metal to e*!and or contract. c. d. A change in tem!erature causes a change in resistance !ro!ortional to tem!erature. )ac'lash. A !ersonal. c. A change in tem!erature causes an electrical current !ro!ortional to tem!erature. b.:-@ : 2 A80:8. a. b. $i3uid level. 2ressure. -. 0he li3uidBinBglass thermometer is based on the !rinci!le of bbbbbbb. 2ressure. e!lacing c. :nstrument 0rainee 0as' 4odule 122/& 1/% . Differential !ressure. +. (. b. Offset. #.DA-"0:O7" @hen a measurement is com!ared to a 'nown standard. 0he measured value. Drift. b. 8ontrolling b. An orifice !late used in measurement causesbbbbbbbbbbbto develo! across it. A change in tem!erature causes volumetric e*!ansion or contraction of a li3uid. 0he true value. c. An installation. b. d. a. eversing d. :nstrumentation systems are used for measuring andbbbbbb!rocess variables. =low. c. a. A systematic. &. %. a. A vacuum. d. Fysteresis. :ncreasing A measurement error caused by !aralla* isbbbbbbbbbbbbberror. d."-$=B8F-8W 1. c. 0he recorded value. c. A random. 0em!erature. b. a. Density. a. d. An error in measurements caused by wear from normal use is bbbbbbbbbbbbb. it re!resents of that measurement. a. =O 4A78. c. $ow !ressure or vacuum. 0em!erature. A rotameter is used in measuring !rocess bbbbbbbbbbbbb.. 0randucers..8:"-" -lements. a. 7arrow band width. d. 8hea!. c. Figh !ressure. b. and transmitters < 4odule 122/& 1/& . $ow tem!erature. b. d. ugged. A bellowsBty!e element is normally used to measure bbbbbbbbbbb a. 1/. . =low. 2ressure. 2. d. =iber o!tics are desirable because: a. b. c.? $A)O A0O E -Z. $ow susce!tibility to electrical noise. $i3uid level.. Figh tem!erature. describe its o!eration. transducer. 2oint out inlet and outlet !orts and describe the installation !rocedure. c 1/. 2oint out different ty!es of each. c #. Given a measurement element. Draw a 1Bline diagram showing the relationshi! between measuring element. b 2. Given a diagram of !ressure to current transducer. discuss the o!eration. b . b &. a %." 0O "-$=B8F-8W 1. a .:-@ ? 2 A80:8. and disadvantages of the device. #. 2. d +.1. c (.DA-"0:O7" :nstrument 0rainee 0as' 4odule 122/& 1/( .a -.. and transmitter. Discuss at least three different devices. advantages.. A7"@.