Description
CHAPTER (4) Sensors and Transducers Objectives: This chapter will consider different types of sensors. After you have read this chapter, you should be able to • • • • • Transform a temperature reading among different scales Explain the operation of different temperature sensors Describe the characteristics and applications of mechanical sensors Describe advantages, disadvantages, and applications of limit switches, photoelectric sensors, inductive sensors, capacitive sensors, and ultrasonic sensors Explain the operation of pressure, flow, and level transducers - 62 - Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 4.1 What is a transducer? The sensor or the sensing element is the first element in a measuring system and takes information about the variable being measured and transforms it into a more suitable form to be measured. The following figure illustrates the difference between sensor and transducer. Physical variable change Passive element Change as ∆R or ∆L or ∆C Sensor Physical variable change Transducer Active signal Change as ∆V or ∆I Figure 4.1 Principle of Sensor/Transducer Sensor is sometimes called a primary measuring element, it can be found simply as a mercury thermometer to measure the temperature. It may be embedded in the transducer to perform its function. That means the transducer consists of a primary element (sensor) plus a secondary element (signal conditioning circuit) that transforms the passive change or small voltage signal into active signal range that can be easily used in other chains of the control loop. Example: with a resistance thermometer, the resistance depends on the temperature value (sensor). It can be inserted into a bridge circuit (secondary element) in order to transform the change in the resistance value to a change in the voltage output. Finally, the output voltage from the bridge circuit express about the temperature change value. In general, we can say that: Transducer = Sensor + Signal conditioning circuit In the following sections, we will present the main features of different transducers to measure: • Temperature - Using mechanical properties to operate - Using electrical properties to operate • Pressure - Static - Dynamic • Level • Flow - Direct measurement - Indirect measurement • Mechanical displacement - 63 - Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 4.2 Temperature sensors and transducers What is Temperature? In a qualitative manner, we can describe the temperature of an object as that which determines the sensation of warmth or coldness felt from contact with it. It is easy to demonstrate that when two objects of the same material are placed together (physicists say when they are put in thermal contact), the object with the higher temperature cools while the cooler object becomes warmer until a point is reached after which no more change occurs, and to our senses, they feel the same. When the thermal changes have stopped, we say that the two objects are in thermal equilibrium. We can then define the temperature of the system by saying that the temperature is that quantity which is the same for both systems when they are in thermal equilibrium. Temperature may be defined as "the condition of a body which determines the transfer of heat to or from other bodies" or the degree of hotness or coldness as referenced to a specific scale of temperature measurement. What is a Thermometer (sensor)? A thermometer is an instrument that measures the temperature of a system in a quantitative way (sensing element). The easiest way to do this is to find a substance having a property that varies regularly with its temperature. The most direct 'regular' way is a linear one: T(x) = ax + b, (4.1) where T is the temperature of the substance and changes as the property x of the substance changes. The constants a and b depend on the substance used and may be evaluated by specifying two temperature points on the scale, such as 32° for the freezing point of water and 212° for its boiling point. 4.2.1 Temperature scales It was in 1724 that Daniel Gabriel Fahrenheit, an instrument maker of Däanzig and Amsterdam, used mercury as the thermometric liquid. Fahrenheit measured the boiling point of water to be 212. He adjusted the freezing point of water to 32 so that the interval between the boiling and freezing points of water could be represented by the more rational number 180. Temperatures measured on this scale are designated as degrees Fahrenheit (°F). In 1745, Carolus Linnaeus described a scale in which the freezing point of water was zero, and the boiling point 100, making it a centigrade (one hundred steps) scale. Anders Celsius (17011744) used the reverse scale in which 100 represented the freezing point and zero the boiling point of water, still, of course, with 100 degrees between the two defining points. In 1948 use of the Centigrade scale was dropped in favor of a new scale using degrees Celsius (° C). To convert from Celsius to Fahrenheit: multiply by 1.8 and add 32. ° F = 1.8° C + 32 (4.2) Lord Kelvin (1824-1907) has proposed another scale its symbol is K. To convert from Celsius to Kelvin, add 273. o K = ° C + 273 (4.3) - 64 - 2) Filled thermal system The device consists of a bulb filled with expanding substance connected to a Bourdon tube mechanism via a capillary tube (≈30m long) as shown in figure.9° C to 356. Using mechanical properties 1) Capillary tube thermometer The element mercury is a liquid in the temperature range of -38.65 - . • Celsius scale is used in scientific measurements and industrial applications. Mercury can be replaced by alcohol for low temperature measurement. • Kelvin scale is based on an extremely low temperature.3 A filled system thermometer .2 Thermometer The mercury-in-glass thermometer illustrated in the above figure contains a bulb filled with mercury that is allowed to expand into a capillary tube.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ We conclude that: • Fahrenheit scale is common used in U. Moving pointer Temperature scale Bourdon tube Control Room Bulb with expanding substance Capillary tube Process Field Figure 4.S. Its rate of expansion is calibrated on the glass scale. Figure 4.7° C As a liquid. 4. its expansion rate is linear and can be accurately calibrated. mercury expands (moves) as it gets warmer. and many other English-speaking countries.2.2 Temperature measurement There are two basic categories either to use mechanical properties (expansion of a substance) or to use electrical properties to operate. This movement is marked using the temperature scale. This sensor consists of two materials (metals) with gross different expansion coefficients and bonded together. Therefore. having relatively slow time of response. and very expensive. this effect can be used to close switch contacts or to actuate an on/off mechanism when temperature increases to some operating set point. the pressure moves the pointer at the moving end of Bourdon tube. the temperature will make each metal to expand with a different length.66 - . to nickel. to convert the curvature into dial rotation. Consequently.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ The pressure inside the bulb changes as the temperature changes. and less expensive. For pure metals. 3) Bimetallic thermometers This type of temperature sensor has the characteristics of being relatively inaccurate. Figure 4. which is very repeatable. having hysteresis. more sensitive. Metals used in these devices vary from platinum. Consequently. quite sensitive. by means of assemblages. which is not quite as repeatable. Temperature scale Moving pointer Bimetallic strip Figure 4. the characteristic relationship that governs resistance is given by: .5 Bimetallic thermometer Using electrical properties 1) Resistance Thermal Detectors (RTDs) It is a temperature sensor that is based on a metal resistance increasing with temperature.4 Bimetallic on/off switch The effect also is used for temperature indicators. c = coefficients calculated on the basis of two or more known resistancetemperature (calibration) points Linear approximation of the above relation is given by: R(T) = R(To )[1 + α o ∆ T] . A typical example of RTD is PT100 sensor as shown in figure. Ω a = temperature coefficient of resistance.67 - .5) R(T) = approximation of resistance at temperature T R(To) = resistance at temperature To ∆T = T.. it can be used for medium range measurements. The entire resistance thermometer is an assembly of parts.4) Ro = resistance at reference temperature (usually at ice. Where: T1 < T < T2 (4. which include the sensing element.004 /ΩoC nickel 0. • Response time. (0. internal supporting and insulating materials. internal lead wires. Ω/Ω (oC) b.6 Temperature sensor PT-100 Industrial RTDs are commonly available with elements of platinum. Where: ( ) (4. and protection tube or case.Ω Rt = resistance at temperature t. The RTDs can be found with two leads. 70% nickel-30 iron.To αO = fractional change in resistance per degree of temperature at To The above equation is valid over the span T1 to T2.5 seconds 5 seconds) • Sensitivity. 70% nickel-30% iron (-180oC 300oC). or four leads for connections. or thee leads. (platinum 0. platinum (.. RTDs Properties: • Range of measurement. nickel. 0oC).Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ R t = R o 1 + at + bt 2 + ct 3 + . Normally.005 /ΩoC) Signal conditioning: The RTDs is generally used in a bridge circuit.100 oC 650oC). The two wires type is used for short distance . Figure 4. or copper. The connection will be as shown in figure. Figure 4. We feed the current to close the circuit with RTD and measure the output voltage across the other two terminals which is proportional to the RTD value. where the process is too near to the controller. R1 +Vs - R2 Vo R3 RTD Terminal block Compensation leads Figure 4.68 - .8 RTD signal conditioning for remote applications The compensation line in the R3 leg of the bridge is required when the lead lengths are so long that thermal gradients along the RTD leg may cause changes on line resistance. These changes show up as false information. By using the compensation line.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ application. Four wires RTD can be used with low current source to avoid the self heating problem in the resistance. where the terminal block is connected to the bridge circuit. The terminal block connection is illustrated in the following figure. the same resistance changes also appear on the R3 side of the bridge and cause no net shift in the bridge null. . suggesting changes in RTD resistance.7 Two wires RTD terminal block connection Three wires RTDs are used for remote applications as shown in figure. When a thermistor is used as a temperature sensing element.6) The following figure illustrates the above relationship.69 - . Normally. (0. Resistance (KΩ) Temperature (oC) Figure 4. It is an electrical device made of a solid semiconductor with a high temperature sensitivity. Maximum up to 300oC to avoid melting of semiconductor materials.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Figure 4. sense the resistance of a thermistor varies as a function of temperature. it can be changed by 1 KΩ for change in 1 oC) .9 Four wires RTD terminal block connection 2) Thermistors The name thermistor is derived from thermally sensitive resistor.5 seconds or faster) • Sensitivity. it can be used for low temperature range measurements.10 Thermistor characteristic Thermistor Properties: • Range of measurement. • Response time. (highly sensitive. the relationship between resistance and temperature can be expressed as: 1 1 B T−T o e Rt = Ro (4. This makes the actual temperature is greater than it should be measured. but in recent years the position of the thermocouple has been increasingly challenged by the RTD.7) ∆T = temperature rise due to self heating in C P = power dissipated in the resistor (sensing element) in Watt Po = dissipated constant of the resistor (sensing element) in Watt/oC 3) Thermocouples For many years the thermocouples was the clear-cut choice of instrumentation and control engineers in the process industries. the thermocouple still is used widely especially in large range of temperature measurement applications.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Signal conditioning: Circuits with instrumentation amplifiers can be or simple potential dividers.11 Seebeck effect Cold junction Peltier effect Jean Peltier (1834) discovered that when an electric current flows across a junction of two dissimilar metals. Alloy A Hot junction I Alloy B ∆T Figure 4. The Peltier effect is the fundamental basis for thermoelectric cooling and heating. But. heat is absorbed at the hotter junction and liberated at the colder junction. When the electric current flows in the same direction as the Seebeck current. Seebeck found that bonding wires of two dissimilar metals together to form a closed circuit caused an electric current to flow in the circuit whenever a difference in temperature was imposed between the end junctions.70 - . Remark: Both RTDs and thermistors suffer a self heating problem due to power dissipation in resistors. To avoid this problem. Seebeck effect As early as 1821. heat is liberated or absorbed. be care with the nonlinearity in the measurement circuit and try to obtain a simple and linear overall relationship. Nevertheless. either to use low current source or a compensation design has to be added to the circuit. The Peltier effect may be defined as the change in heat content when a quantity of charge (1 coulomb) crosses the junction. The excess temperature due to self heating can be computed by the relation: ∆T = Where: P Po o (4. . Chromel and Constantan.8) Where: α = constant in µvolts/oC ε = Seebeck e.12 Thermocouples types The output voltage from the thermocouple can be presented as: ε = α (T2 . measurement range (-200oC to 371oC) • E-type. Chromel and Alumel.13 Characteristics of K-type thermocouple For each type of thermocouple there is standard table that to obtain the output voltage (mV) against the measured junction temperature where the reference junction at 0oC (Appendix A).Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Thermocouple consists of two different materials such as: • J-type.219 – 1.f in mV T1.m. The following figure shows atypical thermocouple response as the temperature increases. measurement range (0oC to 1482oC) Figure 4. then from the table the output voltage = 15.942 mV. Figure 4. measurement range ( -190oC to 760oC) • K-type. J type TC has the measured junction T2 at 280oC and the reference T1 at 25oC. For example.277 = 13.T1 ) (4. 87%platinum+13% rhodium and platinum. . measurement range (0oC to 1482oC) • R-type. measurement range (-100oC to 1260oC) • S-type.71 - . Iron and Constantan. T2 = junction temperatures in oC T1 can be fixed as a reference junction and T2 can be considered as the measured junction temperature. Constantan and Copper. measurement range (-190oC to 1260oC) • T-type. 90%platinum+10% rhodium and platinum. which penetrates into the flow of the fluid being measured (thermowell) • There are standard thermocouple tables that simply give the voltage for a particular type of thermocouple when the reference junctions are at a particular reference temperature. • Response time. such as a zener diode. We have fixed one of them as a reference and the second one is considered to be measured. So. The e. In addition. that means. Signal conditioning: The output from a thermocouple is in mV. The dissipation constant is in the range from 2 to 20 mW/oC.m. and are becoming popular for measurements within a limited range. it depends the thermal contact environment. a group of thermocouples are connected in series so that there are perhaps ten or more hot junctions sensing the temperature.72 - . Therefore. . These sensors are easy to interface to control systems and computers. • Sensitivity.f. produced by each is added together. This value depends on the conditions and the heat sinking. different approaches are used to control its temperature variation such as: • Applying a local temperature control to the reference junction temperature • Design a reference compensation circuit during the measurement • Applying software reference correction in the computer control applications Noise reduction techniques: Different approaches can be applied to reduce the effect of thermocouple measurement noise such as: • Lead wires can be twisted and then wrapped with a grounded foil sheath • Measured junction can be connected to the ground • Use of an excellent amplifier with a good value of CMRR to improve the signal to noise ratio. Remarks • In some applications. This means that considerable amplification will be necessary for a practical application. there is an error will be produced.05 mV /oC . a protective shield for a temperature measuring device. Reference junction compensation: The output signal from the thermocouple depends on the difference between the two junctions temperature. and the measurement junction is at a temperature of interest (Appendix A). the small signal levels make the devices susceptible to electrical noise. • Temperature sensors can be immersed into a moving fluid to measure its temperature. It can be used to provide automatic reference temperature compensation for thermocouples. R-type 0. 4) Solid state temperature sensors Solid state temperature sensors offer voltages that vary linearly with a temperature range (from -50oC to 150 oC). (J-type 0. In poor thermal contact from 10 to 20 seconds. it can be used for large measurements as indicated above.006 mV/oC).Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Thermocouple Properties: • Range of measurement. In a good thermal contact from 10 to 20 ms. In most cases the thermocouple is used with a high gain differential amplifier. If the temperature of the reference junction is not constant. The time constant in a good thermal contact is in the range 1 to 5 seconds. Such arrangement is known as a thermopile. motion Figure 4.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 4.Potentiometer types: 2.73 - .Variable reluctance types (LVDT) 4.15 Potentiometer displacement sensor This sensor suffers from the following drawbacks: • Limited resolution • Friction in the wiper • High electric noise Capacitive types The basic operation of capacitive type sensors can be seen from the familiar equation for a parallel-plate capacitor. a signal conditioning circuit converts the change in resistance value into voltage change. A C = K εo d (4.9) . Then. This device then converts linear or angular motion into change of resistance.14 Transducer principle Different types of mechanical sensors can be considered such as: 1.Capacitive and inductive types 3.Strain gauges Potentiometer types The simplest type of displacement sensor involves the action of displacement in moving the wiper of a potentiometer. This is the basic principle in many transducer devices as shown in figure. Measure variable Displacement Electric signal Primary sensor Converter (signal conditioning) Figure 4. Many physical variables have the capability to produce a displacement that can be converted into active signal.3 Displacement sensors Displacement sensors are widely used not only to measure the distance of a moving object but also it can be embedded in other sensors or transducer devices to measure pressure or level or flow as it will be shown later. variation of the distance between the plates (d). Motion Inductance Figure 4.17 LVDT displacement sensor . null repeatability. that can be used for linear displacements 2. LVDTs can measure displacements from a few microns to several feet in a wide variety of environments. the moving core changes the magnetic flux coupling between two or more coils.74 - . Inductive types If a permeable core is inserted into an inductor as shown in figure. that will be shown later in liquid level measurements. the net inductance is increases. that can bused for angular motion displacements. 3.variation of the common area (A). and environmental ruggedness. Ratiometric wiring scheme Open wiring scheme Figure 4. LVDTs offer many distinct advantages over other displacement measurement devices including: frictionless movement.Variation of the dielectric (K). Variable reluctance (LVDT) In this type.16 Inductive displacement sensor Every new position of the core produces different inductance. The inductor and movable core assembly can be used as a displacement sensor. infinite resolution. An ac bridge or other active electronic circuit sensitive to inductance then may be employed for signal conditioning. A Linear Variable Differential Transformer (LVDT) is an electro mechanical device that produces an output proportional to displacement. temperature stability.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Where: K is the dielectric constant εO is the premitivity (8.85 PF/m) A is the common area d is the plate separation distance There are three ways to change the capacitance: 1. both the Voltage on coil A and coil B are used to determine the position of the core. knowing the modulus of elasticity E and assuming Hook's Law σ= Ee). The resistance of a strain gauge is given by: . The stress (σ= force/area) is computed based on known material properties (e. The metal foil strain gauge is one of the most common methods used to measure the stress in materials. The following equation is computed by the ratiometric signal conditioner to determine the position where G is the gain or sensitivity. A metal foil gauge measures the strain in a material based on the assumptions that the gauge will deform perfectly with the material and that the gauge resistance will change linearly with applied strain. Careful attention to the various error sources will assure accurate measurements. Since stress cannot be measured directly. When used in conjunction with a precision signal conditioner. Strain gauges are the preferred choice in stress analysis due to their small size and relatively low cost. As a result very long cables may be employed with no loss of accuracy around the null position. A magnetically permeable metal core (Ni-Ir) slides through the center of the coils and provides an efficient path for the magnetic flux. the strain (e=∆L/L) of the material is measured through the use of a strain gauge. (4. The amount of Voltage induced in the secondary windings varies with the core's position. Ratiometric LVDT signal conditioning offers several advantages over the open style. over excitation.75 - . Strain Gauges A strain gauge is a thin metal foil that changes resistance with applied strain.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ An LVDT operates on the principal of magnetic coupling between a primary and two secondary windings. Ratiometrically wound LVDTs provide this necessary condition. In ratiometric signal conditioning.10) The above requires that the sum of the secondary coils remain constant for high linearity. the output phase of the secondary winding is compared with the phase of the primary winding. Errors in strain gauge measurements are numerous and if not properly avoided can be quite large.000 micro-strain. bonding errors.11) In order to determine the direction of the displacement. and transverse sensitivity. The ratiometric scheme is highly temperature insensitive. Temperature affects the LVDT signal by changing the magnetic induction efficiency. lead wire effects. Typically. The primary winding produces a magnetic field that passes through the two secondary windings. the net effect is high temperature immunity. accurate strain measurements are possible. The ratiometric scheme is also insensitive to phase shifts between the primary and secondary windings. noise pickup. Some of the more significant errors are those due to temperature effects. In the open type. Since both Voltage A and Voltage B are affected equally. Displacement = G Vc (4.. Strain gauges can measure strain levels from a few micro-strain (µe) to over 100. it must be determined from other measurable parameters. The primary coil is typically energized with a 2-5 Volt sine wave with frequencies between 2-10 kHz. the voltage Vc is directly proportional to the displacement of the core as shown in the following equation where G is the gain or sensitivity.g. Note that. The result is zero output voltage from the amplifier. The dummy sensor is not sensitive to the applied force it compensates only the temperature effect in the null condition. the differential voltage into the amplifier will be zero (i. The gauge factor incorporates the change in resistance due to dimensional stretching and due to the change in resistivity (p) with strain.. The resulting bridge voltage will be amplified by the instrumentation amplifier. GF = ∆R/R ∆L/L (4. Typical values for modern metallic strain gauges (copper-nickel alloy) are around 2. This can be done by calculated the stress by measuring the strain using the above relationships. the more sensitive is the gauge. When the bridge is balanced (i.. three matching resistors (R). A Wheatstone bridge arrangement is commonly used to measure the small changes in resistance associated with strain gauges. we use a dummy stain guage in the bridge as R4 in the figure. When a gauge is stretched all three variables will change.e.76 - .Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ R =ρ L A (4. one can compute the strain level in the strain gage and theoretically in the test specimen. In general.e. By measuring the gage resistance and knowing the relationship between gage resistance and gauge strain. A power source (Vexe). In order to compensate the temperature effect. L is the length.13) The higher the number. Forces (weights) applied to a known area can also be measured using strain gauges (Load cell)s. strain gauge can be affected by temperature that add increases the resistance value. a gauge factor (GF) is defined to be the average ratio of the resistance change to length change (strain) of the gauge.18 Strain gauge signal conditioning circuit . and a high quality instrumentation amplifier make up the simplified circuit. The strain gage (Rgauge) is typically wired in a three wire configuration as shown to minimize lead wire errors (Rlead).12) where p is the resistivity. and A is the cross sectional area. Any change in Rgauge due to strain will upset the bridge balance and the differential input voltage to the amplifier will increase. Strain Gauge Signal Conditioning The changes of resistance associated with strain gauges are small and present measurement situation.V1 = 0 Volts). V2 . Figure 4. R=Rgauge). Object detection sensors are devices used to provide information on the presence or absence of an object as shown in figure.4 Object detector sensors One type of feedback frequently needed by industrial-control systems is the position of one or more components of the operation being controlled.19 Sensors in industry applications There are different types of these sensors in industry such as: • Limit switches • Photoelectric sensors • Inductive proximity sensors • Capacitive proximity sensors • Ultrasonic sensors Table 4.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 4.77 - .1 Industrial sensors . Figure 4. through the pre-travel area.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Limit Switches A typical limit switch consists of a switch body and an operating head. Inductive proximity sensors use an electromagnetic field to detect the presence of metal objects. A spring returns the actuator lever and electrical contacts to their free position when the actuator is no longer in contact with the target. When the target comes in contact with the actuator. Figure 4. The operating head incorporates some type of lever arm or plunger. Figure 4. The switch body includes electrical contacts to energize and deenergize a circuit. The standard limit switch is a mechanical device that uses physical contact to detect the presence of an object (target). When the target comes in contact with the actuator. Capacitive proximity sensors use an electrostatic field to detect the presence of any object. Ultrasonic proximity sensors use sound waves to detect the presence of objects. to the operating position. the actuator is rotated from its normal position to the operating position. referred to as an actuator. capacitive.21 Limit switch operation There are other four types of sensors: inductive. Photoelectric sensors react on changes in the received quantity of light.20 Limit switch One type of actuator operation is momentary. At this point. the electrical contacts in the switch body change state. Some photoelectric sensors can even detect a specific color. This mechanical operation activates contacts within the switch body.78 - . and photoelectric. . it rotates the actuator from the free position. ultrasonic. As the target approaches the sensor the eddy currents increase which is increasing the load on the oscillator and further decreasing the amplitude of the field. This causes a load on the sensor. Figure 4. trigger circuit. oscillator. The sensor will ignore the presence of an object if it is not metal. decreasing the amplitude of the electromagnetic field.2 Sensors technology Inductive proximity sensors Inductive proximity sensors are available in a variety of sizes and configurations to meet varying applications. eddy currents circulate within the target. The sensor incorporates an electromagnetic coil which is used to detect the presence of a conductive metal object.23 Inductive sensor operation When a metal target enters the field. and an output. At a predetermined level the trigger switches the output state of the sensor back to .Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Table 4.79 - . The circuit has just enough feedback from the field to keep the oscillator going. As the target moves away from the sensor. the oscillator’s amplitude increases.22 Inductive proximity sensor This type of sensor consists of four elements: coil. Figure 4. The electromagnetic field produced by the oscillator is emitted from the coil away from the face of the sensor. The oscillator is an inductive capacitive tuned circuit that creates a radio frequency. The trigger circuit monitors the oscillator’s amplitude and at a predetermined level switches the output state of the sensor from its normal condition (on or off). Figure 4. the oscillator begins oscillating. As a result. Capacitive proximity switches will sense metal as well as nonmetallic materials such as paper. As the target moves away from the sensor the oscillator’s amplitude decreases. When an object nears the sensing surface it enters the electrostatic field of the electrodes and changes the capacitance in an oscillator circuit. liquids. and cloth. switching the sensor output back to its original state.6 mm to 75 mm depending on the sensor type.80 - . The basic operating voltage ranges are from 10 to 30 VDC. The trigger circuit reads the oscillator’s amplitude and when it reaches a specific level the output state of the sensor changes. .24 Capacitive proximity sensor The sensing surface of a capacitive sensor is formed by two concentrically shaped metal electrodes of an unwound capacitor. Figure 4. The main difference between the two types is that capacitive proximity sensors produce an electrostatic field instead of an electromagnetic field. Capacitive proximity sensors Capacitive proximity sensors are similar to inductive proximity sensors. glass.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ its normal condition (on or off). and 10 to 65 VDC. 15 to 34 VDC. The rated sensing distance (Sn) is ranged from 0.25 Capacitive sensor operation The basic operating voltage ranges are from 10 to 30 VDC and 10 to 65 VDC. The rated sensing distance (Sn) is ranged from 5 mm to 20 mm depending on the sensor type. echoes are produced. causing it to energize or deenergize the output circuit. This is known as “blanking out the background”. This is between the lower limit and the blind zone. The vibrating disk produces high-frequency sound waves. When the target leaves the preset operating range. a blocking range also exists.81 - . On some sensors. The lower limit can be adjusted only with certain versions. When transmitted pulses strike a sound-reflecting object. the output of the switch changes the state. The operating range can be adjusted in terms of its width and position within the sensing range. The upper limit can be adjusted on all sensors.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Ultrasonic proximity sensors Ultrasonic proximity sensors use a transducer to send and receive high frequency sound signals.26 Ultrasonic sensor The echo can be in micro-volts. It can transmit and receive high-frequency pulses. An object placed in the blind zone will produce an unstable output. . A blind zone exists directly in front of the sensor. the output returns to its original state. Figure 4. There is a signal output assigned to both the operating range and the output range. An object in the blocking range prevents identification of a target in the operating range. A piezoelectric ceramic disk is mounted in the sensor surface. A high frequency voltage is applied to the disk. causing it to vibrate at the same frequency. When a target enters the beam the sound is reflected back to the switch. When the target enters the preset operating range. Figure 4.27 Ultrasonic sensor (Pulse cycle) The time interval between the transmitted signal and the echo is directly proportional to the distance between the object and sensor. Depending on the sensor the blind zone is from 6 to 80 cm. Objects beyond the upper limit do not produce a change at the output of the sensor. The duration of the reflected pulse is evaluated at the transducer. 130 cm.29 Photoelectric sensor The control consists of an emitter (light source). Photoelectric sensor A photoelectric sensor is another type of position sensing device. Photoelectric sensors. Figure 4. We are all familiar with the simple application of a photoelectric sensor placed in the entrance of a store to alert the presence of a customer. use a modulated light beam that is either broken or reflected by the target.30 cm.82 - .600 cm.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Figure 4. Sound travel time can be affected by physical properties of the air. or 80-1000cm. and associated electronics that evaluate and amplify the detected signal causing the photoelectric output switch to change state. Figure 4.28 Ultrasonic sensor ranges The radiation pattern of an ultrasonic sensor consists of a main cone and several neighboring cones. The approximate angle of the main cone is 5°. or 20 . This. This. in turn. The rated operating voltage is 10 to 30 VDC. of course. or 40 300 cm. a receiver to detect the emitted light.30 Photoelectric sensor application . can affect the preset operating distance of the sensor. is only one possible application. Depending on the sensor the sensing range is either 6 . similar to the ones shown below. or 60 . Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Modulated light increases the sensing range while reducing the effect of ambient light. It cannot be used to detect transparent objects. 2) Reflective scan. Lightemitting diode (LED) sources are typically used. The high excess gain of thrubeam sensors make them suitable for environments with airborne contaminants. The maximum sensing range is 35 feet. Light from the emitter is transmitted in a straight line to a reflector and returns to the receiver. In some cases it is necessary to detect a change in color. When the target no longer blocks the light path the sensor returns to its normal state. The photoelectric sensor is able to distinguish the modulated light from ambient light. In addition. Figure 4. In part. The minimum size of the target should equal the diameter of the lens.83 - . Some techniques work well at greater distances while others work better when the target is closer to the sensor. Some targets are opaque and others are highly reflective. separate emitter and receiver units are required for a thru-beam sensor. vibration can cause alignment problems. Scanning distance is also a factor in selecting a scan technique. The effective beam extends from the emitter lens to the receiver lens. The units are aligned in a way that the greatest possible amount of pulsed light from the transmitter reaches the receiver. The maximum sensing range is 300 feet. The effective beam on a thru-beam sensor is the diameter of the emitter and receiver lens. the emitter and receiver are in one unit. causing the receiver’s output to change state. Figure 4. When the target no longer blocks the light path the receiver’s output returns to its normal state. An object (target) placed in the path of the light beam blocks the light to the receiver. When a target blocks the light path the output of the sensor changes the state. Thru-beam is suitable for detection of opaque or reflective objects. 1) Thru-Beam scan. Light sources used by these sensors range in the light spectrum from visible green to invisible infrared. A normal or a cornercube reflector can be used. Modulated light is pulsed at a specific frequency between 5 and 30 KHz. the best technique to use depends on the target.32 Reflective scan technique .31 Thru-beam scan technique The effective beam of a photoelectric sensor is the region of the beam’s diameter where a target is detected. Scan techniques A scan technique is a method used by photoelectric sensors to detect an object (target). Reflectors are ordered separately from sensors. The sensing distance is specified with a particular reflector. Figure 4. the emitter and receiver are in one unit.33 Diffuse scan technique . If the receiver receives enough reflected light the output will switch states.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ The effective beam is tapered from the sensor’s lens to the edges of the reflector. The receiver will be at some angle in order to receive some of the scattered (diffuse) reflection. therefore. Only a small amount of light will reach the receiver. this technique has an effective range of about 40''. When no light is reflected back to the receiver the output returns to its original state. Reflectors come in various sizes and can be round or rectangular in shape or reflective tape.84 - . Light from the emitter strikes the target and the reflected light is diffused from the surface at all angles. In diffuse scanning the emitter is placed perpendicular to the target. 3) Diffuse scan. The minimum size of the target should equal the size of the reflector. we may measure the deflection of the diaphragm with a displacement transducer (such as a capacitive transducer). Its value increases as the liquid velocity increases. Ft =144 square inches . Static pressure is the pressure where no motion is occurring of the liquid. there will be a physical force on the diaphragm given by : Force = (P1 .Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 4. 1 pascal = 1 Newton/ 1 m2 1 bar = 105 pascal In many cases. Units 1 square. it is to our advantage to have a thin diaphragm in order to maximize the deflection that we plan to measure.5 Pressure sensors The measurement and control of fluid (liquid or gas) pressure has to be one of the most common in all the process industries. it is more appropriate to describe the pressure in a relative sense that is compared to atmospheric pressure. then it would exert no effective pressure on the walls of the container because the atmospheric gas exerts the same pressure from the outside. if a diaphragm separates two regions with different pressures on either side.14) The force is directed from the high pressure region to the low-pressure region. metal foil diaphragms are widely used in traditional meteorological instruments (Aneroid barometers). at the surface of the earth of approximately 14. This is called gauge pressure and is given by: . the absolute pressure is not the quantity of major interest n describing the pressure.85 - . Figure 4. Pressure is the force per unit area that a fluid exerts on its surroundings. In order to measure this force. or we may measure the strain in the diaphragm with embedded strain gauges.7 psi. The thickness of the diaphragm is generally also limited by the technology used to manufacture it. There are practical limits to the amount of deflection can measure.P2)(Diaphragm area) (4.7 psi. If a closed vessel at the earth's surface contained a gas at an absolute pressure of 14. because of its weight. Standard technology for metal foil fabrication is capable of thickness down to a few millimeters at low cost and with good reliability. 1 ft = 12 inch 1 psi (pound per square inch) = 1 Ib/in2 = 6. Dynamic pressure is the pressure that it exerts on its surroundings while the fluid is in motion.34 Simple pressure sensor diaphragm Pressure sensors all operate on the basis of the same principle: the detection of a physical force which arises due to pressure. In this case.9 kpa . For example. Its value increases as the liquid head in the tank increases. For example. In either case. The atmosphere of gas that surrounds the earth exerts a pressure. 36 Pressure transducer using LVDT Other types of pressure measurements: • Manometers • Diaphragm with piezoelectric crystal to produce the output voltage • Bourdon tube to express in a mechanical displacement the pressure value .Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Pg = Pabs − Pat (4. the LVDT device will convert the elbow motion into output voltage signal that express about the pressure value. Figure 4. the capacitance between these electrodes depends on the separation between the diaphragm and the electrode at all positions. Another approach to produce the displacement is to use an elbow as shown in figure. motion of the diaphragm towards the fixed electrode increases the device capacitance as the pressure increases. Figure 4.86 - .15) Where: Pg = gauge pressure Pabs = absolute pressure Pat = atmospheric pressure A very common and relatively inexpensive measurement approach involves the measurement of the capacitance between the diaphragm and a fixed electrode.52.35 Capacitance between the Diaphragm In this case. As shown in figure 3. Then. which creates a momentary loss of pressure. a potential is established across the flow. . Figure 4.37 Orifice. This pressure differential is related mathematically to the flow rate.87 - . the flow can be measured by allowing the liquid to flow through a magnetic field and measuring the transverse potential produced. flow nozzle. This device produces an electrical signal directly and is convenient for process control applications involving conducting fluid flow. The output signal from this device is proportional to the squared value of the measured flow. The pipe section in which this measurement is made must be insulated and a nonconductor itself. Square root extractor D/P Cell Output is a function of flow Flow Figure 4. Thus.6 Flow transducers Flow is related to pressure by causing the flowing fluid to pass through some form of restriction in the transport pipe.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 4. perpendicular to the magnetic field. or the potential produced will be canceled by currents in the pipe.38 Indirect flow measurement Magnetic flow meter It can be shown that if charged particles move across a magnetic field. and venturi flow meters The D/P cell is a device to measure the differential pressure between to points. A square root extractor can be used to obtain a linear relation between the flow and the measure signal as shown in figure. if the flowing liquid is also a conductor of electricity (not necessary to be highly good conductor). The polarity of the measured voltage can indicate the direction of the flow. 88 - . The rate of rotation of the turbine is proportional to the flow rate.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Figure 4. The distance to which the float rises in column is proportional to the flow rate.40 Rotameter Turbine type A turbine type flow meter is composed of a freely spinning turbine blade assembly in the flow path.41 Turbine flow meter . If the turbine is attached to a tachometer. the obstruction is a float that rises in a vertical tapered column. a convenient electrical signal can be produced. Figure 4. Fig 4.39 Magnetic flow meter Rotameter In a rotameter. because it is a restriction in the flow. The lifting force is produced by the differential pressure that exists across the float. Thus.43 Discrete points level measurement Capacitance method In this method.89 - . one with the dielectric constant of air (≈1) and the other with that of the liquid. with air in the remaining part. the output voltage from the signal conditioning circuit is proportional to the level value.7 Level transducers In many processes involving liquids contained in vessels. As level increases the contact points that produce ON state increases. . Consequently. evaporators.42 Float type Discrete points method In this method. Thus a continuous measurement of level is required. two concentric cylinders are contained in a liquid. The level of the liquid partially occupies the space between the cylinders. the particular level of liquid in each vessel can be of great importance in process operation. an array of limit switches is mounted in a one side of the tank. Figure 4. provides a direct means of level measurement. a physical hollow ball floats on a liquid and is referred to as a datum point. Figure 4.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 4. This device acts like two capacitors in parallel. such as distillation columns. variation of liquid level causes variation of the electrical capacity measured between the cylinders. Float type In a ball float design. and mixed tanks. 0032 µF With ethyl alcohol.90 - .0032 to 0. Therefore. and then multiply that by 26.005 m) = 3196 pF ≈ 0.85 pF/m) (1.0832 µF The range is 0.0032 pF) = 0.14) (0.806 m2 Thus.44 Capacitive system Example: The level of ethyl alcohol is to be measured fro 0 to 5 m using a capacitive system such that shown in the above figure.005} where H is the liquid level.14) (0. The following specifications define the system: For ethyl alcohol: K = 26 .Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Figure 4.93 m . the capacity becomes C = 26 (0.0575)(H)/0.0575 m) (5 m) = 1.0832 µF The measured capacitance is the equivalent of two capacitances in parallel (air and alcohol).005} + {26 (8.75 cm Distance along cylinder axis is L a) Find the range of capacity variation as the alcohol level varies from 0 to 5 m.5 cm Average radius R= 5. for air K=1 Cylinder separation d=0. A= 2 π R L = 2 (3. So. for air C = 1 (8.85 pF/m)(2) (3. if the measured capacitance is 50 nF (nano Farad). b) Compute the level value. all we need to do is find the capacity for the entire cylinder with no alcohol. we have CTotal = Cair + Calcohol 50 nF = {(8. Solution We saw earlier that the capacity is given by C =KεoA/d. Thus. H= 2.85 pF/m) (2) (3.0575)(5-H)/0.806 m2)/ (0.14) (0. 46 Level measurement in large vessels .91 - . We can measure the level by using square root extractor as shown before. (a) Open tank (b) Closed tank Figure 4.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Head pressure method In this method. the D/P cell measures the differential pressure between the deepest point in the tank and either the atmospheric pressure (open tank) or the upper point in the tank (closed vessel) as shown in figure. The measurement depends on the length of time taken for reflections of an ultrasonic pulse from the surface of the material. The output signal is proportional with the squared value of the level. that is. The use of ultrasonic reflection to measure level is favored because it is a "noninvasive" technique. it does not involve placing anything in the material.45 Head pressure method Ultrasonic method Sonic and ultrasonic technology has been applied to the problem of level measurement of liquids and bulk solids for many years. Figure 4. we can use_______ currents A) High B) Medium C) Small .8 Basic concepts (MCQ) Place the letter of statement that best completes the sentence in space provided.92 - . 1] The transducer can be considered as _______________. A) Sensor B) Sensor + signal conditioning C) Feedback information 2] In industry.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 4. A) Kelvin B) Fahrenheit C) Celsius 3] ______ is used to protect the temperature sensor while measuring the temperature in fluids. A) RTD B) Amplifier C) Thermowell 4] _________ is used to measure high temperature ranges A) Thermistor B) RTD C) Thermocouples 5] The temperature sensor that has a fastest speed of response is ___________. ________ is often used as a temperature scale. A) RTD B) Thermocouple C) Thermistors 6] Three wires RTD is used for _______________ A) Remote applications B) High temperature measurements C) Low temperature measurements 7] To avoid the self heating effect in temperature sensors. A) Greater B) Less C) No difference 14] When measuring very cold temperatures. A) Mercury B) Ether C) Alcohol 15] A rotameter is the device that measures ________. freezing is 32o A) Fahernheit B) Celsius C) Kelvin 13] At 1000 feet above sea level. thermometers are filled with __________. A) Two polarized metal wires B) Two wires of the same alloy C) Two wires composed of different metals 10] Two vessels with different diameters will have the same hydrostatic pressure reading if the height of liquid in each is ___________. A) Reaction Timing Device B) Resistance Thermal Detector C) Resistance Thermometer Device 9] Thermocouples are temperature measuring devices that consist of ________ connected to an electronic circuit.93 - . A) Different B) Higher in the larger diameter vessel C) Equal 11] Pressure acts ________ A) Only in one direction at a time B) In all directions C) Only in a downward direction 12] On the ________ temperature scale. A) Flow B) Pressur C) Level . atmospheric pressure is _______ than at sea level.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 8] RTD refers to ___________. A) Metallic B) Insulated C) Long . A) Diffuse B) Reflective C) Thru-Beam 21] Manometers are devices to measure ___________. A) Pressure B) Flow C) Level 22] The liquid level in a closed tank can be measured using ________ A) Float B) D/P cell C) Capacitance 23] Magnetic flow meter is used to measure the flow in _______ pipes. A) Potentiometer B) LVDT C) Strain Gauge 17] The dummy strain gauge is used in signal conditioning circuit to compensate the effect of ________. the emitter and receiver are not in one unit.94 - . A) Pressure B) Force C) Temperature 18] A sensor that detects the presence of metal only is __________. A) Inductive proximity B) Capacitive proximity C) Limit switch 19] _________ sensors can be used in collision avoidance applications A) Limit switches B) Ultrasonic C) Photoelectric 20] In ________ scan technique for photoelectric sensors.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 16] A displacement transducer that can measure small values is ____________. . R2 = 3445 Ω. and εo =8. and R3= 1560 Ω.004/oC.RTD and Thermistor .85 PF. Compute the level value. for air =1. The bridge is not lead compensated. if the measured capacitance = 4nF.9 Problems 1] What are the most common temperature scales and write the relation between them? 2] Convert 150 oC into oF and oK and -200oF into oC and oK 3] A temperature sensor RTD has αo(20oC)=0.45 Ω/ft.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 4.Sensor and Transducer . What is the sensor resistance? 5] A K-type thermocouple with 25oC reference temperature will be used to measure the temperature between 500oC to 700oC.1 V/ oC b) Plot the output volt versus the given temperature range. If R=106 Ω at 20oC.Seebeck effect and Peltier effect 12] A thermistor Rth is used to convert temperature into resistance according to the following relation: Rth = Ro (1/ T) Where: Ro is constant and equal 100 k ohm T is the measured temperature from 10 oC to 100 oC a) Develop a signal conditioning circuit to obtain a linear output voltage signal with a gain of 0. 13] A rectangular electrodes of 1 cm separation with height of 10 meters are used to measure the level of a liquid in a tank. 6] Explain how a D/P cell can be used to measure the liquid level in an open tank? 7] How to obtain a linear displacement transducer using variable distance capacitive type? 8] Explain the main principle of LVDT transducer? 9] What is the difference between Manometer and Rotameter? 10] What is meant by object detectors and give some examples? 11] What is the difference between: .Psig and Psia . 4] A bridge circuit is used with a sensor located 100m away. K for the liquid=10. and the cable to the sensor has a resistance of 0.95 - .meter. .Active SG and Dummy SG . Develop a signal conditioning that produce a corresponding output from 0 to 10 volt. The bridge nulls with R1 = 3400 Ω. find the resistance at 30oC. what resistance will the RTD have? What then is the indicated temperature? . Calculate the resistance change from a strain of 140 µin/in 15] A temperature sensor has the relation R (ohm) = 100 + T (oC) Develop a signal conditioning circuit to provide a voltage signal in the range (0-5) volts when the temperature varies from 100 oC to 200 oC and keep the relation to be linear. 16] Suppose the RTD in the previous problem that has a dissipation constant of 25 mW/oC and is used in a circuit that puts 8 mA through the sensor.Sensors and Transducers ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ 14] A strain gauge has GF=2 and the nominal resistance = 120 ohm.96 - . If the RTD is placed in a bath at 100oC. 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HendersonThe Theory, Design and Construction of Induction Coilsby Henri ArmagnatExperiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illuminationby Nikola TeslaMagnetic Induction in Iron, and Other Metalsby Sir James Alfred EwingFooter MenuBack To TopAboutAbout ScribdPressOur blogJoin our team!Contact UsJoin todayInvite FriendsGiftsLegalTermsPrivacyCopyrightSupportHelp / FAQAccessibilityPurchase helpAdChoicesPublishersSocial MediaCopyright © 2018 Scribd Inc. .Browse Books.Site Directory.Site Language: English中文EspañolالعربيةPortuguês日本語DeutschFrançaisTurkceРусский языкTiếng việtJęzyk polskiBahasa indonesiaSign up to vote on this titleUsefulNot usefulMaster Your Semester with Scribd & The New York TimesSpecial offer for students: Only $4.99/month.Master Your Semester with a Special Offer from Scribd & The New York TimesRead Free for 30 DaysCancel anytime.Read Free for 30 DaysYou're Reading a Free PreviewDownloadClose DialogAre you sure?This action might not be possible to undo. 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