Resistance Temperature Detectors (RTDs)

Transcription

1 Exercise 2-1 Resistance Temperature Detectors (RTDs) EXERCISE OBJECTIVES To explain how resistance temperature detectors (RTDs) operate; To describe the relationship between the temperature and the electrical resistance of the most common types of RTDs; To define the following terms: nominal resistance, temperature coefficient, and sensitivity. To explain how a Wheatstone bridge can be used to measure the voltage produced across an RTD. DISCUSSION Electrical resistance An important characteristic of all metals is their electrical resistance. Electrical resistance is the opposition of the metal to the flow of electrical current. Electrical resistance is measured in ohms (Ω) in both the S.I. and U.S. systems of units. The electrical resistance of a metal is dependent upon the temperature at which the metal is. Figure 2-4, for example, shows what happens to the relative resistance of different metals as their temperature increases. The relative resistance is the ratio between the resistance at the applied temperature to the resistance at a reference temperature of 0EC (32EF). As the figure shows, the relative resistance of the metals increases as their temperature gets higher. Moreover, the relative resistance increases almost linearly with temperature, at least over a substantial range of temperatures. Besides, the relative resistance of nickel increases more sharply with temperature than that of copper or platinum. 2-9

2 Figure 2-4. Relative resistance-versus-temperature relationship of different metals. Temperature coefficient All metals have a specific temperature coefficient that indicates their average change in relative resistance per unit of temperature between 0 and 100EC (between 32 and 212EF). The temperature coefficient is symbolized by the Greek letter alpha (α). It is usually measured in ohms per ohm degree Celsius (EC -1 ) or in ohms per ohm degree Fahrenheit (EF -1 ). Figure 2-4, for example, indicates that the temperature coefficient of platinum is EC -1 ( EF -1 ). Consequently, the relative resistance of platinum varies by between 0 and 100EC (32 and 212EF). Beyond 100EC (212EF), the shape of the platinum curve indicates that the temperature coefficient decreases slightly as the temperature gets higher. 2-10

3 Resistance temperature detectors A resistance temperature detector (RTD) is a primary element that is used to sense temperature. The RTD works on the principle that the electrical resistance of metals changes with temperature. The RTD consists of a metallic conductor usually wound into a coil. The RTD is to be connected to an electrical circuit in order to make a constant excitation current flow through it. As the temperature increases, the electrical resistance of the metallic conductor increases and, therefore, the voltage across the RTD increases. Consequently, by measuring the voltage across the RTD, a signal proportional to the temperature of the RTD can be obtained. This signal can be conditioned into a current, voltage, or pressure of normalized range that is suitable for instrumentation and control, the combination of the RTD and the conditioning circuit thus forming a temperature transmitter. RTD metals The selection of a metal for use as an RTD depends on several factors. Among these, the most important are the capability to follow rapidly changing temperatures, a good linearity, a good reproducibility, and a relatively high change of resistance for a given change in temperature (i.e. a high temperature coefficient). The metals most commonly used for RTDs are platinum, nickel, and copper (refer to Figure 2-4): Platinum is the preferred metal for RTDs. It has been chosen as the international standard metal for RTD temperature measurement. Platinum has a nearly linear resistance-versus-temperature relationship over a wide temperature range. Platinum offers good stability and reproducibility. It is well-suited for the measurement of high temperatures up to 650EC (1200EF). Nickel is the second mostly used metal for RTDs. It is less expensive than platinum and it is more sensitive because of its higher temperature coefficient. However, nickel has a narrower sensing range than platinum and is limited to the measurement of temperatures below 300EC (570EF). Copper is the least expensive of the three metals and it has the most linear relationship. Similar to platinum, copper is well suited for the measurement of high temperatures. However, copper is subject to oxidation, and it has poorer stability and reproducibility than platinum. RTD characteristics Two important characteristics of RTDs are their nominal resistance and their temperature coefficient: The nominal resistance is the resistance of the RTD at a given reference temperature, as specified by the manufacturer. Platinum RTDs, for example, are usually designed so that their nominal resistance is 100 Ω at the ice reference point of 0EC (32EF). 2-11

4 The temperature coefficient is the mean change in relative resistance of the metal per unit of temperature between 0 and 100EC (32 and 212EF), as previously explained. The nominal resistance and the temperature coefficient of an RTD determine the sensitivity of the RTD within the 0-100EC (32-212EF) temperature range. The sensitivity is the amount by which the resistance of the RTD will change per unit of temperature, in Ω/EC (or Ω/EF). For example, a platinum RTD having a nominal resistance of 100 Ω at 0EC (32EF) and a temperature coefficient of EC -1 ( EF -1 ) will have a sensitivity of Ω/EC (0.218 Ω/EF) within the 0-100EC (32-212EF) temperature range. Measurement of the voltage across an RTD As previously mentioned, the voltage produced across an RTD, which is directly proportional to temperature, can be used for process instrumentation and control. The traditional method of measuring the voltage across an RTD is to use a Wheatstone bridge, as Figure 2-5 (a) shows. The RTD and its two lead wires constitute one leg of the bridge. Resistors R 1 and R 2 are of equal resistance, while resistor R 3 is adjustable and is used as a reference. A DC voltage source supplies an excitation current to the RTD. A differential amplifier produces a voltage V O proportional to the bridge output voltage (measured between points a and b). With the RTD placed in an ice bath at 0EC (32EF), resistor R 3 is initially adjusted in order to obtain a null voltage (0 V) at the output of the differential amplifier. In this condition, the bridge is said to be null balanced. Once the bridge has been null balanced, the amplifier output voltage will vary in direct proportion to the temperature of the RTD. 2-12

5 Figure 2-5. Measurement of the voltage across an RTD. If the two leads that connect the RTD to the bridge are more than a few centimeters (inches) long, they will introduce an error in the temperature measurement. This occurs because the resistance of the leads will contribute to the voltage produced at the output of the bridge, causing the measured temperature to be higher than that actually measured. To minimize this error, RTDs are available in a three-wire version. The extra wire is used to cancel the resistances of lead wires 1 and 2 by balancing the bridge, as Figure 2-5 (b) shows. This has the effect of removing the error produced by lead wires 1 and 2 as long as these wires are of equal resistance (i.e. of equal length and temperature). Advantages and limitations of RTDs RTDs have the following advantages: they provide a good sensitivity, a good reproducibility, and a good stability. They also provide a high accuracy, some platinum RTDs being able to measure a few thousandths of a degree. However, RTDs are relatively expensive, and they have a slower response time than thermocouples. Moreover, the measurement accuracy of RTDs is dependent upon the thermal stability of the resistors and power supply used in the Wheatstone bridge. The RTD probe and the RTD Temperature Transmitter of the Process Control Training System The Process Control Training System comes with a three-wire RTD probe that uses a platinum RTD of 100 Ω at 0EC (32EF). The RTD probe is intended to be used with the RTD Temperature Transmitter to measure the temperature of the water in the trainer Column, as Figure 2-6 shows. 2-13

6 The tip of the RTD probe, which contains the RTD, is to be inserted into the Column through the opening of the Float Switch. The other end of the RTD probe, which has three leads, is to be connected to the "100-Ω RTD" terminals of the RTD Temperature Transmitter. The RTD Temperature Transmitter produces an excitation current through the RTD and it measures the resulting voltage produced across the RTD. This voltage, which is proportional to the temperature of the RTD, is conditioned into normalized voltages and current that are available at the transmitter OUTPUTS. The transmitter also contains a calibration source that can be used to simulate the voltage produced across the RTD for any RTD temperature comprised between 0 and 100EC (32 and 212EF). The source eliminates the need to set the RTD at a wellknown temperature when performing calibration of the transmitter OUTPUTS. The following is a detailed description of the RTD Temperature Transmitter terminals and adjustments (refer to Figure 2-6): Î Ï Ð Ñ Ò POWER INPUT terminals: used to power the transmitter with a DC voltage of 24 V. CALIBRATION SOURCE adjustment knob: sets the probe temperature to be simulated by the calibration source signal. This temperature can be adjusted between 0 and 100EC (32 and 212EF). INPUT SELECTOR: selects between the actual probe signal or the simulated probe signal produced by the calibration source. CALIBRATION SELECTOR switch: places the 0-5 V and 4-20 ma OUTPUTS in either fixed or variable calibration mode. ZERO and SPAN adjustment knobs: used in the variable calibration mode (CALIBRATION SELECTOR switch at VARIABLE) to set the temperature range for which the 0-5 V and 4-20 ma OUTPUTS will pass from minimum to maximum: The ZERO knob sets the temperature for which the outputs will be minimum (0 V and 4 ma), i.e. the minimum temperature to be detected. The minimum temperature can be adjusted between 0 and 50EC (32 and 122EF). The SPAN knob sets the temperature for which the outputs will be maximum (5 V and 20 ma), i.e. the maximum temperature to be detected. The maximum temperature can be adjusted between 15 and 30EC (27 and 54EF) above the minimum temperature set by the ZERO knob. 2-14

7 Figure 2-6. The RTD probe and RTD Temperature Transmitter of the Training System. Ó CALibrated OUTPUT: provides a voltage proportional to the temperature sensed by the probe or to the simulated probe signal produced by the calibration source, depending on the position of the INPUT SELECTOR switch. 2-15

8 This output has a fixed calibration of 100 mv per sensed EC above 0EC (or 56 mv per sensed EF above 32EF). It will pass from 0 to 10 V when the actual or simulated temperature changes from 0 to 100EC (32 to 212EF). Ô 0-5 V and 4-20 ma OUTPUTS terminals: provide a voltage and a current proportional to the temperature sensed by the probe or to the probe temperature signal simulated by the calibration source, depending on the position of the INPUT SELECTOR switch. The calibration of the 0-5 V and 4-20 ma OUTPUTS can be either fixed or variable, depending on the position of the CALIBRATION SELECTOR switch: In the fixed calibration mode (CALIBRATION SELECTOR switch at FIXED), the temperature range for which the outputs will pass from minimum to maximum is fixed and is 0-100EC (32-212EF). In the variable calibration mode (CALIBRATION SELECTOR switch at VARIABLE), the temperature range for which the outputs will pass from minimum to maximum can be adjusted by means of the ZERO and SPAN adjustment knobs. Õ 100-Ω RTD input terminals: used to connect the RTD probe to the transmitter. Procedure summary In the first part of the exercise, you will familiarize yourself with the operation of an RTD Temperature Transmitter in the fixed calibration mode. In the second part of the exercise, you will familiarize yourself with the operation of an RTD Temperature Transmitter in the variable calibration mode. In the third part of the exercise, you will set up and operate a temperature process. You will use an RTD Temperature Transmitter to measure the temperature of the water in a column. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart in Appendix A of the manual to obtain the list of equipment required to perform this exercise. PROCEDURE Operation of the RTD Temperature Transmitter in the fixed calibration mode G 1. Get the RTD Temperature Transmitter and 24-V DC Power Supply from your storage area. Mount these components on the Main Work Surface. 2-16

9 G 2. Power up the RTD Temperature Transmitter. G 3. Get the RTD probe from your storage location and connect it to the 100-Ω RTD input of the RTD Temperature Transmitter. Let the probe tip lie on the Work Surface. G 4. Make the following settings on the RTD Temperature Transmitter: INPUT SELECTOR... RTD CALIBRATION SELECTOR... FIXED This selects the RTD probe signal as the transmitter input signal and places the transmitter OUTPUTS in the fixed calibration mode. G 5. Connect a DC voltmeter to the 0-5 V OUTPUT of the RTD Temperature Transmitter. Since this output is in the fixed calibration mode, it generates a fixed voltage of 50 mv per sensed EC above 0EC (or 28 mv per sensed EF above 32EF). According to the voltmeter reading, what is the ambient temperature? G 6. Further experiment with the operation of the transmitter in the fixed calibration mode: Fill a suitable container with ice water (a mixture of ice cubes and water). Immerse the tip of the RTD probe into the ice water. The 0-5 V OUTPUT voltage should decrease and stabilize at about 0.0 V, which, in the fixed calibration mode, corresponds to an RTD temperature of 0EC (32EF). Fill a suitable container with boiling water heated by an electric kettle or a microwave oven. Immerse the tip of the RTD probe into the boiling water. The 0-5 V OUTPUT voltage should increase and stabilize at about 5.0 V, which, in the fixed calibration mode, corresponds to an RTD temperature of 100EC (212EF). Note: The 0-5 V OUTPUT of the RTD Temperature Transmitter will stabilize at a voltage lower than 5.0 V if the atmospheric pressure is lower than kpa, absolute (14.7 psia). 2-17

10 Record below your observations. Operation of the RTD Temperature Transmitter in the variable calibration mode Note: In the following steps, you will use the calibration source of the RTD Temperature Transmitter to calibrate its 0-5 V OUTPUT so that the voltage at this output passes from 0.0 to 5.00 V when the probe temperature simulated by the calibration source passes from 25 to 55EC (77 to 131EF), respectively. G 7. Make the following settings on the RTD Temperature Transmitter: INPUT SELECTOR... CAL. SOURCE CALIBRATION SELECTOR... VARIABLE ZERO adjustment knob... MAX. SPAN adjustment knob... MAX. This selects the calibration source signal as the transmitter input signal and places the transmitter OUTPUTS in the variable calibration mode. G 8. Set the probe temperature to be simulated by the calibration source of the transmitter at 25EC (77EF). To do so, adjust the CALIBRATION SOURCE knob of the transmitter until you obtain a voltage of 2.5 V at the CAL. OUTPUT of the transmitter. G 9. While monitoring the voltage at the 0-5 V OUTPUT of the transmitter, turn the ZERO adjustment knob counterclockwise and stop turning it as soon as the voltage ceases to decrease, which should occur around 0.01 V. Then very slowly turn the knob in the clockwise direction and stop turning it as soon as the voltage starts to increase. This sets the minimum temperature to be detected at 25EC (77EF) approximately. G 10. Now set the probe temperature to be simulated by the calibration source of the transmitter at 55EC (131EF). To do so, adjust the CALIBRATION SOURCE knob of the transmitter until you obtain a voltage of 5.5 V at the CAL. OUTPUT of the transmitter. G 11. Adjust the SPAN knob in order to obtain a voltage of 5.00 V at the transmitter 0-5 V OUTPUT. 2-18

11 This sets the maximum temperature to be detected at 55EC (131EF) approximately. G 12. Now that the RTD Temperature Transmitter is calibrated, proceed to the next part of the exercise. Measuring temperature with an RTD Preliminary setup G 13. Get the Expanding Work Surface from your storage location and mount it vertically (at an angle of 90E) to the Main Work Surface, if this has not already been done. G 14. Connect the system shown in Figure 2-7, being careful not to modify the calibration settings just made on the RTD Temperature Transmitter. Figure 2-8 shows the suggested setup. The speed of the variable-speed drive of the Pumping Unit will be controlled with a controller, FIC1, placed in the manual (open-loop) mode. The Heating and Cooling Units will be controlled manually. (This is the reason why there is no temperature controller, or "TC" instrumentation symbol illustrated next to these units in the flow diagram of Figure 2-7.) The Column will first be operated in the pressurized mode in order to purge air from the components downstream of the Column. Consequently, let the tip of the RTD probe lie on the Work Surface for now. Note: Make sure to mount the Heating Unit at the highest possible location on the Expanding Work Surface, in order for this unit to be above the other process components, as Figure 2-8 shows. Failure to do so may result in water entering the Heating Unit upon disconnection of the hoses, which in turn might cause damage to the Heating Unit. Moreover, mount the 24-V DC Power Supply and the RTD Temperature Transmitter in such a manner that water cannot enter these components and their electrical terminals when hoses are disconnected. The Heating Unit must be connected for the direction of flow indicated by the arrow heads in the symbol on its front panel. On the other hand, the Cooling Unit will operate regardless of the direction of water flow through it. However, to minimize the risk of cavitation caused by air suction within the pump when the water becomes hot, connect the Cooling Unit as indicated in Figure 2-8, that is, with the upper unit port used as the hot water inlet and the lower unit port used as the cooled water outlet. For the same reason, mount the Column at the highest possible location on the Expanding Work Surface in order to create a substantial head of water upstream of the Cooling Unit. 2-19

12 Note: If the controller you are using as flow controller FIC1 is the Lab-Volt Process Control and Simulation Software (LVPROSIM), model 3674, you can refer to Figure B-1 of Appendix B for details of how to connect the LVPROSIM computer to the variable-speed drive (SC1) of the Pumping Unit. Figure 2-7. Measuring temperature with an RTD temperature transmitter. 2-20

13 Figure 2-8. Suggested setup for the diagram of Figure 2-7 (see table next page for the detail of the components). 2-21

14 Î : Column Ï : Heating Unit Ð : Paddle Wheel Flow Transmitter Ñ : Pumping Unit Ò : Cooling Unit Ó : RTD probe Ô : RTD Temperature Transmitter Õ : DC Power Supply G 15. Make the following settings On the Heating Unit: S1 switch...2 Manual control knob...turned fully counterclockwise On the Cooling Unit: S1 switch...2 Manual control knob...turned fully counterclockwise S2 switch... On the RTD Temperature Transmitter: SELECTOR switch... RTD Note: The 0-5 V OUTPUT of the RTD Temperature Transmitter should still be calibrated for a temperature measurement range of 25-55EC (77-131EF) from the first part of the exercise. G 16. Power up the Heating Unit: Connect the AC line cord of this unit to a wall outlet. Set the POWER switch at I. G 17. Power up the Cooling Unit and the Paddle Wheel Flow Transmitter by connecting their POWER INPUT terminals to the 24-V DC Power Supply. Purging air from the components downstream of the Column G 18. Make sure flow controller FIC1 is in the manual (open-loop) mode. Set the output of this controller at 0% (0 V). G 19. On the Column, make sure the cap of the insertion opening of the Float Switch is tightened firmly. 2-22

15 G 20. Make sure the reservoir of the Pumping Unit is filled with about 12 liters (3.2 gallons US) of water. Make sure the baffle plate is properly installed at the bottom of the reservoir. G 21. Turn on the Pumping Unit by setting its POWER switch at I. G 22. On the Pumping Unit, adjust valves HV1 through HV3 as follows: Open HV1 completely; Close HV2 completely; Set HV3 for directing the full reservoir flow to the pump inlet (turn handle fully clockwise). G 23. Set the variable-speed drive of the Pumping Unit to the maximum speed: with controller FIC1 in the manual (open-loop) mode, set the controller output at 100% (5 V). G 24. Allow the level of the water to rise in the pressurized Column until it stabilizes at some intermediate level. This will force air out of the components downstream of the Column. Note: If the cap of the insertion opening of the Float Switch on the Column has not been tightened firmly, air will be allowed to escape from the Column and the water level will not stabilize in the Column. Should this case occur, stop the variable-speed drive of the Pumping Unit. Open valves HV1 and HV2 of the Pumping Unit in order to drain the Column to the reservoir. When the Column is empty, tighten the cap of the insertion opening of the Float Switch on the Column with more force. Then resume the procedure from step 22. Placing the system in the water recirculating mode Note: In the following steps, you will place the system in the water recirculating mode by setting the Pumping Unit valves so as to direct the return flow directly to the pump inlet, not to the reservoir. This will reduce the time required to raise or decrease the temperature of the process water. For the same reason, the water level in the Column will be set at a low, minimum level of 7.5 cm (3 in). G 25. On the Pumping Unit, close valve HV1, which will cause the water level to rise further in the Column. Then set valve HV3 for directing the full return flow directly to the pump inlet (turn handle fully counterclockwise). G 26. On the Column, remove the cap of the insertion opening of the Float Switch to depressurize the Column. (The water level in the Column will remain stable). 2-23

16 G 27. On the Pumping Unit, open valve HV2 in order to decrease the water level in the Column to 7.5 cm (3 in), then close this valve. G 28. Readjust the output of controller FIC1 until you read 4.0 V approximately at the "F (cal.)" output of the Paddle Wheel Flow Transmitter. This will set the flow rate at about 4 l/min (1.1 gal US/min). Note: Small, continuous variations of a few tenths of volts around the adjusted mean value of 4.0 V are normal at the flow transmitter output. However, large variations of one volt or more are abnormal, and indicate that air has entered the system through an untight connector or component on the suction side of the pump. Should that case occur, stop the variable-speed drive of the Pumping Unit in order to drain the column to the reservoir. When the Column is empty, check the inside of the connector on the Pumping Unit return line hose for any dirt or particles. Also, check the o-rings on the two hose connectors of the Cooling Unit for any fissure or crack. Once you have located and eliminated the cause of the leak, reconnect the system as in Figure 2-7 and resume the procedure from step 19. Measuring temperature with the RTD G 29. Insert the RTD probe all the way into the Column in order for its tip to be submerged in the water. G 30. Have the signal at the 0-5 V OUTPUT of the RTD Temperature Transmitter plotted on the trend recorder of controller FIC1. Adjust the update rate of the trend recorder (sampling interval) in order to be able to monitor the transmitter signal over a period of 10 minutes approximately. Note: If the controller you are using as controller FIC1 is the Lab- Volt Process Control and Simulation Software (LVPROSIM), model 3674, refer to Figure B-5 of Appendix B for details of how to connect the LVPROSIM computer to the RTD Temperature Transmitter. On the I/O Interface, make sure the RANGE switch of ANALOG INPUT 1 is set at 5 V. In LVPROSIM, select Analog Input 1 from the Trend Recorder selection list to have the RTD Temperature Transmitter signal plotted on the trend recorder. Set the LVPROSIM sampling interval at 1500 ms. Access the Configure Analog Inputs window and set the minimum and maximum range values of Analog Input 1 at 25 and 55EC (77 and 131EF), respectively, which corresponds to the current measurement range of the RTD Temperature Transmitter. Set the filter time constant of this input at 0.5 second. Make sure the square root extracting function is unselected. Accept setup and return to main screen. 2-24

17 G 31. On the trend recorder, observe the RTD Temperature Transmitter output signal. Since no electrical power is applied to the heating element of the Heating Unit, theoretically, the water in the Column should be at ambient temperature. Assuming that the ambient temperature is below 25EC (77EF), the level of the RTD Temperature Transmitter signal should be at 0% of span on the trend recorder, since the minimum temperature the transmitter can detect has been adjusted to 25EC (77EF). Yet, you may observe that the RTD Temperature Transmitter signal is at some higher level, thermal energy being transferred to the recirculated water mainly from frictional resistance of the pump internal parts. G 32. On the Heating Unit, set the manual control knob to the mid position. On the trend recorder, observe what happens to the temperature of the water in the Column. Now that electrical power is applied to the heating element of the Heating Unit, thermal energy is transferred from this element to the recirculated water. Consequently, the temperature of the water should increase in the Column. Is this your observation? G Yes G No G 33. Let the temperature of the water in the Column increase to about 45EC (113EF), or 67% of span, then turn the manual control knob of the Heating Unit fully counterclockwise to remove electrical power from its heating element. According to the RTD Temperature Transmitter output signal on the trend recorder, did the temperature of the water in the Column increase linearly over time? How long did it take for the temperature to increase from the initial temperature to the final temperature of 45EC (113EF)? G 34. On the Cooling Unit, turn the manual control knob fully counterclockwise. What happens to the temperature of the water in the Column? 2-25

18 G 35. Allow the temperature of the water in the Column to cool down. According to the RTD Temperature Transmitter output signal, does the temperature of the water in the Column decrease linearly over time? Explain. G 36. Stop the variable-speed drive of the Pumping Unit by setting the output of controller FIC1 at 0%. G 37. Turn off the Pumping Unit, the Heating Unit, and the 24-V DC Power Supply by setting their POWER switch at O. G 38. Open valve HV1 of the Pumping Unit completely and let the water in the Column drain back to the reservoir. The Column can also be drained by disconnecting the end of the hose connected to the Cooling Unit inlet port and reconnecting it to either of the auxiliary return ports on the Pumping Unit. G 39. Disconnect the system. Return all leads, hoses, and components to their storage location. CAUTION! Hot water may remain in the hoses and components. Be careful not to allow water to enter the electrical components and their terminals upon disconnection of the hoses. G 40. Wipe off any water from the floor and the Process Control Training System. CONCLUSION In this exercise, you familiarized yourself with the operation of an RTD temperature transmitter in the fixed and variable calibration modes. You learned that, in the fixed calibration mode, the temperature measurement range is fixed and is equal to 0-100EC (32-212EF). In the variable calibration mode, the temperature measurement range can be adjusted, and a maximum span of 30EC (54EF) can be obtained. Since this span is narrower than the 100EC (180EF) span of the fixed calibration mode, the variable calibration mode provides a greater measurement accuracy for any given transmitter output range. 2-26

19 REVIEW QUESTIONS 1. What is an RTD? How does an RTD work? 2. What are three metals commonly used for RTDs? What are the advantages and limitations of each metal? 3. Name and describe two important characteristics of RTDs. 4. How is the voltage produced across an RTD traditionally measured? 5. Why are RTDs available in three-wire version? Explain. 2-27