Resistance Temperature Detectors (RTDs)

Transcription

1 Exercise 2-1 Resistance Temperature Detectors (RTDs) EXERCISE OBJECTIVE In this exercise, you will become familiar with the mode of operation of resistance temperature detectors (RTDs) and you will be able to describe the relationship between the temperature and the electrical resistance of the most common types of RTD. You will also be able to explain the mode of operation of a Wheatstone bridge to measure the resistance of the RTD. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Introduction Electrical resistance RTD metals Measurement with an RTD Wheatstone bridge. Wheatstone bridge applied to RTDs. Advantages and limitations DISCUSSION Introduction An RTD (resistance temperature detector) probe is a temperature measurement device which takes advantage of the physical properties of metals to obtain precise and reliable measurements. All metals have a physical characteristic known as electrical resistance. The electrical resistance describes the opposition of the metal to a flow of electricity. It is measured in ohms () in both the SI and the U.S. customary system of units. The observation that the electrical resistance of a metal depends on its temperature (i.e., it is a thermometric property) was made in 1821 by Sir Humphry Davy ( ). In 1871, it was suggested by Sir William Siemens ( ) to use platinum to demonstrate the application of the relationship between electrical resistance and temperature. The progress towards commercial fabrication took a long time: the first platinum resistance temperature detector was finally constructed in 1932 by C. H. Meyers. Figure 2-8. Sir Humphry Davy, Bt. The development of RTDs might have been slow before the second half of the 20 th century, but platinum RTDs are now the international standard for temperature measurement between the triple point of hydrogen (13.81 K or C [24.86 R or F]) and the freezing point of antimony (903.9 K or C [ R or F]). RTDs are recognized as precise and very stable temperature measurement devices with numerous uses in laboratory experiments and industrial applications alike. Electrical resistance Solid-state physics has models which describe the different properties of matter for different temperatures of the solid. Such models show that the electrical Festo Didactic

2 Ex. 2-1 Resistance Temperature Detectors (RTDs) Discussion resistance of a metal is dependent upon the temperature of the metal. Figure 2-10 presents experimental results of how the relative electrical resistance evolves as a function of the temperature for different metals. The relative resistance is simply the ratio of the resistance at a specific temperature over the resistance at a reference temperature. 7 Figure 2-9. Sir Carl Wilhelm Siemens. Relative Resistance Temperature ( C) Platinum RelativeResistance = C -1 at 0 C Platinum Copper RelativeResistance = C -1 at 0 C Copper Nickel RelativeResistance = C -1 at 0 C Nickel Figure 2-10.Relative resistance of different metals as a function of the temperature 8. As shown in Figure 2-10 and as a general rule of thumb, the relative electrical resistance of a metal increases with the temperature. Moreover, in the case of platinum, the relative resistance varies almost linearly with the temperature, at least over a substantial range of temperatures. This allows postulating, to a very good approximation, that the resistance varies according to the following equation: (2-11) where is the resistance of the metal () is the nominal resistance () is the temperature coefficient (1/ C or 1/ F) is the temperature of the metal ( C or F) is the reference temperature (0 C or 32 F) Therefore, RTDs can be said to have two important characteristics: The nominal resistance is the resistance of the RTD at a given reference temperature. Platinum RTDs, for example, are usually designed so that their nominal resistance is 100 at the reference temperature of 0 C (32 F). The temperature coefficient indicates the average change in relative resistance per unit change of temperature. This temperature coefficient is symbolized by the Greek letter alpha (). It is usually written in units of 1/ C or sometimes as /( C) (or 1/ F in the U.S. customary system). 8 To convert the temperature to degree Fahrenheit use 34 Festo Didactic

3 Ex. 2-1 Resistance Temperature Detectors (RTDs) Discussion The platinum curve in Figure 2-10 shows that the temperature coefficient of the platinum used in the provided RTD is C -1 ( F -1 ) at 0 C (32 F). However, the relative resistance ( ) of the platinum probe increases by only when the temperature increases from 0 C to 100 C (32 F to 212 F). This means that the average temperature coefficient in the 0 C to 100 C range is C -1 ( F -1 ). This indicates that the actual relationship is not exactly linear. The linear approximation will remain nonetheless quite sufficient for our needs. Note that, if the need arises, the relationship between the resistance and the temperature can be described by more advanced relationships, such as the Callendar-van Düsen equation, to allow for better precision. Observe in Figure 2-10 that the curves are not so linear for all metals in some temperature ranges. These metals can nonetheless be used in RTDs, as long as they are used in a temperature range where the relationship is almost linear. When the relationship given in Equation (2-11) applies, and the different constants are known ( determining the temperature is then only a matter of measuring the resistance of the metal in the RTD. The temperature is then directly inferred from the equation. RTD metals The selection of a specific metal for use as an RTD depends on several factors such as its usable temperature range and price. Other factors include the capability to rapidly follow temperature changes, the linearity of its resistanceversus-temperature relationship, the reproducibility of its measurement, and the size of its resistance change for a given change in temperature (i.e., a high temperature coefficient implies a large change in resistance for a given change in temperature). The metals most commonly used for RTDs are platinum, nickel, and copper: Platinum is the preferred metal for RTDs. It has been chosen as the international standard for RTD temperature measurement. Platinum has a nearly linear resistance-versus-temperature relationship over a wide range of temperatures. It also offers good stability and reproducibility. It is well suited for the measurement of temperatures up to about 650 C (1200 F) even though platinum RTDs that can be used in temperature ranges from -251 C to 899 C (-420 F to 1650 F) are commercially available. Nickel is the second most used metal for RTDs. It is less expensive than platinum and it is more sensitive due to its higher temperature coefficient. However, nickel is limited to a narrower sensing range than platinum: -196 C to about 300 C (-320 F to 600 F). Copper is the least expensive of the three metals and it has the most linear resistance-temperature relationship over a wide range of temperatures. Similar to platinum, copper could be well suited to the measurement of high temperatures. It is, however, subject to oxidation and it has poorer stability and reproducibility than platinum. Consequently, the typical temperature ranges of commercially available copper RTDs are located between -196 C to 120 C (-320 F to 250 F). Festo Didactic

4 Ex. 2-1 Resistance Temperature Detectors (RTDs) Discussion Measurement with an RTD The RTD probe measures the temperature of a process fluid due to the change of resistance of the device. Measuring the temperature with this method implies determining the precise resistance of the RTD. To do so, a Wheatstone bridge is usually employed. The section below gives a short introduction to the concept of a Wheatstone bridge. Wheatstone bridge In 1843, the English physicist Charles Wheatstone devised a method to precisely measure the electrical resistance of a conductor. This method uses an electrical circuit called a bridge circuit. In such a circuit, two branches are connected together (bridged) at an intermediate point by a third branch. The figure below shows a simple Wheatstone bridge where two parallel branches formed by - and - are bridged together by the central branch that contains a measuring instrument (to evaluate ). The measuring instrument is usually a galvanometer, which is a sensitive current measurement device. a b The principle of a Wheatstone bridge is quite simple. Four resistors are connected as shown above. The values of resistors and are known, the third resistor is a variable resistor, and is the unknown resistor. To precisely measure the resistance value of, one must adjust the variable resistor until the galvanometer indicates zero, that is, when no current flows between a and b (and is null). When no current flows in the third branch, the bridge is balanced and the value of the unknown resistor is calculated as follows: Wheatstone bridge applied to RTDs Evaluating the resistance of an RTD can be achieved with a Wheatstone bridge, with the unknown resistance here labeled as. The idea is to balance the two legs of the bridge by adjusting the variable resistance until the electrical potential difference is null. In the Wheatstone bridge, and are chosen to have exactly the same resistance value. When becomes null, is then equal to the resistance of the RTD ( ), with the effects of lead resistances ignored (refer to the previous equation). Different wiring configurations can be used to measure the resistance. The simplest configuration is the two-wire configuration, shown in Figure 2-11, which can be used when high accuracy is not needed. In this setup, two leads (A 36 Festo Didactic

5 Ex. 2-1 Resistance Temperature Detectors (RTDs) Discussion and B) connect the RTD to the bridge. The resistance of the wires ( and ) connecting the RTD to the Wheatstone bridge causes errors in the temperature measurement which can be large. Lipták (see the bibliography) mentions a typical error due to the resistance of cables resulting in an offset error of 26 C (47 F) for a platinum RTD. Lead B Lead A Figure 2-11.Wheatstone bridge used to measure R RTD - Two-wire configuration. When, we find using Kirchhoff s laws that: (2-12) Since, the equation reduces to: (2-13) We obtain the result that the value of resistance is equal to the resistance of the RTD ( ) plus the resistance due to the electric wires used to connect the RTD to the bridge. The three-wire configuration can attenuate the error due to the lead resistances. In a three-wire setup, shown in Figure 2-12, the wire-resistance error is cancelled when the two lead resistances are exactly equal. This is the type of configuration which is the most frequently used (it is the one used by the temperature transmitter). R 2 R 1 R B Lead B R RTD R 3 R A Lead A R C Lead C Figure 2-12.Wheatstone bridge used to measure R RTD - Three-wire configuration. Festo Didactic

6 Ex. 2-1 Resistance Temperature Detectors (RTDs) Discussion When, we find using Kirchhoff s laws that: (2-14) Since, the equation reduces to: (2-15) We obtain the anticipated result: the value of resistance is equal to the resistance of the RTD ( ) plus the resistance of wire B minus the resistance of wire A. The resistance of wires A and B should be pretty much equal for wires of the same type and quality so that and. This is a great improvement over the two-wire result. Finally, a four-wire configuration can be used when high-precision measurements are required (for example, in scientific laboratory experiments or in high-precision devices). With this configuration, the effect of lead resistance is cancelled out completely to allow for a very precise measurement of. The interested reader should consult an electric circuit reference book for details on four-wire Wheatstone bridge configuration. Advantages and limitations RTDs have many advantages such as: good thermal sensitivity, reproducibility, and stability. They also provide high precision as some platinum RTDs are able to measure with a resolution of up to a few thousandths of a degree. On the other hand, 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 components used in the Wheatstone bridge. An RTD is also sensitive to self heating: it is important to avoid heating the metal with too great an electrical current or for too long while measuring its resistance. If heat is generated because of power dissipation in the RTD, the measured temperature will not be the temperature of the fluid of interest, but the temperature of the self-heating RTD. 38 Festo Didactic

7 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Operating the RTD temperature transmitter Fixed calibration mode. Variable calibration mode. Setup and connections Preliminary setup. Purging air from the components downstream of the column. Placing the system in the water recirculating mode. Measuring temperatures with the RTD Ending the exercise PROCEDURE Operating the RTD temperature transmitter Fixed calibration mode 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. Power up the RTD temperature transmitter. Refer to the previous exercise if necessary. 3. Connect the 100 RTD probe to the RTD input of the temperature transmitter. Let the probe tip lie on the work surface. 4. Adjust the transmitter to the settings shown in Table 2-1. This selects the RTD probe signal as the transmitter input signal and places the transmitter outputs in the fixed calibration mode. Figure 2-13 illustrates the relation between the current (or voltage) and the temperature in the fixed calibration mode. Table 2-1. RTD temperature transmitter settings (fixed calibration mode). Knob or switch INPUT CALIBRATION Setting RTD FIXED Festo Didactic

8 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure Current ma Slope: 0.16 ma/ C (0.089 ma/ F) 20 5 Voltage V Slope: 50 mv/ C (28 mv/ F) 4 0 C (32 F) 100 C (212 F) Temperature 0 C (32 F) 100 C (212 F) Temperature (a) 4-20 ma output (b) 0-5 V output Figure Fixed calibration mode. 5. Connect a multimeter to the 4-20 ma output of the RTD temperature transmitter. Since this output is in the fixed calibration mode, it generates a fixed current of 0.16 ma per sensed C above 0 C (or ma per sensed F above 32 F), plus an offset of 4 ma. For a temperature of 75 C (167 F), you would obtain: a In C, a reading of: (0.16 ma/ C) (75 C - 0 C) ma = ma In F, a reading of: (0.089 ma/ F) (167 F - 32 F) ma = ma The 0-5 V output in fixed calibration mode generates a fixed voltage of 50 mv per sensed C above 0 C (or 28 mv per sensed F above 32 F), with no offset. According to the multimeter reading, what is the ambient temperature? 6. Further experiment with the operation of the transmitter in the fixed calibration mode: Fill a container with a mixture of ice cubes and water. Immerse the tip of the RTD probe into the cold water. The 4-20 ma output current should decrease and stabilize at about 4.00 ma, which corresponds to a temperature of 0 C (32 F) in the fixed calibration mode. Fill a container with boiling water. 40 Festo Didactic

9 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure a Immerse the tip of the RTD probe into the hot water. The 4-20 ma output current should increase and stabilize at about 20.0 ma, which corresponds to a temperature of 100 C (212 F) in the fixed calibration mode. The 4-20 ma output of the RTD temperature transmitter will stabilize at a current lower than 20.0 ma if the atmospheric pressure is lower than kpa, absolute (14.7 psia). Variable calibration mode In the following steps, you will use the calibration source of the RTD temperature transmitter to calibrate its 4-20 ma output so that the current at this output goes from 4.00 ma to 20.0 ma when the probe temperature simulated by the calibration source goes from 25 C to 55 C (77 F to 131 F). a Use the same procedure to adjust the range of the 0-5 V output. Simply replace mentions of the 4-20 ma output with the 0-5 V output. Also, replace mentions of 4 ma with 0 V, 20 ma with 5 V, and current with voltage. 7. Adjust the temperature transmitter using the settings shown in Table 2-2. This selects the calibration source signal as the input signal and places the outputs in the variable calibration mode. Figure 2-14 illustrates the relation between the current (or voltage) and the temperature in the variable calibration mode performed in this procedure. The variable calibration mode presents a narrower span and thus has a greater measurement accuracy compared to the fixed calibration mode. Table 2-2. RTD temperature transmitter settings (variable calibration mode). Knob or switch INPUT CALIBRATION ZERO SPAN Setting CAL. SOURCE VARIABLE MAX. MAX. Festo Didactic

10 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure Variable calibration Slope: ma/ C (0.296 ma/ F) Variable calibration Slope: 167 mv/ C (92.6 mv/ F) Current Fixed calibration Voltage ma Slope: 0.16 ma/ C V (0.089 ma/ F) 20 5 Fixed calibration Slope: 50 mv/ C (28 mv/ F) 4 25 C (77 F) 55 C (131 F) Temperature 25 C (77 F) 55 C (131 F) Temperature (a) 4-20 ma output (b) 0-5 V output Figure Variable calibration mode vs fixed calibration mode. 8. Set the temperature to be simulated by the calibration source at 25 C (77 F). To do so, adjust the CALIBRATION SOURCE knob of the transmitter until you read a voltage of 2.5 V at the CAL. output using a multimeter. 9. While monitoring the current at the 4-20 ma output with a multimeter, turn the ZERO adjustment knob counterclockwise until you read 4.00 ma. This sets the minimum temperature to be detected at 25 C (77 F). a Alternatively, you could monitor the 0-5 V output, 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. 10. Set the temperature to be simulated by the calibration source at 55 C (131 F). To do so, adjust the CALIBRATION SOURCE knob of the transmitter until you read a voltage of 5.5 V at the CAL. output. 42 Festo Didactic

11 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure 11. Adjust the SPAN knob in order to obtain a current of 20.0 ma at the 4-20 ma output. Turn the SPAN knob counterclockwise to increase the current value. This sets the maximum temperature to be detected at 55 C (131 F). a You could monitor the 0-5 V output and adjust the SPAN knob to obtain a reading of 5.00 V. 12. Due to interaction between the ZERO and SPAN adjustments, repeat steps 8 through 11 until the 4-20 ma output of the transmitter actually varies between 4.00 ma and 20.0 ma when the CALIBRATION SOURCE is varied between 2.5 V and 5.5 V to simulate a temperature between 25 C (77 F) and 55 C (131 F). 13. Once the RTD temperature transmitter is calibrated, proceed to the next part of the exercise. Setup and connections Preliminary setup 14. Get the small expanding work surface and install it vertically (at an angle of 90 ) on the main work surface. Proceed as shown in Figure 2-15 to prepare your setup for this exercise. Install the equipment, but do not perform the hose connections nor the electrical wiring yet. Make sure to mount the heating unit at the highest possible location on the expanding work surface. Failure to do so may result in water entering the heating unit upon disconnection of the hoses, which might cause damage to the heating unit. Festo Didactic

12 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure Cap Back view Plug Pumping unit Figure Setup - Temperature measurement with an RTD probe. 1 - Column 4 - Rotameter 2 - Heating unit 5 - RTD temperature transmitter 3 - Power supply 6 - Cooling unit 44 Festo Didactic

13 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure 15. Connect the system as shown in Figure 2-16 (flow diagram with hose connections) and Figure 2-17 (electrical wiring diagram), but leave the pumping unit, the power supply, and the heating unit turned off. Be careful not to modify the calibration settings just made on the RTD temperature transmitter. Connect the heating unit so that water flows in the direction indicated by the arrowheads in the symbol on its front panel (refer to Figure 1-10). The cooling unit will operate regardless of the direction of water flow through it. However, to minimize the risk of cavitation, connect the cooling unit as indicated in Figure 2-15, that is, with the upper port used as the hot water inlet and the lower 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. The variable speed drive of the pumping unit will be regulated with the controller FC1 placed in the manual mode. The heating and cooling units will be controlled manually; this is why there is no temperature controller, or TC instrumentation symbol, illustrated next to these units in the flow diagram of Figure 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, but connect the probe to the RTD temperature transmitter. Cooling unit RTD temperature transmitter Plug Reservoir Column Heating unit Variable speed drive Controller Rotameter Plug RTD probe Do not insert in the column Figure Flow diagram Temperature measurement with an RTD probe. Festo Didactic

14 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure Figure Wiring diagram Temperature measurement with an RTD probe. LVProSim Figure 2-17 shows how to connect the computer running LVProSim to the pump and RTD temperature transmitter via the I/O interface. To control the pump speed using LVProSim, connect the variable-speed drive to output 1 of the I/O interface. With this configuration, you can modify the pump speed by changing the output signal manually in the appropriate PID controller section of LVProSim. Refer to Appendix B for details on how to use LVProSim. 46 Festo Didactic

15 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure 16. Adjust the equipment to the settings shown in Table 2-3. Table 2-3. Equipment settings. Equipment Knob or switch Setting Heating unit S1 2 Manual control knob Fully counterclockwise S1 2 Cooling unit RTD temperature transmitter Manual control knob S2 INPUT CALIBRATION Fully counterclockwise RTD VARIABLE The 4-20 ma output of the RTD temperature transmitter should still be calibrated for a temperature measurement range of C ( F) from the first part of the exercise. 17. Make sure flow controller FC1, which regulates the variable-speed drive of the pumping unit, is in the manual mode (refer to Appendix B for details). Set the output of this controller to 0% (4 ma). Purging air from the components downstream of the column 18. On the column, make sure the cap is tightened firmly and the plugs are in place (one at the bottom and one at the top). 19. Verify that the reservoir of the pumping unit is filled with about 12 liters (3.2 gal) of water and that the baffle plate is properly installed at the bottom of the reservoir. a To keep the water in the reservoir clean and hygienic for an extended period of time, add one capful of algaecide (P/N 38097). To prevent rust, add about 40 ml (1.3 oz) of rust inhibitor (P/N 38096). Add these products when you fill the reservoir. 20. Adjust valves HV1 through HV3 of the pumping unit using Table 2-4. Table 2-4. Valves settings. Valve HV1 HV2 Position Open Closed HV3 Fully clockwise 9 9 To direct the full reservoir flow to the pump inlet Festo Didactic

16 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure 21. Turn on the power supply by setting its power switch to I (on). This powers up the I/O interface, the cooling unit, and the RTD temperature transmitter. Leave the heating unit off. 22. Turn on the pumping unit and adjust the parameters of the drive to allow the FC1 signal to control the frequency of the drive. To do so, set the drive parameters P10 to 0.3 and P05 to 0.4. Refer to Appendix F for details on how to use the variable-speed drive of the pumping unit. 23. Press the Run button on the drive keypad to start the pump. Set the output of controller FC1 to 100% (20 ma) in order for the pump to reach its maximum rotational speed. 24. Allow the level of the water to rise in the pressurized column until it stabilizes at some intermediate level. This forces air out of the components downstream of the column. a If the cap of the column opening has not been tightened firmly, air will be allowed to escape and the water level will not stabilize. Should this occur, stop the variable-speed drive of the pumping unit. Open valves HV1 and HV2 to drain the column to the reservoir. When the column is empty, tighten the cap more firmly. Then, resume the procedure from step 20. Placing the system in the water recirculating mode 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 to a low, minimum level of 7.5 cm (3 in). 25. On the pumping unit, close valve HV1. This causes 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). 26. 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. 27. Remove the cap to depressurize the column (the water level remains stable). 28. Adjust the variable-speed drive of the pumping unit (via the output of controller FC1) until you have a flow rate of about 4 L/min (1 gal/min). a Large variations (1 L/min or more) in flow rate are abnormal and indicate that air has entered the system through a non-tight connector or component on the suction side of the pump. 48 Festo Didactic

17 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure Should this occur, stop the variable-speed drive of the pumping unit and 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 and resume the procedure from step 20. Measuring temperatures with the RTD 29. Insert the RTD probe into the column in order for its tip to be submerged in the water. 30. Have the signal at the 4-20 ma output of the RTD temperature transmitter plotted on the trend recorder of the software, as explained below. LVProSim Refer to Figure 2-17 for the electrical connections between the I/O interface and the RTD temperature transmitter. Follow the steps below to plot the transmitter output signal on the trend recorder of the software. Press the Set Channels icon in the LVProSim menu bar and, in the Set Channels window, select the channel number corresponding to the input on the I/O interface to which the RTD temperature transmitter is connected. Then: Enter the name you want to give to the channel in the Label text box. Select Temperature as the type of measured variable. Select Celsius (or Fahrenheit) as the measurement unit. Enter 25 C (77 F) in the Minimum value field and 55 C (131 F) in the Maximum value field. These values correspond to 4 ma and 20 ma signals, respectively. From the Settings menu, change the sampling interval to 1000 ms. Add the channel to the curves list at the bottom of the trend recorder and press the play button in the menu bar to start recording data. To add a channel to the curves list, select the label from the drop-down list that corresponds to the channel you want to add and press ADD. Refer to Appendix B for details on how to use the trend recorder. 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 25 C (77 F), 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 25 C (77 F). Yet, you may observe that the RTD temperature transmitter signal is at some higher level, because thermal energy is transferred to the recirculated water mainly from frictional resistance of the pump internal parts. Festo Didactic

18 Ex. 2-1 Resistance Temperature Detectors (RTDs) Procedure 32. On the heating unit, set the power switch to I (on) and 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, thermal energy is transferred from this element to the recirculated water. Consequently, the temperature of the water increases. 33. Let the temperature of the water increase to about 45 C (113 F), 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 signal recorded, did the temperature of the water increase linearly over time? How long did it take for the temperature to increase from the initial temperature to the final temperature of 45 C (113 F)? 34. On the cooling unit, turn the manual control knob fully clockwise. What happens to the temperature of the water in the column? 35. Allow the water in the column to cool down. According to the RTD temperature transmitter output signal, did the temperature of the water decrease linearly over time? Explain. Ending the exercise 36. Set the output of controller FC1 to 0% to stop the variable-speed drive of the pumping unit. Even if the heating unit is protected against overheating, electrical power should not be applied to the heating element in the absence of water flow through this unit. This means that the manual control knob of the unit should be turned fully counterclockwise or that the current or voltage applied by the controller to the control input terminals of the unit should be minimum (4 ma or 0 V) in the absence of water flow. Failure to do so might cause the heating unit to wear out prematurely. 50 Festo Didactic

19 Ex. 2-1 Resistance Temperature Detectors (RTDs) Conclusion 37. Turn off the pumping unit, the heating unit, and the power supply by setting their power switch at O (off). 38. Open valve HV1 of the pumping unit completely and let the water in the column drain back to the reservoir. 39. Disconnect all leads from the training system. Remove from the work surface the power supply, the temperature transmitter, and any electrical equipment not included in the water loop. 40. Disconnect the hoses. Return all leads, hoses, and components to their storage location. Hot water may remain in the hoses and components. The training system is not equipped with dripless connectors, so be careful not to allow water to enter the electrical components and their terminals upon disconnection of the hoses. 41. 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. In the variable calibration mode, the span is narrower than the 100 C (180 F) span of the fixed calibration mode. The variable calibration mode thus provides greater measurement accuracy. REVIEW QUESTIONS 1. What is an RTD? How does an RTD work? Festo Didactic

20 Ex. 2-1 Resistance Temperature Detectors (RTDs) Review Questions 2. Name 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 versions? Explain. 52 Festo Didactic