Exercise 2: Temperature Measurement EXERCISE OBJECTIVE When you have completed this exercise, you will be able to explain the use of a thermocouple in temperature measurement applications. DISCUSSION the relationship of output voltage to temperature. On your circuit board, the components are selected for a C. What is the gain of the second op amp? A V = (Recall Value 1) The voltage divider in the non-inverting input circuit allows you to adjust for a temperature offset. The circuit is designed for you to zero the output at 30 C. 184 FACET by Lab-Volt
The Thermocouple You can calculate the temperature T from the output voltage by using the following formula: T = (V OUT At what temperature does the calibrated circuit output 6.8 V? T = (V OUT T = C (Recall Value 2) Your circuit board also has provisions to demonstrate the results of the ice point reference temperature differing from the thermocouple measuring junction temperatures. When you insert a two-post connector in the HEATER position in the THERMOCOUPLE circuit block, reference circuit. FACET by Lab-Volt 185
As the resistor heats up, it increases the thermistor temperature and a measurement error results. PROCEDURE In this PROCEDURE, you will calibrate the thermocouple temperature measurement circuit. You will verify your results by comparing calculated and measured output voltages at several set points. Set the temperature controller for 40 C and insert a two-post connector in the OVEN ENABLE position. Complete the following steps as you allow the oven to reach its set point. Connect the +OUT and OUT in the THERMOCOUPLE circuit block to IN and +IN, respectively, in the INSTRUMENTATION AMPLIFIER circuit block. 186 FACET by Lab-Volt
The Thermocouple Set your multimeter to measure Vdc and connect its leads to IA OUT (+) and GND ( ) in the INSTRUMENTATION AMPLIFIER circuit block. To what output voltage should you calibrate the circuit at 40 C? IA OUT = x T x A V = 51 A V = 100 V OUT = mv (Recall Value 1) Observe the OVEN ON LED and allow it to complete several cycles to make sure the oven has reached its set point. FACET by Lab-Volt 187
Observe the meter and adjust the REF pot for 204 mv at IA OUT. setting for the remainder of this PROCEDURE. Move the + meter lead to AMP OUT in the THERMOCOUPLE circuit block. Add a connection from IA OUT in the INSTRUMENTATION AMPLIFIER circuit block to AMP IN in the THERMOCOUPLE circuit block. 188 FACET by Lab-Volt
The Thermocouple You will set the relationship of temperature to output voltage according to this formula: T = (V OUT To determine the correct output voltage, you can rearrange the equation to solve for V OUT : (V OUT V OUT V OUT = (T 30) x 0.5 To what output voltage should you calibrate the circuit at 40 C? V OUT = V (Recall Value 2) Observe the meter and adjust the CAL pot for 5.00 V ±0.01 V at AMP OUT. the remainder of this PROCEDURE. FACET by Lab-Volt 189
Change the set point to 35 C and complete the following steps as you allow the oven to reach its new set point. Calculate the output voltage at 35 C. V OUT = (T 30) x 0.5 V OUT = V (Recall Value 3) Observe the OVEN ON LED and allow it to complete several cycles to make sure the oven has reached its set point. Measure the output voltage. V OUT = V (Recall Value 4) Change the set point to 45 C and complete the following steps as you allow the oven to reach its new set point. Calculate the output voltage at 45 C. V OUT = (T 30) x 0.5 V OUT = V (Recall Value 5) Observe the OVEN ON LED and allow it to complete several cycles to make sure the oven has reached its set point. Measure the output voltage. V OUT = V (Recall Value 6) 190 FACET by Lab-Volt
The Thermocouple Change the set point to 50 C and complete the following steps as you allow the oven to reach its new set point. Calculate the output voltage at 50 C. V OUT = (T 30) x 0.5 V OUT = V (Recall Value 7) Observe the OVEN ON LED and allow it to complete several cycles to make sure the oven has reached its set point. Measure the output voltage. V OUT = V (Recall Value 8) This table shows your calculated and measured voltages at the four set points. Temperature Calculated Output Voltage Measured 35 C 2.5 (Step 13, Recall Value 4) 40 C 5.0 5.0 45 C 7.5 50 C 10.0 (Step 17, Recall Value 6) (Step 21, Recall Value 8) Are the two values about equal at each temperature? a. yes b. no a. yes b. no FACET by Lab-Volt 191
Insert a two-post connector in the HEATER position of the THERMOCOUPLE circuit block. thermistor in the ice point reference circuit. Observe the output voltage for about 30 seconds. The output voltage has a. increased. b. decreased. c. remained about the same. 192 FACET by Lab-Volt
The Thermocouple Remove all two-post connectors from the circuit board. You have demonstrated that the thermocouple measurement circuit is not accurate unless the thermistor in the ice point reference circuit is at the same temperature as the a. measuring junctions. b. thermocouple. CONCLUSION In a thermocouple temperature measurement circuit, you can use an op amp to increase the sensitivity of the ice point reference circuit. You can select components for the desired relationship of temperature to output voltage. The thermistor must be at the same temperature as the thermocouple measuring junctions to correctly cancel the measuring junction voltages. FACET by Lab-Volt 193
REVIEW QUESTIONS NOTE: Enable the oven and set the temperature controller for 40 C. Complete Review Questions 1 through 4 as you allow the oven temperature to stabilize for review question 5. 1. You can compensate for measuring junction voltages in a thermocouple temperature measurement circuit by using a a. Wheatstone bridge. b. voltage divider. c. current divider. d. voltage multiplier. 2. One leg of a Wheatstone bridge used in a reference junction circuit should include a(n) a. thermocouple. b. thermistor. c. IC temperature transducer. d. Any of the above 3. a. set the temperature-to-voltage conversion factor. b. establish a temperature offset at the circuit output. c. amplify the bridge output voltage. d. All of the above 4. You can adjust this circuit for a 0 V output at a certain reference temperature by a. adjusting the CAL pot. b. adjusting the REF pot. c. changing the ratio of R FB to R I. d. shorting the thermistor. 194 FACET by Lab-Volt
The Thermocouple Observe the OVEN ON LED and allow it to cycle several times to be sure the oven has reached its 40 C set point. Set your multimeter to measure Vdc. Set the INSTRUMENTATION AMPLIFIER gain to 100. Connect the meter and the THERMOCOUPLE and INSTRUMENTATION AMPLIFIER circuit blocks as shown in the image below. Adjust the REF pot for a reading of 0.204 V as IA OUT. FACET by Lab-Volt 195
Place CM 1 in the ON position to disconnect the +0.5 V reference voltage to the bridge. Read IA OUT again. IA OUT = mv (Recall Value 1) What accounts for the output voltage difference when the reference voltage is removed from the bridge? a. The thermocouple output has decreased. b. The measuring junction voltages are included in the reading. c. The non-inverting input is at 0 V. d. NOTE: Make sure all CMs are cleared (turned off) before proceeding to the next section. 196 FACET by Lab-Volt