Exercise 1: Thermocouple Characteristics

Transcription

1 The Thermocouple Transducer Fundamentals Exercise 1: Thermocouple Characteristics EXERCISE OBJECTIVE When you have completed this exercise, you will be able to describe and demonstrate the characteristics of a thermocouple. You will verify your results with a multimeter. DISCUSSION two wires joined at their ends. a. positive b. negative Since a thermocouple generates a voltage, it is considered a self-powered device. Thermocouples are simple, rugged, low-cost tranducers that operate over a very wide temperature range. 174 FACET by Lab-Volt

2 Transducer Fundamentals The Thermocouple Disadvantages include a low output level, stability, and sensitivity, and the need for a reference for accurate temperature measurement. Manufacturers supply tables of voltage versus temperature for their thermocouples. = 51 C) to calculate voltage at a given temperature. This schematic shows how the thermocouple is connected in a thermistor bridge on your circuit board. The thermistor and the measuring junctions are located on the underside of the circuit board so that they are in the same temperature environment. a reference junction circuit. C, which is the freezing point of water. For this reason, the circuit is also called the electronic ice point reference. FACET by Lab-Volt 175

3 The Thermocouple Transducer Fundamentals The thermocouple on your circuit board has an output of 0 V at 0 C. What is the output voltage of the calibrated bridge at 0 C? a. 0 V b V c. Cannot be determined V = 100) to boost the voltage to more practical levels. ) equals 51 the circuit output at any temperature T: IA OUT = x T x A V What voltage would the circuit output at 28.6 C? IA OUT = mv (Recall Value 1) PROCEDURE In this PROCEDURE, you will calibrate the electronic ice point reference to read the thermocouple voltage output. You will verify your results by calculating and measuring the output voltage at different set points and comparing the values. Enable the oven and select the 40 C set point. Complete the following steps as you allow the oven to reach the set point. 176 FACET by Lab-Volt

4 Transducer Fundamentals The Thermocouple Set your multimeter to measure Vdc and connect the leads across the output of the In the THERMOCOUPLE circuit block, connect +OUT and OUT from the reference circuit to +IN and IN respectively, in the INSTRUMENTATION AMPLIFIER circuit block. FACET by Lab-Volt 177

5 The Thermocouple Transducer Fundamentals Calculate the voltage to which you should calibrate the circuit output at 40 C: IA OUT = x T x A V = 51 A V = 100 IA 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. Adjust the REF potentiometer for a voltmeter reading of 204 mv at the IA OUT terminal. You have calibrated the circuit for a bridge output of 51 for the remainder of this PROCEDURE. Move the TEMP shunt to the 35 C position. Complete the following steps as you allow the oven to reach the new set point. 178 FACET by Lab-Volt

6 Transducer Fundamentals The Thermocouple Calculate the circuit output voltage at 35 C: IA OUT = x T x A V = 51 A V = 100 IA OUT = mv (Recall Value 2) Observe the OVEN ON LED an allow it to complete several cycles to make sure the oven has reached its set point. Measure the output voltage at 35 C. IA OUT = mv (Recall Value 3) Move the TEMP shunt to the 45 C position. Complete the following steps as you allow the oven to reach the new set point. Calculate the circuit output voltage at 45 C: IA OUT = x T x A V = 51 A V = 100 IA OUT = mv (Recall Value 4) FACET by Lab-Volt 179

7 The Thermocouple Transducer Fundamentals 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 at 45 C. IA OUT = mv (Recall Value 5) Move the TEMP shunt to the 50 C position. Complete the following steps as you allow the oven to reach the new set point. Calculate the circuit output voltage at 50 C: IA OUT = x T x A V = 51 A V = 100 IA OUT = mv (Recall Value 6) Observe the OVEN ON LED and allow it to complete several cycles to make sure the oven has reached its set point. 180 FACET by Lab-Volt

8 Transducer Fundamentals The Thermocouple Measure the output voltage at 50 C. IA OUT = mv (Recall Value 7) This table shows the output voltages you calculated and measured at the four set points. Temperature Calculated Output Voltage (mv) Measured V (mv) 35 C (Step 11, Recall Value 3) 40 C C C (Step 15, Recall Value 5) (Step 19, Recall Value 7) Are the two values about the same at each temperature? a. yes b. no The last column shows the voltage difference ( V) between the calculated values spaced at 5 C temperature intervals. The data shows that the thermocouple is a. linear over its entire operating range. b. linear over the measuring range in this procedure. c. non-linear. What information in the table indicates that the thermocouple has a positive temperature a. equal V values b. increasing voltage c. Neither of the above FACET by Lab-Volt 181

9 The Thermocouple Transducer Fundamentals CONCLUSION A thermocouple is a temperature transducer that generates a voltage that is proportional to its temperature. Thermocouples have a very wide temperature range with a non-linear response. However, over short temperature ranges, the response is nearly linear. REVIEW QUESTIONS 1. What thermocouple parameter changes with temperature? a. resistance b. voltage c. capacitance d. inductance 2. a. voltage source. b. current source. c. voltmeter. d. ohmmeter. 3. has a a. linear response. b. c. resistance that is a function of temperature. d. All of the above 4. A thermistor bridge is often used in thermocouple circuits to a. b. c. d. offset the measuring junction voltages. 182 FACET by Lab-Volt

10 Transducer Fundamentals The Thermocouple 5. For an accurate reading of the thermocouple output voltage, the thermistor should be at the same temperature as the a. other resistors in the bridge. b. thermocouple. c. measuring junctions. d. Does not matter FACET by Lab-Volt 183