Exercise 2: Temperature Measurement

Transcription

1 Exercise 2: Temperature Measurement EXERCISE OBJECTIVE When you have completed this exercise, you will be able to explain and demonstrate the use of an RTD in a temperature measurement application by using a resistance-to-voltage converter circuit. DISCUSSION This is the schematic of the resistance-to-voltage converter in the RTD circuit block. You can calibrate the current by adjusting the voltage across R4 with potentiometer R2. FACET by Lab-Volt 149

2 Transducer Fundamentals circuit? a. R2 b. R3 c. RTD1 resistance. voltage is 0 V at a reference temperature of 30 C. Therefore, you can calculate the output (in volts) at any temperature T by using the following formula: V OUT = (T 30) x What is the output voltage at 58 C? V OUT = V (Recall Value 1) 150 FACET by Lab-Volt

3 To increase the output to more practical levels, you can connect the resistance-to-voltage converter PROCEDURE In this PROCEDURE, you will calibrate the RTD resistance-to-voltage converter and measure the output to determine oven temperature. You will observe the temperature response of the RTD resistance-to-voltage converter. Select 40 C on the TEMP header in the IC TRANSDUCER circuit block. Insert a two-post connector in the OVEN ENABLE position. Complete the following steps as you allow the oven to reach its set point. In the RTD circuit block, insert a two-post connector to connect the RTD in the op amp feedback loop. FACET by Lab-Volt 151

4 Transducer Fundamentals jack. In the INSTRUMENTATION AMPLIFIER circuit block, insert a patch lead between IN and Set the multimeter for volts dc. 152 FACET by Lab-Volt

5 Connect the multimeter leads between IA OUT (+) and GND ( ) in the INSTRUMENTATION AMPLIFIER circuit block. Observe the LED and allow it to cycle on and off several times to make sure the oven has reached its 40 C set point. V OUT = V (Recall Value 1) procedure. Move the TEMP shunt to 35 C. Complete the following steps as you allow the oven to reach its new set point. FACET by Lab-Volt 153

6 Transducer Fundamentals V CALCULATED = V (Recall Value 2) Observe the LED and allow it to cycle on and off several times to make sure the oven has reached its set point. V MEASURED = V (Recall Value 3) Move the TEMP shunt to 45 C. Complete the following steps as you allow the oven to reach its new set point. V CALCULATED = V (Recall Value 4) Observe the LED and allow it to cycle on and off several times to make sure the oven has reached its set point. V MEASURED = V (Recall Value 5) Move the TEMP shunt to 50 C. Complete the following steps as you allow the oven to reach its new set point. 154 FACET by Lab-Volt

7 V CALCULATED = V (Recall Value 6) Observe the LED and allow it to cycle on and off several times to make sure the oven has reached its set point. V MEASURED = V (Recall Value 7) This table shows your output voltage calculations and measurements at the four set points. Temperature Calculated Output Voltage Measured 35 C 2.5 (Step 11, Recall Value 3) 40 C C C 10.0 (Step 15, Recall Value 5) (Step 19, Recall Value 7) Are the two values about the same at each temperature? a. yes b. no Set the oven controller for 40 C. Complete the following steps as the oven approaches its set point. FACET by Lab-Volt 155

8 Transducer Fundamentals When you calibrated the circuit by adjusting R2, you set a reference voltage (V REF ) at the op If you were to remove R4 from the input circuit, the op amp would have unity gain. You could then read V REF directly at the output. Observe the OVEN ON LED and allow it to complete several cycles to be sure it has reached its 40 C set point. 156 FACET by Lab-Volt

9 Connect the voltmeter to OUT in the RTD circuit block. The voltage should be about 50 mv. Place CM 2 in the ON position to open R4. Read the reference voltage that now appears at the op amp output. V REF = V (Recall Value 8) FACET by Lab-Volt 157

10 Transducer Fundamentals Place CM 2 in the OFF position. You should again read about 50 mv at the op amp output. Since the op amp forces the ( ) input voltage to equal the (+) input voltage, V REF V (Step 23, Recall Value 8) also appears at the inverting input. You can verify this by switching R4 from 5 V to ground. With 0 V as the input voltage to R4, the op amp output equals the non-inverting input voltage (V REF ) multiplied by the non-inverting gain (A V = 1 + R FEEDBACK INPUT ): V OUT = V REF x (1 + R RTD1 You determined that the RTD resistance (R RTD1 ) at 40 C is 1154 output with 0 V at R4: V OUT = V REF x (1 + R RTD1 V OUT = (Step 23, Recall Value 8) x ( ) V OUT = V (Recall Value 9) 158 FACET by Lab-Volt

11 Place CM 13 in the ON postion to switch R4 from 5 V to 0 V. Measure the op amp output voltage. V OUT = V (Recall Value 10) Remove the two-post connector from the OVEN ENABLE position. V OUT (calculated) = (Step 24, Recall Value 9) V OUT (measured) = (Step 25, Recall Value 10) Are your calculated and measured values of V OUT about equal? a. yes b. no 5 V supply multiplied by the inverting gain (A V = R FEEDBACK INPUT ): R RTD1 = 40 C V1 = 5 x [ (R RTD1 = 5 x = V The second voltage is the reference voltage multiplied by the non-inverting gain: V2 = V REF x [1 + (R RTD1 = x = V FACET by Lab-Volt 159

12 Transducer Fundamentals The output is the sum of the two voltages: V OUT = V1 + V2 = ( 2.079) = V This is the approximate output voltage you currently see at 40 C. Make sure all CMs are cleared (turned off) before proceeding to the next section. CONCLUSION You can use an RTD to measure temperature by using a resistance-to-temperature conversion circuit. temperature. output voltage. REVIEW QUESTIONS 1. in this circuit controls the a. voltage gain. b. current gain. c. temperature offset. d. All of the above 2. The circuit used in this exercise allows you to use an RTD to measure temperature by converting a. temperature to resistance. b. resistance to temperature. c. resistance to voltage. d. current to voltage. 160 FACET by Lab-Volt

13 3. The voltages you calculated at the four oven set points show that the RTD a. b. is a nearly linear device. c. Both of the above d. None of the above 4. The purpose of R2 is to calibrate the circuit for a 0 V output a. b. when the RTD is removed from the circuit. c. at a reference temperature. d. at every set point. 5. If a circuit has the voltage-temperature relationship shown, what is its output voltage at 0 C? a. 0 V b. 7.5 V c. 15 V d. +15 V FACET by Lab-Volt 161