Transcript
2. Temperature measurement with thermocouples Task 1. Measure the output voltage of a thermocouple a) directly, with the use of hand-held DMM or bench-top DMM, e.g. U3401A. Apply the real temperature of the thermocouple’s cold junction (measured by a room thermometer) while calculating the temperature. b) with the same instruments as in a) but with a compensation box OMEGA CJ connected between the thermocouple and the DMM. 2. Design a suitable op-amp circuit for amplification of the thermocouple’s output voltage (desired amplification A=100). Do not connect the compensation box. Apply correction for possible measurement method error. Measure the amplifier’s offset voltage by commutating the thermocouple output and apply correction to the temperature calculation. 3. Measure the temperature with a dedicated USB DAQ module with an isothermal terminal block.
Optional task Calculate the extended uncertainty of the thermocouple voltage measurement (coefficient of expansion k= 2). Calculate the uncertainty for direct measurement with DMM as well as for measurement of the amplified voltage. While evaluating the uncertainty uB for the amplified voltage reflect also the effect of the op-amp’s input offset, the effect of input bias currents is considered to be negligible.
Questions 1. Compare the results from tasks 1-3. 2. What is the temperature of the thermocouple’s cold junction which is corrected by the compensation box? Is it possible to measure the temperature simply? 3. Is it possible to somehow compensate/correct the amplifier’s input offset? (commutation?) 4. Do you know any other method how to measure the amplifier’s offset voltage?
Instructions The coefficient of the thermocouples (∆ T / output voltage) is in the order of tens of microvolt per degree of Celsius. Type K thermocouple [(chromel: 90% nickel and 10% chromium) – (alumel: 95% nickel, 2% manganese, 2% aluminium and 1% silicon)] used in this setup has the coefficient c= 40,8 µV/°C. -> 1) Compensation box is used to correct the cold junction temperature. It consists of a powered bridge with a temperature dependent element. If the cold junction (ambient) temperature differs from 0 °C the bridge becomes to be unbalanced and there is a voltage added to the thermocouple’s output. The internal resistance of the compensation box output is approximately 250 Ω (and it changes slightly with a temperature, see Figure 1). Therefore it is necessary to use DMM with a sufficiently high input resistance for the output voltage measurement.
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Fig. 1: Omega CJ compensation box and its inner circuit. !!! The compensation box is powered by a primary lithium battery. Please switch the device off after the measurement has been finished. -> 2) A thermocouple is a voltage source with a specific internal resistance. The resistance is in the order of Ohms (0.1-10Ω), therefore the error by measurement of the output voltage is negligible for a typical DMM (input resistance >10MΩ). The error is circuit dependent if the op-amp amplifier is used (inverting vs. non-inverting circuit).
Fig. 2: Direct thermocouple output voltage measurement.
Fig.3: An inverting op-amp amplifier circuit
Fig.4: A non-inverting amplifier
It is apparent from Fig.4 that the input resistance of the non-inverting circuit is given by the 100kΩ resistor (what is the purpose of this resistor?). The input resistance is only 1000Ω for the inverting circuit (Fig.3).
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Note: It is not convenient to use significantly higher value for R1 than approx. 10 kΩ. The calculated value of R2 would be then too high. This is unlikely for two reasons: 1. High noise voltage generated by a high resistance. 2. The op-amp input bias currents can significantly influence the measurement.
Measurement of the op-amp’s input voltage asymmetry (offset) There are two ways how to measure the input offset voltage: 1. commutation of the input voltage source 2. input short-circuiting The op-amp datasheet mentions the INPUT voltage offset value. The measured value (measured at op-amp output) must be recalculated to the amplifier input if you want to compare it with the datasheet value (divide by amplification). -> 3. USB based DAQ module contains ADC and an isothermal terminal block. There is a special temperature sensor (semiconductor resistive sensor from Siemens) close to the terminal box which allows to measure precisely the cold-junction temperature. The cold-junction compensation is then done by software. Run AD24Control program to do the temperature measurement (desktop icon). It is possible to measure the room temperature by short-circuiting the input of the DAQ module.
Parameters of used instruments 1. Hand-held DMM Mastech MY-64
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2. Bench-top DMM Agilent U3401A
3. Compensation box OMEGA CJ. Accuracy of the cold-junction compensation: 0,25 °C at 25 °C 0,5 °C from 15 to 35 °C 0,75 °C from 10 °C to 50 °C Power: lithium cell 3,6 V 4. OP-AMP OP 07
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5. USB DAQ module AD24USB Technical parameters ADC – galvanically decoupled integration converter with programmable resolution 22 to 26 bits, 8 differential inputs or 16 SE, input range 0-10 V/ ± 5 V, programmable amplification 1 to 128 (1 to 512 for differential version), noise 15 nVp-p @ 1 Sa/s, expanders for thermocouples connection or RTDs (Pt100). http://www.janascard.cz/
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AD24Control software In Configuration menu choose item “Mode”, select “Continual” and set “Time axis length” to approximately 60 s.
In “Mathematics” menu choose “Thermocouple”, this will trigger the temperature calculation. The program will otherwise simply measure the applied voltage.
In “Input” menu select item “Input 0”, the thermocouple is connected to this input. Set the “Range” to maximum sensitivity (78 mV). You can select between two display modes in “Display” menu – text window or graph. Press the “Thermocouple” button and choose K type thermocouple.
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Start the measurement by pressing “Start” button in the main window or by pressing F2. F3 stops the measurement.
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