Tmp 4103

AN EXPERIMENTAL ANALYSIS OF TUE METROLOGICAL PERFORMANCES OF AN AUTOMA.TIC THERMOCOUPLE INSTRUMENTATION Sl'STEM CONDUCTED BY A SIMPLE CALIBRATION SETUP by P. Cappa and SA. Sciuto NOMENCLATURE AT

AV

T

T MEAS

v

VcAL,,.

V CALme..

Vc1 VHJ V1v

difference between the set calibration temperature and the measured value; difference between the calibration voltage and the value measured by the digitai voltmeter; resistance of the thermistor in the multiplexer; generic temperature measured by the data acquisition contro! system; temperature calibration value; temperature of the cold junction of the thermocouple; temperature of the hot junction of the thermocouple; temperature measured by the data acquisition contro! system dependently on the calibration value; sum of the voltages corresponding to the temperatures of the cold and the hot junctions; voltage corresponding to the temperature calibration value; voltage applied by the universal source; voltage measured by the digitai voltmeter; voltage corresponding to the cold junction temperature; voltage corresponding to the hot junctiontemperature; voltage measured by the integrating digitai voltmeter.

P. Cappa (SEM Member) is Associated Professar and S.A. Sciuto (SEM member) is Research Associate, Department of Mechanics and Aeronautics, University of Rome "La Sapienza," Rome, ltaly.

Thermocouples, as it is well known, have been used widely in the utility industries, nuclear power station, etc. because they are simple to use, inexpensive, versatile, durable, relatively accurate sensors and, finally, low-impedence device. Thermocouples and the associated system are usually calibrated l) by establishing a reference temperature at their cold junctions by means of an ice bath, or an isothermal block monitored with another thermal sensor, or a solid-state heat pump; 1 2) by impressing to their hot junctions one or more known temperatures by means of calibration fumaces. Another possible methodology is based on a special thermocouple called "self-calibrating. " 2 For this thermocouple, the hot junction is located near an incapsulated meta!, so that a single calibration point is made possible when the temperature reaches the phase of transition of the meta!. Moreover, many portable, multifunction microprocessor-based digitai calibrators are recently commercially available: 3 .4 these units can be easily carried to any user location for on site calibrations. The calibrator measures the cold junction temperature and, depending on the selected thermocouple type and the simulated temperature, provides a DC signa!. A commerciai calibrator is available at the laboratory of the Department of Mechanics and Aeronautics of the University of Rome "La Sapienza" but the metrological performances (accuracy: ±(0.1% of reading + 0.5°C)) are judged insufficient for the purpose of this work, which consists in the dynamic characterization of a transducer recently proposed,S-7 called duplex gage, that measures both specimen' s surface temperature and surface strains. The duplex gage is based on two electrical resistance strain gages: one is temperature compensated for the test materia!, the latter for a materia! with a different coef-

Experlmental Tecbnlques 19

ficient of linear expansion. In preliminary experimental analysis some specimens were equipped with duplex gages and were subjected to static and quasi-static temperature variations: a comparative examination of theoretical and experimental results showed satisfactory agreement. In the incoming research project, it is planned the examination of the duplex gage performances when dynamic strain and temperature variations are imposed. In order to check the temperature measurement capabilities, the thermocouples will be applied on the specimen surface and an automatic digitai data acquisition system, with software compensation of non-linear thermocouple outputs, will be used even though, for exampie, Shubba and Ramesh 8 designed a simple analogue fourth-order polynomial curve-fitting circuit capable of assuring an error of ±0.1 oc in the temperature ranges of O to 400°C and 400 to 1000°C, dependently on the adopted coefficients of the polynomials. Thus, it appears the necessity of a preliminary metrological characterization of the automatic thermocouple instrumentation system. The main objectives of the present work are then: l . the development of a simple and programmable calibration procedure capable of assuring selectable intervals of uncertainties, for example, up to a vaiue of 0.02°C; 2. the determination of the dependence of the metrological performances of an automatic thermocouple data acquisition system on the chosen integration time of the utilized digitai voltmeter.

THE EXPERIMENTAL APPARATUS, PROCEDURES AND RESUL TS The automatic digitai data acquisition and contro) system, examined the present work, can be schematized as a relay multiplexer equipped with a reference junction whose temperature is monitored by means of a thermistor, a programmable integrating digitai voltmeter and a mainframe controller, which has built-in intelligence capabilities. These devices are connected via backplane. The software compensation technique implemented in the data acquisition and contro) system can be schematized as it follows: l. the data acquisition and contro) system measures the resistance RT of the thermistor placed on the isotherrnal block, determines the cold junction temperature T c1 an d, depending on the thermocouple type, converts it in a reference voltage Vc1 ; 2. it measures the hot junction voltage VHJ of the thermocouple; 3. it computes the voltage V = Vc1 + VHJ and converts it to the corresponding temperature value T. Unfortunately, the global metrological performances of the system are documented by the manufacturer only

20 September /October 1994

for operating temperatures that lie in the range of 18 to 28°C. Furthermore, the available data for the global system accuracy (:5 ±0.8°C) are only relative to an integration time of the integrating voltmeter equa) to 0.1 and 1.0 number of power line cycles, that means multipies of base cycle time (i.e. 0.002 s and 0.02 s, respectiveiy, with the European frequency of 50Hz). The previous observations are the reasons for performing a calibration of the system with an accuracy of :::; ±0.02°C in an ordinary laboratory where, during the tests, the ambient temperature changes in an uncontrollable manner from about 22 to 26°C, and with different integration times for the integrating voltmeter, which affect speed, accuracy and resolution of the measurement and the noise rejection capabilities. The adopted calibration procedure is simply based on a programmable universal source with the following performances when operating in high resolution at l V range: resolution of l J.LV and accuracy of ±(0.0007 percent of reading + 15 J.LV). The universal source is controlled by means of a digitai voitmeter with the following characteristics when operating at a range of 100 mV: maximum resolution of 10 nV and accuracy of ±(0.0003 percent of reading + 0.05 J.LV). To monitor the environmental temperature, with a giobal accuracy of ±0.1 °C, a platinum resistance thermometer (PRT) is placed near the system. The devices are interfaced by means of the IEEE-488 bus to a computer that controls the measurement instruments, stores and analyzes the data. Figure l shows a block diagram of the utilized experimental setup. After having been started, the calibration program asks for some experimental inputs referred to: l) the calibration procedure, as thermocouple type, temperature calibration range, incrementai temperature value, required maximum inaccuracy value associated to the simulated temperature value; 2) the parameter related to the integrating voitmeter, the integration time. Afterwards, the calibration program provides for an autocalibration of the digitai voltmeter. Then, for each selected temperature calibration value TcAL• the experimental procedure can be schematized as it follows: l. the controller measures TcJ of the thermistor mounted on the isothermal connector block, converts this value in VcJ by means of the appropriate polynomials; 9 2. the controller evaluates the calibration voltage VcAL and sets the universal source to force VcAL; 3. the universal source applies a VcALapp; 4. the controller compares the actual voltage VcALm... • measured by the digitai voltmeter, and its set value VcAL; using the answer, depending on the .lV= VcAL - VcALme.,• it regulates a new value for the voltage applied by the universal source, until VCALme,. is such that .lV is Iess or equal to the desired precision;

Digitai Multimeter Signal Conditioner

Universal Source Digitai VoltMeter

IEEE-488

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DACS

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Fig. 1-Schematic of the experimental setup

5. the integrating digitai voltmeter measures V1v. TcJ and the data acquisition and contro! system automatically determines the temperature TMEAs actually measured; 6. finally the controller compares TcAL and TMEAs· The tests were performed so that the calibration procedure: l) simulates a cromel-alumel thermocouple in a range of O to 200°C; 2) accepts a maximum difference