Objective
To meet the requirements of several technical applications of temperature measurements it is necessary to calibrate industrial type platinum resistance thermometers (IPRT) at least to within 0.1 C in the range 0 C to 630 C and to within 0.5 C in the range 0 C to 850 C.
2 groups, each of 6 IPRT were selected to cover the ranges 0 C to 630 C and 0 C to 850 C. Selection was made on grounds of stability. These groups were now calibrated and comparisons made with calibrations made at the National Physical Laboratory. The calibration data was interpolated with different equations and a final selection was made of the best calibration and interpolation procedures for each temperature range.
With the most stable IPRT and using a fourth order fitting it is possible to measure the temperature to within 0.05 C in the range 0 C to 630 C.
PROJECT MTR00116 PROJECT DEFINITION The use of platinum resistance thermometers of various designs has considerably increased in industry. However there is no established practice to calibrate them. This would require the selection of appropriate calibration points and the establishment of an extrapolation curve. The project will consider the main types of industrial resistance thermometers and after selecting only those which are stable, it will determine the empirical curve describing the variations of the resistance of the thermometer with temperature. It is expected that there could be one single curve for thermometers of different designs. The temperature ranges 0-631 C and 631 to 850 C will be considered separately. Another consequence of this investigation is expected to make it possible to select only a limited number of points for the calibration. The uncertainty contemplated is of the order of 50 mK. The work is based on previous experience of IMGC in this field. The confirmation of the uncertainty will require contribution of NPL and INM to the final calibration. RESULTS A number of platinum resistance thermometers were obtained from seven European suppliers. They were first tested for stability and several of them failed. For those thermocouples which were sufficiently stable, six for the range 0-630 C and six for the range 0-850 C calibration were done at IMGC showed that the calibration curve can be described adequately by a quadratic curve. Additional metrology laboratories will participate to substantiate the statement of uncertainties. The work should be terminated before the end of 1990.