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New standard reference tables for thermocouples are necessary as a result of the introduction of the new international temperature scale, ITS-90, which interpolates between fixed points by means of a standard platinum resistance thermometer instead of thermocouples. Unfortunately, in the case of S, R, and B thermocouples, they cannot be produced by a mathematical transformation of existing data because of anomalies which arise at discontinuities in the thermal EMF versus temperature t-90 characteristic.
New measurements were therefore undertaken in four laboratories to calibrate a number of platinum rhodium/platinum thermocouples with reference to the new international temperature scale, ITS-90. The thermocouples were calibrated at fixed points and also compared with high temperature standard platinum resistance thermometers that were calibrated in agreement with ITS-90.
RESULTS
Reference functions, describing the EMF characteristics of the different types of thermocouples investigated in the project in terms of the new international temperature scale ITS-90, were obtained on the basis of the measurements. A fourth order polynomial function was found to give a good fit in the range 630 to 1064 C for the measurements whose reproducibilities were generally +/- 1uV. The results of the project are being used to revise the international reference tables for thermocouples (IEC584-I).
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