VARIABILITY OF DEVIATION FUNCTIONS AND EASE OF INTERPOLATION FOR PT-BASED THERMOCOUPLES

The deviation functions for thermocouples, representing the difference between thermocouple emf and the corresponding reference function, ar e examined using data on 367 thermocouples of types R and S, submitted for calibration over the period 1984 to 1996. Calibrations were condu cted with the CSIRO National Measurement Laboratory (NML) calibration facility, which thermoelectrically scans the wires, automatically coll ects comparison data to 1100 degrees C and incorporates data from the melting point of palladium. It is shown that deviation functions are w ell behaved, changing gradually with temperature, and that relatively accurate calibrations may be carried out with few calibration temperat ures. For example, a calibration to similar to 1064 degrees C may rely on two axed points, those of aluminium and gold, and a calibration to 1550 degrees C on three fixed points, those of Al, Au and Pd. Such an approach is feasible for Pt-based thermocouples because linear interp olation over temperature intervals of similar to 500 degrees C contrib utes no more than about 3 mu V to the expanded uncertainty of calibrat ion.