Phase transition behavior of pure Fe cells for thermocouple calibration

Abstract

The fixed points adopted in the international temperature scale of 1990 have generally been used to calibrate contact thermometers, such as thermocouples, to get the highest accuracy. The fixed point for Pd is located at the upper limit range of the Pt/Rh noble metal thermocouple calibration. However, the high price of Pd itself restricts its wide use in calibration laboratories. To overcome this difficulty, this study develops an alternative fixed point having a close transition temperature to the Pd freezing temperature of 1554.8 °C. The alternative is pure Fe with a quoted melting temperature of 1538 °C. To make a thermometric cell, an alumina crucible was used to contain the molten liquid and it was found to be robust enough to last for more than 60 cycles of melting and freezing tests without any chemical and mechanical damage. Two cells were fabricated, and their melting and freezing behaviors were studied using type B thermocouples. By considering the advantage and disadvantages of melting and freezing, the melting temperature was selected as a thermometric transition point for the calibration of thermocouples because of its easy operational processes and acceptable uncertainty level. Using a reference thermocouple that had been calibrated versus a standard radiation thermometer, the average melting temperature of the two Fe cells was 1534.0 °C with an uncertainty of 1.2 °C (k = 2). From the studies, an Fe cell made in an alumina crucible can successfully replace the Pd fixed point.