Evaluation of the Pd–C eutectic fixed point and the Pt/Pd thermocouple

Abstract

A Pd–C eutectic fixed point cell (1492 °C) was constructed to investigate its utility for thermocouple calibration. The primary aim of the study was to evaluate the long-term stability, immersion characteristics (influence of heat conduction along the thermocouple stem) and robustness of a Pd–C fixed point using a Pt/Pd thermocouple, especially constructed for this purpose. The performance of both devices at this relatively high temperature could therefore be tested. The melting and freezing plateaux at the Pd–C eutectic point were measured using the Pt/Pd thermocouple. The total exposure to the Pd–C melting temperature was about 850 h for the fixed point cell and 550 h for the thermocouple. The standard deviations of the melting and freezing points were 1.03 µV (0.041 °C) and 0.77 µV (0.031 °C) respectively. The emfs of the thermocouple at the melting point were observed to drift by about 0.1 °C. The immersion measurements show that for the current cell design, the measuring junction should be at most 30 mm from the bottom of the thermowell to be properly immersed. The long-term performance and robustness of the fixed point indicate a promising future for its use as a fixed point for calibration of noble metal thermocouples.