Thermoelectric properties of the Au/Pt thermocouple

Abstract

An Au/Pt thermocouple having a gold-bridged junction instead of a Pt coil is fabricated and its thermoelectric properties are investigated at the Sn, Zn, Al, and Ag freezing points. Reproducibility is found to be excellent and standard deviation is as small as ±4 mK at the Ag fixed point. Immersion properties, which indicate homogeneity of the thermocouple wire, are also shown to be very good, with standard deviation estimated as about 10 mK. In order to investigate interchangeability, 20 sets of Au/Pt thermocouples are made and their emfs are measured at the same fixed points. Thermocouples from the same spool are reproducible within ±0.02 °C for the whole temperature range. For practical use of the Au/Pt thermocouple, an inverse function and a deviation function for calibration are proposed. The inverse function has two temperature ranges and is accurate within ±1 mK, while the deviation function is a third-order polynomial without a constant term. The Au/Pt thermocouple is stable at high temperatures for long periods. After 1500 h at 1000 °C, the thermal emf at the Ag fixed point is changed by about 0.9 μV (about 40 mK). After the other experiments, calibration uncertainties at the fixed points are evaluated. At the Ag fixed point, the maximum expanded uncertainty is calculated and is about ±30 mK at a confidence interval of 95%. It is shown that the gold-bridged Au/Pt thermocouple is reproducible, accurate, and stable. At present, the Korea Research Institute of Standards and Science (KRISS) uses Au/Pt thermocouples as secondary standard thermometers; they are supplied to industry as certified references.