Development in constructing Au/Pt thermoelectric sensors

Au/Pt thermocouples are considered as an alternative to High Temperature Platinum Resistance Thermometers and are one of the prime candidates to replace them as the interpolating instrument of the International Temperature Scale of 1990 (ITS-90) in the temperature range between about 660 °C and 962 °C. This work presents the results of performance investigation of two Au/Pt thermocouples made with a new sensor construction consisting solely of quartz glass (SiO2) insulation material. The conducted experiments undertaken were made to test the performance of these Au/Pt thermocouples in different positions and elevated temperatures as previous experience has shown a non-negligible effect on the performance of sensors with this new construction. The measured electromotive forces were found to be sensitive to changes of the measuring conditions, for instance when inserting the Au/Pt thermocouples into additional metallic protection tubes. Temperature error measurements above about 650 °C have shown to be caused by using a Pt20%Rh protection tube with increasing negative deviations from the reference temperature up to about −1 °C at about 950 °C. The purpose of these protective tubes was to protect the Au/Pt thermocouples from the previously recorded effect that caused the temperature error measurements during the comparison measurements in the sodium heat pipe furnace. The final replacement of the quartz glass by Al2O3 as an insulation material of the Au/Pt thermocouples resulted in a temperature increase up to about 0.5 K measured at the freezing point of silver. The decreasing insulation resistance of quartz glass at higher temperatures is believed to be the source of thermoelectric instability. These findings have resulted in recommendations for the quartz glass construction concept of Au/Pt thermocouples.