Abstract. The work reported explores the effect of Pd oxidation on the behaviour of Pt/Pd thermocouples within the temperature range 250 °C to 1000 °C. The results confirm that oxidation of Pd takes place primarily in the temperature range 600 °C to 800 °C. Uniform (isothermal) heat treatment in the critical temperature range can increase the thermoelectric emf by 1.5 µV (equivalent to 0.15 °C) at the zinc freezing point, with the peak change occurring for a heat-treatment temperature of 750 °C. When subjected to non-uniform heat treatment, an initially homogeneous thermocouple may develop a thermoelectric inhomogeneity of 1 µV (equivalent to 0.14 °C) at the tin freezing point when subjected to temperatures between 600 °C and 800 °C. The oxidation of palladium is more easily interpreted from our parallel investigation of a Pd resistor fashioned as a resistive temperature detector (RTD). For the Pd RTD, the peak of the palladium oxide formation occurs at 725 °C and oxide does not form at or above 800 °C. The changes in thermoelectric emf of the Pt/Pd thermocouple are less than would be expected for a Pt-Rh/Pt thermocouple such as a type S. However, the Pt/Pd thermocouple does not exhibit the stability of the gold/platinum thermocouple due to oxidation of the Pd component and is not recommended in its stead unless the maximum temperature of use exceeds 1000 °C.

Print publication: Issue 1 (February 2002)