Behaviour of Au/Pd thermocouple in various gases

Thermoelectric properties of Au/Pd thermocouple in various atmospheres are investigated at the Ag, Al and Zn fixed points in order to find the optimum conditions of use. Gas compositions are selected as pure argon, argon +5% oxygen, pure nitrogen and nitrogen +5% oxygen. In pure argon and nitrogen atmospheres, Au/Pd thermocouple shows smaller fixed point EMFs and poorer reproducibilities and immersion characteristics than in oxygen mixture. When thermocouples in pure argon and nitrogen are exposed to air, their properties become the same as those of oxygen-containing thermocouples. It is confirmed that any oxygen existing in oxide form on the palladium wire surface is sufficient to stabilize the Au/Pd thermocouple. It is believed that oxidized palladium has a more negative Seebeck coefficient than oxide-free palladium. Based on the experimental results, it is recommended to oxidize the palladium wire during the preparatory heat treatment stage before use in order to obtain a stable and reproducible thermal EMF. The use of the Au/Pd thermocouple in air can also be recommended.