Thermal EMF hysteresis depending on the heating and cooling conditions of thermocouples from different manufacturers of platinum-rhodium wire

Studies have been conducted to identify patterns of changes in the thermal EMF values of platinum-rhodium type B thermocouples made of PR30 (30% Rh) and PR6 (6% Rh) alloys from different manufacturers. They were studied at the stages of heating to a temperature of 1300 oC and cooling to 300 оC, as well as during repeated cycles of thermal exposure at high heating and cooling rates of 10 оC/s, in order to determine the possible causes of the discrepancy between the thermocouple parameters and the required measurement accuracy. The effects of thermal EMF hysteresis are revealed, consisting in the discrepancy of thermocouple parameters during heating and cooling, as well as a narrowing of the thermal EMF hysteresis loop during repeated heating and cooling cycles. No impurity elements were detected in the wires of PR30 (30% Rh) and PR6 (6% Rh) compositions from different manufacturers, and there are no significant changes in the chemical composition after thermal exposure when heated to a temperature of 1300 оC. It has been established that there is no discrepancy between the thermocouple parameters and the actual temperatures of the technological conditions due to deviations in the regulated requirements for the chemical composition of the wire making up the thermocouple or incorrectly selected welding modes. It is shown that the degree of penetration of the welded joint of the elements of the studied products does not affect the value of thermal EMF. It was found that during the assembly of thermocouples, there are traces of deformation effects from the rollers feeding the wire into the welding unit. It is determined that this may be the reason for the incomplete course of the recrystallization process and the occurrence of temperature discrepancies in thermocouples.