Determination of the recrystallization onset temperatures of platinum-rhodium thermocouples using differential scanning calorimetry and thermal EMF methods

The temperatures of the recrystallization onset of platinum-rhodium thermocouple alloys of different chemical compositions PR30 (30% Rh) and PR6 (6% Rh) were determined by differential scanning calorimetry (DSC) and thermal EMF. A new technique for determining of the recrystallization onset temperature has been developed, which consists in constructing a polynomial approximation curve with high accuracy (R2> 0.99) describing the thermal EMF curve, the critical points on which were identified by changes in the curves of regular residues, i.e. by the difference between the experimental value of thermal EMF and the approximation curve of these values. The technique showed a good correlation of the results determining the recrystallization onset temperatures by the thermal EMF method with the data obtained by the DSC method (the relative difference in the recrystallization onset temperatures for these methods is less than 0.12%). The absence of a statistically significant effect of the heating rate of the tested thermocouple samples on the temperature of the recrystallization onset of platinum-rhodium thermocouple alloys was established by comparing the data obtained by the DSC method, when the heating rate is 10 °C/min, and the thermal EMF method, where the heating rate is 10 оC/s. There was no statistically significant correlation between the values of the recrystallization onset temperature for PR30 and PR6 thermocouple alloys having different chemical compositions, which is associated with the predominant effect of the degree of deformation on the recrystallization onset temperature in relation to the influence of the wires chemical composition. A comparison of the critical point values obtained by different test methods confirms the hypothesis that the temperature of the recrystallization onset for platinum-rhodium thermocouple alloys can be determined with a high degree of reliability using the thermal EMF method. Based on the results of studies conducted to determine the recrystallization temperatures, a technological mode of thermocouples annealing in an industrial enterprise is proposed.