Some predictions of a validated physical model of Pt–Rh thermocouple drift above 1200 °C

Abstract

A simple model was recently presented which relates the electromotive force (emf) drift rate of Pt–Rh thermoelements to the vapour pressure of Pt and Rh oxides. The model assumes that the evaporation of these oxides gives rise to a continuously changing concentration of Pt and Rh, at different rates along the length of the wires, which causes a change in the Seebeck coefficient. The model was tested by comparison with high precision measurements under comparable circumstances. By considering various thermocouples of different compositions, it was demonstrated that the calculated drift rate is proportional to the measured drift rate, which represented a validation of the model. In the current study, the model is used to make some predictions concerning the set of optimum ‘zero-drift’ thermocouple wire compositions above 1200 °C. It is shown that for a wire of Pt–Rh with more than a few %Rh, there is a corresponding wire to make a thermocouple which has nearly zero thermoelectric drift, and that this is almost independent of temperature. Remarkably, this optimum relation is found to agree very well with a previous optimisation that was based on an empirical technique. An intriguing finding is that when the measurement junction is at around 1285 °C, the drift rate is very low, regardless of wire composition; the reason for this is explained by the model. This has implications for thermocouple drift testing at temperatures close to 1285 °C, which may be unreliable if the drift is inherently low regardless of the composition of the two thermoelements, as suggested by the model. The melting point of Co–C, 1324 °C, commonly used for thermocouple drift assessment, is far enough away from 1285 °C for this effect not to be a problem.