A validated physical model of the thermoelectric drift of Pt-Rh thermocouples above 1200 °C

Abstract

Abstract Pt and Rh oxide vapour transport is thought to dominate the thermoelectric drift of Pt-Rh thermocouples above about 1200 °C. A simple model is 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 has been tested by comparison with high precision measurements of the thermoelectric drift of a Pt-Rh thermocouple assembly consisting of five thermoelements, repeatedly exposed to the melting of a Co-C high temperature fixed point (1324 °C) for a total duration of 500 h. By considering various thermocouples of different compositions, it is demonstrated that the calculated drift rate is proportional to the measured drift rate, which represents a validation of the model.