Pressure Dependence of the emf of Thermocouples to 1300°C and 50 kbar

The relative and absolute effects of pressure on the emf of commonly used thermocouples were measured to 1300°C and 50 kbar. The relative temperature corrections due to the effect of pressure are presented for Pt/Pt10Rh, Pt/Pt13Rh, Chromel-Alumel, and iron-constantan thermocouples as a function of temperature and pressure. The accuracy of these relative temperature corrections is approximately ±1°C and their magnitudes can exceed 40°C in some cases. The absolute corrections of these thermocouples are approximately determined by: (1) correlating experimental and theoretical results on the boundary conditions for the diamond ⇄ graphite equilibrium, (2) relating experimental and calculated results for the effects of pressure on other single-component phase transformations, and (3) evaluating theoretical and experimental results for diffusion at high pressures and high temperatures. The absolute temperature corrections for all of the thermocouples investigated are proportional to the pressure at a given temperature. The magnitude of the absolute correction ΔT can exceed 50°C, as in the case of Pt/Pt10Rh at 50 kbar and T>1300°C.