Behavior of thermocouples under high pressure in a multi-anvil apparatus

Abstract

We have conducted experiments to study the behavior of W5%Re–W26%Re (type C) and Pt10%Rh–Pt (type S) thermocouples under high pressure in a multi-anvil apparatus. The electromotive force (emf) between four different or three identical thermocouple wires was measured up to 15 GPa and 2100 °C. Mechanical and chemical stability of the thermocouples was examined during and after the experiments. Due to the effect of pressure on the emf/temperature relation, the temperature reading of the type C minus that of the type S thermocouple rises to +5 °C then falls to −15 °C between room temperature and 1500 °C at 5 GPa, and to +25 °C and then −35 °C between room temperature and 1800 °C at 15 GPa. In addition, we observed variations in the emf/temperature relation caused by uncertainties in the position and geometry of hot junctions in a steep temperature gradient, and by variable distribution of pressure gradient and non-hydrostatic stress on the thermocouple wires. These errors are estimated at 1.6% for the type S thermocouple up to 1700 °C, and 0.8% for the type C thermocouple up to 2100 °C. Self-diffusion and chemical contamination of the thermocouples by high-purity insulating ceramics appear negligible for the type S thermocouple at 1700 °C for one hour, and for the type C thermocouple at 2100 °C for half an hour. In contrast, large-scale displacement of the hot junction due to dislocation of the type C thermocouple wires and plastic deformation of the type S thermocouple wires may lead to large errors in temperature measurement (±200 °C).