Miniature Co–C eutectic fixed-point cells for self-validating thermocouples

Miniature fixed-point cells have been constructed using ingots of Co–C eutectic alloy, with the aim of constructing self-validating thermocouples for in situ calibration. The design consists of a cylindrical graphite crucible, diameter 6 mm, containing the ingot, integrated into the measuring junction of a type R thermocouple. Two sizes of cell were studied, with internal volumes 124 and 64 mm3 respectively. A detailed study of the characteristics of the two designs shows a correlation between the emf at the melting temperature, and the furnace temperature. This correlation is dramatically reduced for the smaller cell. The correlation is characterized extensively. The smaller cell provides the best overall performance. The repeatability of both cells is of the order of 0.1 °C.