Multi-Mini-Eutectic Fixed-Point Cell for Type C Thermocouple Self-Calibration

Abstract

Thermocouples are generally calibrated using a series of standard fixed-point cells. However, thermocouples in use, particularly base metal and refractory thermocouples, exhibit significant calibration drift due to factors such as inhomogeneity growth. It is not possible to reliably determine the magnitude of this drift by removing the thermocouple for recalibration and instead, must be quantified by some form of in situ calibration. Here, a multi-mini-cell for the use with a metal sheathed W–Re thermocouple (Type C) was developed. The cell contains two layers of different eutectic materials in the same crucible, one in each compartment, separated by a thin graphite disk. The cobalt–carbon and iron–carbon eutectic materials were selected to prove the concept. In addition, thermal modeling was performed to predict the performance of this design of this multi-fixed point. A prototype multi-mini-fixed-point cell was constructed and tested, and results are reported. The overall performance, despite only very small amounts of each fixed-point material being used, is that the repeatability of the W–Re thermocouple (melt-to-melt) was found to be 1.8 $$\upmu \hbox {V}$$ or $$0.11\,^{\circ }\hbox {C}$$ for the Fe–C $$(1153\,^{\circ }\hbox {C})$$ and 1.5 $$\upmu \hbox {V}$$ or $$0.10\,^{\circ }\hbox {C}$$ for the Co–C $$(1324\,^{\circ }\hbox {C})$$ . The lack of drift in the thermocouple enabled the repeatability and stability of the principle of multi-mini-fixed points for self-calibration to be demonstrated.