Long-Term Monitoring of Thermocouple Stability with Miniature Fixed-Point Cells

Abstract

In the framework of the European Metrology Research Programme ENG08 “MetroFission” project, two National Measurement Institutes, LNE-Cnam (France) and NPL (UK), have cooperatively developed methods of in situ validation of thermocouple output for application in next-generation nuclear fission power plants. Miniature fixed-point cells for use at three temperatures were constructed in the first step of this project: at the freezing point of silver ($961.78\,^{\circ }\mathrm{C}$), the freezing point of copper ($1084.62\,^{\circ }\mathrm{C}$), and the melting point of the iron–carbon eutectic ($1154\,^{\circ }\mathrm{C}$). This paper reports the results of a second step in the study, where the robustness of the self-validation method has been investigated. Typical industrial Type N thermocouples have been employed with each of the miniature fixed-point devices installed, and repeatedly thermally cycled through the melting and freezing transitions of the fixed-point ingots. The devices have been exposed to a total of up to 90 h in the molten state. Furthermore, the LNE-Cnam devices were also subjected to fast cool-down rates, on five occasions, where the rate is estimated to have been between $150\,^{\circ }\mathrm{C}\,{\cdot }\min ^{-1}$ and $200\,^{\circ }\mathrm{C}\,{\cdot } \min ^{-1}$. The devices are shown to be repeatable, reliable, and robust over the course of these tests. The drift of the Type N thermocouple has been identified separately to the behavior of the device. A reliable method for improving thermocouple performance and process control is therefore demonstrated. Requirements for implementation and the advantages of each approach for monitoring and correcting thermocouple drift are discussed, and an uncertainty budget for self-validation is presented.