Performance of Pt–C, Cr $$_7$$ 7 C $$_3$$ 3 –Cr $$_3$$ 3 C $$_2$$ 2 , Cr $$_3$$ 3 C $$_2$$ 2 –C, and Ru–C Fixed Points for Thermocouple Calibrations Above 1600 $$^{\circ }$$ ∘ C

Abstract

A series of high-temperature fixed points (HTFPs) Pt–C (1738 $$^{\circ }\mathrm {C}), \text {Cr}_{7}\text {C}_{3}{-\text {Cr}}_{3}\text {C}_{2}\,(1742\,^{\circ } \mathrm{C}), \text {Cr}_{3}\text {C}_{2}{-\text {C}}\,(1826\,^{\circ }\mathrm{C})$$ , and Ru-C (1953 $$^{\circ }\text {C}$$ ) have been constructed at the National Physical Laboratory (NPL) and the Laboratoire National de metrologie et d’Essais and Conservatoire national des arts et metiers (LNE-Cnam). These are required for the calibration of high-temperature thermocouples in the framework of work package 6 of the European Metrology Research Programme IND01 project “HiTeMS.” The goal of this work package is to establish a European capability that can determine low-uncertainty reference functions of non-standard high-temperature thermocouples. For reference functions to be widely applicable, measurements must be performed by more than one institute and preferably by more than one method. Due to the high price of the ingot materials, miniature HTFP cells are used. NPL and LNE-Cnam constructed their HTFP cells with different designs; these are described here, together with the performance of the cells using both radiation thermometry and thermocouples. The melting temperature of the Ru–C cells (for thermocouple calibrations) was determined using radiation thermometry at both NPL and LNE-Cnam, and the two results are compared. The suitability of the cells for calibration of W–Re and Rh–Ir thermocouples is evaluated, and some results are presented. Some discussion is given regarding the materials challenges when calibrating Rh–Ir thermocouples up to 2000 $$^{\circ }$$ C.