Evaluation and Selection of High-Temperature Fixed-Point Cells for Thermodynamic Temperature Assignment

Abstract

A multi-partner project to determine the thermodynamic temperatures of a selected set of high-temperature fixed points based on metal-carbon eutectics is underway as a working group activity within the Comite International des Poids et Mesures. The investigation focuses on four fixed-point types, namely, the three metal-carbon eutectic points of Re-C \((2474\,^{\circ }\hbox {C})\), Pt-C \((1738\,^{\circ }\hbox {C})\), and Co-C \((1324\,^{\circ }\hbox {C})\), and the Cu point \((1084.62\,^{\circ }\hbox {C})\). This paper describes the construction, pre-evaluation, and screening stage of the cells prior to their thermodynamic temperature determinations. The construction of the HTFP cells was undertaken by nine national metrology institutes (NMIs) according to instructions laid out in a pre-agreed protocol that ensures production of best quality cells. Four NMIs conducted the evaluation, each for a certain fixed-point type, and screened out cells that did not meet pre-determined selection criteria while assuring sufficient variety in the sources of the cells in the final selected sets. In autumn 2012, the selected cells were successfully passed on to the final stage of the project, the thermodynamic temperature measurement, and assignment.