Influence of the Cavity Length on the Behavior of Hybrid Fixed-Point Cells Constructed at INRIM

Abstract

Hybrid cells with double carbon/carbon sheets are used at the Istituto Nazionale di Ricerca Metrologica (INRIM) for the realization of both pure metal fixed points and high-temperature metal–carbon eutectic points. Cells for the Cu and Co–C fixed points have been prepared to be used in the high-temperature fixed-point project of the Comite Consultatif de Thermometrie. The results of the evaluation processes were not completely satisfactory for the INRIM cells because of their low transition temperatures with respect to the best cells, and of a rather large melting range for the Co–C cell. A new design of the cells was devised, and considerable improvements were achieved with respect to the transition temperature, and the plateau shape and duration. As for the Cu point, the duration of the freezing plateaux increased by more than 50 % and the freezing temperature increased by 18 mK. As for the Co–C point, the melting temperature, expressed in terms of the point of inflection of the melting curve, increased by about 70 mK. The melting range of the plateaux, expressed as a difference \((T_{\mathrm{liquidus}} - T_{\mathrm{solid}})\) was reduced from about 180 mK to about 130 mK, with melting times \((t_{\mathrm{liquidus}} - t_{\mathrm{solid}})\) increased by about 50 %, as a consequence of an improvement of flatness and run-off of the plateaux.