NIST Realization of the Gallium Triple Point

摘要

The gallium triple point (302.9166 K) instead of the gallium melting point (302.9146 K) is used at NIST in the realization of the International Temperature Scale of 1990 (ITS-90). Four different sources of gallium with purities of at least 99.999 995 wt. % were compared at NIST over the last eight years. One of these four sources was a 30 kg lot designated for use as a Standard Reference Material. Three different fixed-point cell designs were used to fabricate twelve cells from these sources. Different methods of preparation of the gallium triple point were investigated to determine the optimum realization method and to determine if different methods affect the realization temperature of the triple point. Pressure and immersion characteristics were tested to check the agreement with the ITS-90 assigned values for the effect of pressure and hydrostatic head. Comparison of the twelve cells with the previous NIST reference gallium triple-point cell (Ga-1) containing 99.999 99+ wt. % pure gallium shows that the triple-point temperatures of the twelve cells are about 0.03 mK hotter than that of Ga-1. Additionally, repeated measurements of the twelve gallium triple-point cells give a reproducibility that is equivalent to that of a similar group of water triple-point cells. The uncertainty (k = 2) assigned to the current NIST reference gallium triple-point cells is the same as that assigned to the NIST reference water triple-point cells, i.e. 0.04 mK. This paper presents the fixed-point cells designs, fabrication method, realization technique and the uncertainty of the NIST-realized gallium triple-point temperature.