The gallium trip 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% were compared at NIST over the last eight years. One of these four sources was a 30 kg single log 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 effect 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+ % pure gallium shows that the triple-point temperature of the twelve cells are about 0.03 mK hotter than that of Ga-1.
International Symposium on Temperature and Thermal Measurements
NIST Realization of the Gallium Triple Point, International Symposium on Temperature and Thermal Measurements, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=830622
(Accessed May 29, 2023)