Six new Hg triple-point (TP) cells (234.3156 K) were manufactured in the NIST PRT Laboratory for use in the realization of the International Temperature Scale of 1990 (ITS-90). The fixed-point cells consisted of both glass and stainless steel designs with either an 8 mm or 12 mm inner diameter reentrant well. Triple-distilled Hg with nominal purities of 99.999 999 wt.% were used to fill the cells using a vacuum distillation filling process. The certification method of the new cells included measuring melting curve plateaus, direct comparisons with the PRT Laboratory reference Hg TP cell (Hg SS-1), and immersion profiling. The estimated amount of impurities leached out of the stainless steel crucible into the Hg sample of Hg SS-1 over 25 years was found to be 1 μg/g. This paper presents the fixed-point cell designs, fabrication method, results from the cell certification procedures, and the estimated uncertainty assigned to the Hg TP cells.
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