- Conference date: 21-24 October 2002
- Location: Chicago, Illinois (USA)
Part of the metrological basis for thermometry at the Swiss Federal Office of Metrology and Accreditation (METAS) is formed by eight sets of three sealed fixed‐point cells covering the range from −38 °C to 961 °C according to ITS‐90. This basis is metrologically supervised by means of international comparisons with other National Metrology Institutes (NMIs). Between such comparisons, the stability of a given fixed point is verified by comparing the corresponding METAS cells two at a time in three measurement series of paired, simultaneous fixed‐point realizations. Our measurements show that the differences between the cells are, although not zero, quite small. Repeating these internal comparisons for the three indium cells after about two and then four years, the observed variations in the mutual temperature differences between these cells are small and remain far within the estimated overall uncertainty of that fixed point. The uncertainty values considered are mainly determined by the contributions attributed to a certain shortcoming of information on chemical impurities and on the cell gas pressure when using commercially available sealed cells. It is demonstrated for our indium fixed point that based on our results the overall estimated uncertainty can be lowered from 2.04 mK to 1.04 mK (k=2) due to reductions of the chemical impurity and the cell gas pressure contributions to the uncertainty. In the whole, the described concept to check the sealed fixed‐point cells by comparisons turned out to provide a high level of confidence in the thus monitored portion of the temperature scale basis.
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