Full-range interpolations for long-stem standard platinum resistance thermometers down to the triple point of argon.

This paper offers a possible solution to the present and potential future difficulties with the International Temperature Scale of 1990, ITS-90, regarding the triple point of mercury, TP Hg. This is discrepant within the ITS-90 and also may become unavailable for regulatory reasons on account of its toxicity. The suggestion is that for long-stem Standard Platinum Resistance Thermometers (L-SPRTs) requiring calibration below the triple point of water, TPW, the ITS-90 subranges above the TPW (ITS-90 Section 3.3.2) could simply be extended down to the triple point of argon, TP Ar, with no replacement for the TP Hg, but with an extra term in the interpolation equations. Such interpolations based on the TP Ar, the TPW, and the freezing points of Sn, Zn and Al, as far as is required, are found to be well-behaved and consistent with each other and with the ITS-90 above TPW, within about 0.5 mK. Below TPW the agreement with the ITS-90 is less good (∼1 mK at TP Hg), mainly due to the problem with the mercury point. These new interpolations have been tested for internal consistency and for consistency with calibration data for L-SPRTs, and we suggest that they should be considered for inclusion in any amendment or revision of the ITS-90. The paper discusses some complications arising from this suggestion. [ABSTRACT FROM AUTHOR]