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Adjustments to the NIST Realization of the ITS-90 from 5 K to 24.5561 K



Weston L. Tew, Christopher W. Meyer


Recent clarifications issued by the Consultative Committee on Thermometry (CCT) for the definitions of the equilibrium hydrogen (e-H2) triple points (TPs) and vapor-pressure points (VPs) have resulted in adjustments to the NIST-disseminated ITS-90 (T90) in the range of the interpolating constant volume gas thermometer (ICVGT) from 5 K to 24.56 K. The NIST-disseminated ITS-90 is derived from capsule standard platinum resistance thermometers (SPRTs) calibrated over their lowest-defined sub-range of 13.8 K to 273.16 K and from realization of the ICVGT from 5 K to 24.556 K. The SPRT subrange-1 uses e-H2 VPs and both definitions use the e-H2 TP as a calibration points. These calibrations are traceable to NIST realizations of the e-H2 TP and VPs which were performed using highly-depleted hydrogen gas. The revised CCT definitions for all e-H2 fixed points now call for a less-depleted composition equivalent to that of Standard Light Antarctic Precipitation (SLAP). This has necessitated adjustments in both the ICVGT range and the SPRT subrange-1. One effect of this adjustment is a larger difference between the ICVGT and NPL-75 in the region from ~ 10 K to 20 K. The differences T-T90 reported by Pitre et. al.  in the range of the ICVGT have likewise been adjusted.
Proceedings Title
Report of the 24th Meeting to the International Committee for Weights and Measures | BIPM
Conference Dates
May 18-23, 2008
Conference Location
Sèvres Cedex, FR
Conference Title
24th Meeting of the Consultative Committee for Thermometry for the International Committee for Weights and Measures


Temperature, ITS-90, hydrogen fixed points, gas thermometry, isotopic composition, triple point, SPRT
Created May 1, 2008, Updated February 19, 2017