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.
Report of the 24th Meeting to the International Committee for Weights and Measures | BIPM
May 18-23, 2008
Sèvres Cedex, FR
24th Meeting of the Consultative Committee for Thermometry for the International Committee for Weights and Measures