From ITS-90 to thermodynamic temperature: Hybrid CSPRT calibrations with LNE-Cnam acoustic gas thermometry.

In the cryogenic temperature range between 13.8033 K and 273.16 K, eight fixed points are required by the International Temperature Scale of 1990 (ITS-90 or T₉₀) to fully calibrate a standard platinum resistance thermometer (SPRT). Two of them are often no longer realized according to the prescriptions defined in the scale but by comparison with a "wire scale": they are the two temperatures close to 17 K and 20.3 K. On the other hand, some primary methods, such as acoustic gas thermometry (AGT), are now able to measure any thermodynamic temperature T in this range with user-adjustable spacings. We investigate here the suitability of "hybrid" calculations for the temperature with an SPRT carrying both ITS-90 and AGT calibrations. The ITS-90 equations are applied to a modified fixed-point set, where the two mentioned ITS-90 points are replaced by (alternative) thermodynamic temperatures close to those of the original fixed-points. Several AGT points are tested as possible replacements, by substituting them in the calculation procedure of the calibration coefficients. The results consider different scenarios, where only one or both ITS-90 points are replaced, to find the optimal combination for the difference T - T₉₀ with the lowest uncertainty. [ABSTRACT FROM AUTHOR]