The non-uniqueness of ITS-90 above the silver point and its impact on values ofT-T-90

摘要

Non-uniqueness is a fundamental limit to the reproducibility of a defined temperature scale. In this paper, the non-uniqueness of temperatures,T-90, on the International Temperature Scale of 1990 (ITS-90) above the silver point is fully investigated for the first time. The non-uniqueness arises from three causes: inconsistency in extrapolation with the choice of the three allowable ITS-90 reference fixed points; a wavelength dependence ofT(90)due to the difference between thermodynamic temperature,T, andT(90)at these reference fixed points; and a further wavelength dependence arising from the discrepancy between the value of the second radiation constant defined on ITS-90 and its value derived from fundamental constants. The non-uniqueness is quantified by deriving an exact expression forT - T(90)above the silver point and calculating the differences in this expression as a function of reference fixed point and wavelength. In the worst case, the non-uniqueness is about 100 mK, andT - T(90)is as much as 400 mK, at 3000 degrees C.