New interpolating equations of ITS-90 for SPRTs and HTSPRTs for the temperature range 419.527°C-961.78°C

The subrange inconsistency (SRI) of the International Temperature Scale of 1990 (ITS-90) arises from the use of different interpolating equations for the same standard platinum resistance thermometers (SPRTs), called Type 1 non-uniqueness, and is a significant component of the uncertainty in the realization of the scale. In order to eliminate the SRI, new interpolating equations were proposed for the subranges 0.01 degrees C to 660.323 degrees C and 0.01 degrees C to 961.78 degrees C, in which the subrange to the zinc point (419.527 degrees C) was extended using two additional terms without any overlap. This study investigated 30 SPRTs calibrated over the temperature range of 0.01 degrees C-660.323 degrees C and 19 high-temperature SPRTs (HTSPRTs) calibrated over 0.01 degrees C-961.78 degrees C at the National Institute of Metrology (NIM). The feasibility of the proposed equations were evaluated by comparing them with the equations defined in ITS-90 and calculating the propagation uncertainties. The results show that the differences between the defined interpolating equations in ITS-90 and the proposed interpolating equations are generally within +/- 0.4 mK for temperature range of 419.527 degrees C-961.78 degrees C, which is close to or less than the SRIs between the water-zinc and water-aluminium subranges. The propagated uncertainties of the proposed equations were within reasonable limits. It is concluded that these new interpolating equations can be taken as equivalent to the ITS-90 interpolating equations with the SRI eliminated.