A study on ITS-90 type 3 non-uniqueness between freezing points of Al and Ag

This work presents an investigation about the type 3 non-uniqueness of the International Temperature Scale of 1990 (ITS-90) between the aluminium (660 °C) and silver (962 °C) freezing points, carried out at the Italian National Institute of Metrology (INRiM) within the framework of the EMRP-JRP SIB10 NOTED (Novel Techniques for Traceable Temperature Dissemination). For this purpose six High Temperature Standard Platinum Resistance Thermometers (HTSPRTs) from different manufacturers were calibrated at the ITS-90 fixed points, then intercompared in a sodium Gas Controlled Heat Pipe (GCHP) at 25 K intervals between the aforementioned fixed points; a temperature uniformity and stability within ±0.5 mK was achieved in the whole range between 660 °C and 962 °C. This paper presents the results of type 3 non-uniqueness analysis of these test HTSPRTs, together with problems deriving from their stability, which is the largest source of uncertainty in this work.