NIST Assessment of ITS-90 Non-Uniqueness for 25.5. ohm SPRTs at Gallium Indium, and Cadmium Fixed Points

摘要

The use of temperature subranges in the definition of the international Temperature Scale of 1990 (ITS-90) permits standard platinum resistance thermometers (SPRTs) to be measured at defining fixed points of a particular subrange as well as at fixed points not utilized in that subrange. These redundant fixed points allow analysis of the non-uniqueness of the scale. Specifically, with 25.5. ohm SPRTs at gallium, indium, and cadmium fixed points have been made and examined to determine the non-uniqueness of the ITS-90 at the three redundant fixed points does not contribute significantly to the total error associated with the calibration of an SPRT.