Research Article

A new approach to the analysis of Type 1 non-uniqueness of the ITS-90 above 0 °C

¹ Formerly: National Institute of Metrology (INM), Bucharest, Romania
² Formerly: Institut National de Métrologie – Laboratoire National de métrologie et d'essais/ Conservatoire National des Arts et Métiers, La Plaine Saint-Denis, France

^* Corresponding author. sonia.gaita@temperature.ro

Received: 15 March 2017
Accepted: 15 November 2017

Abstract

The Type 1 non-uniqueness (NU-1) is the difference between interpolated values at the same temperature in the resistance thermometer subranges of the International Temperature Scale of 1990 (ITS-90) that overlap. The paper argues for a method of evaluating the NU-1 at a given temperature which considers all subranges of the Scale that contain the respective temperature, not only combinations of two, and it proposes mathematical models to determine the values of NU-1 for temperatures above 0 °C. The paper demonstrates that NU-1 is not the right contributor to the uncertainty associated with the realisation of the ITS-90. Therefore, a new concept of Correction for the Type 1 non-uniqueness of the Scale, C_NU-1, is introduced and its mathematical model is established. Also, the estimate of C_NU-1 and its standard uncertainty are defined and they are assessed through statistical analysis. The values of standard uncertainty determined by the novel methodology do not exceed 0.26 mK and they are smaller than the values given in the specific Guides developed by the Consultative Committee for Thermometry. The proposed models allow authors to single out and analyse the factors that generate Type 1 non-uniqueness of the Scale and influence its value.