| Summary: | Starting from the end of the past century, the importance has been recognised of the effect of isotopic composition on some of the temperature fixed points for the most accurate realisations of the ITS-90. In the original definition of the latter, dating back to 1990, only a generic reference was made to “natural” composition of the substances used for the realisation of the fixed points, except for helium. The definition of a reference isotopic composition for three fixed points, e-H2, Ne and H2O, while eliminating the non-uniqueness of the Scale in this respect, induced detectable differences in the present and future realisations of the Scale, at the highest accuracy level, with respect to the previous realisations, when they affected the results of past MRA key comparisons, namely the CCT K1 (and K1.1) and CCT K2 (and K2.1–K2.5) and the related regional and supplementary ones. The paper provides evidence of the extent of this effect by using the results of the relevant key comparisons for neon archived in the BIPM KCDB, and of other comparisons existing in the literature (1979–1984, 2007–2012 and 2009–2010 sealed cell comparisons), and discusses the meaning and the outcomes of this evaluation.
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