Progress of Cryogenics and Isotopes Separation , ISSN: 1582-2575
2001, Volume 4, Issue 7+8

Isotopic Problems in the ITS-90

Abstract

The International Temperature Scale (ITS-90) uses 15 different substances for its definition fixed points (phase transitions: vapour pressure points, triple points, melting points and freezing points). For some important substances, isotopic problems are becoming, the single major limitation to further improvement of thermometry, especially below 273.16 K. The paper makes a summary of the current knowledge about natural isotopic mixtures for water, helium, neon, oxygen, hydrogen and deuterium, their variability in different inventories and the uncertainties that arise in thermometry due to this variability. In particular, the problems concern water (the triple point of water is the definition of the kelvin), neon, hydrogen and deuterium.

Help is needed from specialists for producing isotopic mixtures, for certifying them in an accurate and traceable way and for limiting the amount of some isotopic impurities or their increase with time in the storage cylinders or measuring equipment.