The official publication of the International Temperature Scale of 1990 (ITS-90) gives a concise definition of temperatures T₉₀ and is highly prescriptive. To accompany the text, supplementary information concerning the practical realization of the scale was published in 1990 by the BIPM in a monograph entitled Supplementary Information for the ITS-90; this monograph provided the non-prescriptive details in a comprehensive form.

A first revision of the initial edition was completed in 2018 and takes into account the progress in thermometry that have been achieved since 1990 concerning both temperature fixed points and interpolation instruments. The new edition has been drafted by the CCT Task Group for the Realization of the Kelvin (CCT-TG-K) and each chapter has been approved by the Consultative Committee for Thermometry.

This new edition, Guide to the Realization of the ITS-90, is published uniquely in electronic form on the BIPM website. The chapters are self-contained but harmonized allowing a higher flexibility for future revisions to follow the pace of new developments. The presented material could be assembled only thanks to the continual assistance of many colleagues in the temperature-measurement community.

There are many sources of information which requires a subjective selection of the references and unintentional errors of omission are certainly inevitable; comments and proposals for future revisions are most welcome.

This document gives a general guidance for realizing the Provisional Low Temperature Scale of 2000 (PLTS-2000) by establishing and measuring melting pressures of ³He in the range from 0.902 mK to 1 K. It draws on published techniques and designs, and is intended to be suitable for application in national measurement institutes, as well as more generally in ultra-low temperature laboratories where absolute pressure standards are not normally available.

The document describes methods by which the PLTS-2000 can be realized successfully. However, it should not be taken as laying down how it must be done. The description of any particular apparatus is more for illustration than prescription, and considerable variation is often seen. Likewise, quoted numerical data and dimensions are mostly for guidance only.

The Mise en Pratique for the definition of the kelvin (MeP) the International Temperature Scale of 1990 (ITS-90), and the Provisional Low Temperature Scale of 2000 (PLTS-2000), provide the means for measurement and dissemination of temperature at the highest levels of accuracy.

The Guide on Secondary Thermometry, accessible through the links below, and the booklet Techniques for Approximating the International Temperature Scale of 1990, provide guidance for thermometry at lower levels of accuracy. These guides should enable temperature measurement with greater convenience and lower cost than the more formal and primary realizations described in the MeP.

The Guide on Secondary Thermometry is available only in electronic format, in standalone documents that will gradually replace the corresponding sections of the booklet. This will enable us to keep the guides as up-to-date as is practical.

The material contained in the documents has been collated from information provided by the CCT and experts in the temperature-measurement community world-wide, but is not exhaustive. Suggestions for improvements and proposals for future revisions are welcomed, and can be submitted directly to the CCT or to the CCT Task Group for Guides on Thermometry (CCT-TG-GoTh).