In this paper, we specify the purpose of an international temperature scale and present some definitions that are basic to the International Temperature Scale of 1990. These definitions include those for non-uniqueness and for the temperature fixed points underlying the scale. Three types of non-uniqueness are identified.

The triple point of water serves to define the kelvin, the unit of thermodynamic temperature, in the International System of Units (SI). Furthermore, it is the most important fixed point of the International Temperature Scale of 1990 (ITS-90). Any uncertainty in the realization of the triple point of water contributes directly to the measurement uncertainty over the wide temperature range from 13.8033 K to 1234.93 K.The Consultative Committee for Thermometry (CCT) decided at its 21st meeting in 2001 to carry out a comparison of water triple point cells and charged the BIPM with its organization.Water triple point cells from 20 national metrology institutes were carried to the BIPM and were compared with highest accuracy with two reference cells. The small day-to-day changes of the reference cells were determined by a least-squares technique. Prior to the measurements at the BIPM, the transfer cells were compared with the corresponding national references and therefore also allow comparison of the national references of the water triple point.This report presents the results of this comparison and gives detailed information about the measurements made at the BIPM and in the participating laboratories. It was found that the transfer cells show a standard deviation of 50 µK; the difference between the extremes is 160 µK. The same spread is observed between the national references.The most important result of this work is that a correlation between the isotopic composition of the cell water and the triple point temperature was observed. To reduce the spread between different realizations, it is therefore proposed that the definition of the kelvin should refer to water of a specified isotopic composition.The CCT recommended to the International Committee of Weights and Measures (CIPM) to clarify the definition of the kelvin in the SI brochure by explicitly referring to water with the isotopic composition of Vienna Standard Mean Ocean Water (VSMOW). The CIPM accepted this recommendation and the next edition of the SI brochure will include this specification.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

This is a summary of the Consultative Committee for Thermometry (CCT) Key Comparison CCT-K3, i.e. the comparison of realizations of the fixed points of the International Temperature Scale of 1990 (ITS-90) over the range 83.8058 K to 933.473 K. The differences in the realizations of the various fixed points in this range of the ITS-90 and the uncertainties of those differences are given for the fifteen standards laboratories participating in the comparison.

Techniques are recommended for comparisons, at the highest levels of accuracy, of fixed-point cells of the defining fixed points, excluding the vapour-pressure points, of the ITS-90, which are used for contact thermometry. The authors are the members of Working Group 1 of the Consultative Committee for Thermometry (CCT) of the Comité International des Poids et Mesures, dealing with Defining Fixed Points and Interpolating Instruments underlying the realization of the ITS-90.

The bilateral comparison of the ITS-90 realizations at the National Metrological Institutes of Germany (PTB) and the Russian Federation (VNIIM) in the temperature range from 273,16 K to 692,677 K was initiated by the Technical Committee on "Thermometry and thermal physics" TC1.10 of COOMET. The purpose of the comparison was to evaluate the differences between the realizations of the national standards in the most essential temperature range with the highest requirements to the measurement uncertainty. This temperature range includes five ITS-90 fixed points: the triple point of water, the melting point of gallium, the freezing points of indium, tin and zinc. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCT, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).