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We present accurate measurements of the triple point of nitrogen. The quality of this triple point is once more confirmed, and its temperature value is found to be T 90 = 63.152 6 K (expanded uncertainty U = 0.30 mK, reproducibility better than 0.1 mK). This is the first direct determination of this point on ITS-90 since the introduction of the scale. This point is useful as a non-uniqueness checkpoint.

Miniature glass and stainless-steel cells have been made at the Institute of Low Temperature and Structure Research for the realization of the triple point of mercury (T90 = 234.3156 K) to be used for the calibration of capsule-type standard platinum resistance thermometers. This paper gives details on the construction of these cells and some measurement results obtained at the triple point of mercury.

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In the temperature range from 0.65 K to 24.5561 K, the ITS-90 is defined by specified vapour-pressure equations for 3He and 4He and interpolation equations for a constant-volume gas thermometer using 3He or 4He. The definitions are given in sections 3.1 and 3.2 of the ITS-90 text, and methods for realising the scale in this range are outlined in chapters 4 and 5 of the Guide to the Realization of the ITS-90 (formerly Supplementary Information for the ITS-90). Direct realisations of the ITS-90 by these methods require relatively sophisticated apparatus and time-consuming experiments, and consequently they are rarely carried out. The situation is acceptable in practice only because rhodium-iron resistance thermometers (RIRTs) are available that, once calibrated, are able to maintain their calibrations reliably for long periods of time. They are the practical thermometers on which realisations of the ITS-90 are most accurately maintained, disseminated and compared. The key comparison EURAMET.T-K1 is therefore a comparison of calibrated RIRTs. 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).