| Résumé | Phase transitions of pure substances provide the key reference temperatures, otherwise known as “defining fixed points”, of the International Temperature Scale. At temperatures below the triple point of mercury (234.3156 K), the substances involved are gases at room temperature and include the triple points of hydrogen (13.8033 K), neon (24.5561 K), oxygen (54.3584 K) and argon (83.8058 K). The Consultative Committee for Thermometry also provides a list of “secondary reference points” whose quality approaches (or is equivalent to) that of the defining fixed points of the ITS. Although the list of secondary reference points includes vapor-pressure/temperature relations for selected gases, triple points can be realized more precisely than boiling points and other vapor-pressure-dependent temperatures. The 1996 CCT Working Group 2 list of secondary reference points describes the triple points of deuterium (18.724 K), neon-20 (24.541 K), nitrogen (63.151 K), methane (90.694 K), xenon (160.405 K) and carbon dioxide (216.592 K) as “first-quality points” while the triple point of krypton (115.775 K) is considered a “second-quality” point. The recommended temperatures in the CCT WG2 publication were obtained by taking original measurements reported as IPTS-68 temperatures and converting them to the ITS-90. This process introduced additional uncertainty due to the non-uniqueness of IPTS-68. Here, we report direct measurements of the triple point of krypton on the ITS-90, review the historical measurements that the CCT relied upon for its 1996 assessment, and demonstrate that the triple point of krypton is a “first quality” secondary reference point. Based on our measurements, the temperature of the triple point of krypton is 115.7755 K. |
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