metrologia Key Comparison CCT-K2: key comparison of capsule-type standard platinum resistance thermometers from 13.8 K to 273.16 K A. G. Steele, B. Fellmuth, D. I. Head, Y. Hermier, K. H. Kang, P. P. M. Steur and W. L. Tew Abstract. Calibrated capsule-type standard platinum resistance thermometers were used to compare national realizations of the International Temperature Scale of 1990 (ITS-90) from 13.8033 K, the triple point of equilibrium hydrogen, to 273.16 K, the triple point of water, for seven countries in CIPM Key Comparison CCT-K2. Measurements were made at temperatures close to the eight low-temperature defining fixed points of the ITS-90, using a copper comparison block capable of simultaneously holding nine thermometers. Two separate measurement runs were performed, allowing two different groups of capsules from each laboratory to be examined. The results are used to determine the degree of equivalence of the independent national realizations of the scale for use in the Mutual Recognition Arrangement Appendix B database. In addition, measurements were made with the first group of thermometers at approximately eighty temperatures throughout the cryogenic range, which provide information to evaluate some of the so-called scale non-uniqueness issues inherent in the ITS-90 interpolation scheme. 1. Introduction In September 1996, the Consultative Committee for Thermometry (CCT) agreed to initiate a series of five international comparisons designed to investigate the degree of equivalence among nations of their independent realizations of the International Temper- ature Scale of 1990 (ITS-90) [1]. These comparisons, covering temperatures from 0.65 K, the lowest point on the ITS-90, to 1700 C, were selected to probe the highest quality measurement capabilities in use A. G. Steele: National Research Council of Canada (NRC), Institute for National Measurement Standards, Ottawa, K1A 0R6 Ontario, Canada. B. Fellmuth: Physikalisch-Technische Bundesanstalt (PTB), Abbestrasse 2-12, 10587 Berlin, Germany. D. I. Head: National Physical Laboratory (NPL), Teddington, Middlesex TW11 0LW, UK. Y. Hermier: Bureau National de M´ etrologie-Institut National de M´ etrologie (BNM-INM), 292 rue Saint Martin, 75003 Paris, France. K. H. Kang: Korea Research Institute of Standards and Science (KRISS), Yuseong, Daejeon 305-600, Republic of Korea. P. P. M. Steur: Istituto di Metrologia “G. Colonnetti” (IMGC), Strada delle Cacce 73, 10135 Turin, Italy. W. L. Tew: National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899, USA. around the world, and to test the quality of the scale itself. Subsequent to the CCT meeting, in October 1999, many of the member countries of the Metre Convention signed a Mutual Recognition Arrangement (MRA) [2], which put in place a formal scheme for the signatories to recognize their national standards. The technical basis for this mutual recognition was deemed to be just such international comparisons as were already taking place under the auspices of the CCT. The term “CIPM Key Comparison” was coined to identify these studies, and the CCT comparisons were given designations CCT-K1 to CCT-K5 for use in the Appendices to the MRA (www.bipm.org/kcdb). This report covers the work done in CCT-K2: comparison of capsule-type standard platinum resistance thermometers from 13.803 K to 273.16 K. A total of nine national metrology institutes (NMIs) expressed interest in participating in this study: BNM-INM (France), IMGC (Italy), KRISS (Republic of Korea), NIST (United States), NMi- VSL (Netherlands), NPL (United Kingdom), NRC (Canada), PTB (Germany) and VNIIFTRI (Russian Federation). Each laboratory was asked to supply two calibrated capsule-type standard platinum resistance thermometers (CSPRTs) to the pilot laboratory (NRC), Metrologia, 2002, 39, 551-571 551