Comparison of local realizations of Silver freezing-point temperatures between NMIA and NMIJ AIST

The key comparison CCT-K4.1 was initiated on the request of the National Measurement Institute, Australia (MNIA) to re-link their standards to the Consultative Committee for Thermometry (CCT) Key Comparison (KC) and facilitate the NMIA CMC submission. National Metrology Institute of Japan, Japan (NMIJ) provided the linkage to the CCT-K4 at the temperature of the freezing point of silver. At the time of the CCT-K4 NMIA (formerly CSIRO-NML) had only one silver cell and this was found to be contaminated shortly after completion. Subsequently, more silver cells were fabricated at different times to form an ensemble of five cells before the start of CCT.K4.1. In the CCT-K4.1 bi-lateral comparison, a silver cell provided by NMIA and used as the artefact and circulated in accordance with the pilot-participant-pilot scheme. It is standard practice for NMIJ to use argon at 6N purity and NMIA argon 5N purity to regulate the pressure in the silver cell. To mitigate this, the effect of different argon purity was assessed in the post-circulation measurements. First the comparison was made under 6N purity atmosphere to preserve the cell ambience after it left NMIJ then the measurement was repeated under 5N argon atmosphere. The linkage was based on the temperature of NMIJ reference Ag fixed point relative to the key comparison reference value (KCRV) of CCT-K4 at the Ag fixed point via the difference in temperature between the NMIA reference Ag fixed point and NMIJ reference Ag fixed point (links to the KCRV of the CCT-K4). The temperature difference between the standards of NMIA and the KCRV of the CCT-K4 was within the evaluated comparison uncertainties of the CCT-K4.1. This report provides detailed information on the comparison results, linkage mechanism, and the Degree of Equivalence of NMIA relative to the institutes having participated in the CCT-K4. 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 https://www.bipm.org/kcdb/. 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).