Progress Report on NMIJ Acoustic Gas Thermometry at the Triple Point of Water
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Herein, progress in the development of an acoustic gas thermometry (AGT) system at the National Metrology Institute of Japan is reported. This AGT system is an initial low-cost version that uses a 1-l quasi-spherical resonator (QSR) made of oxygen-free copper. The system was tested by measuring the speed of sound in argon at the temperature of triple point of water. Measurements were conducted at ten different pressures, ranging from 60 kPa to 420 kPa. The ideal gas limit of the squared speed of sound was obtained through extrapolation, and a preliminary calculation of the Boltzmann constant, which was 12 ppm below the CODATA2014 value, was made. Large inconsistencies among microwave and acoustic modes were observed, which are dominant sources of uncertainty in speed of sound measurements. The system will be improved by replacing the present QSR with another one that is more precisely fabricated.
KeywordsAcoustic gas thermometry Acoustic resonance Boltzmann constant Microwave resonance Primary thermometry
We would like to thank Keith Gillis and Michael R. Moldover of NIST for useful advices and constructive discussion on acoustic analysis and apparatus design. We also thank Laurent Pitre of LNE-INM and Inseok Yang of KRISS for collaborating on argon gas analysis and discussion. Laurent Pitre also shared his abundant experience on microwave and acoustic analysis. Last but not least, we appreciate the encouragement that our colleagues Atsushi Onae, Kenichi Fujii, Nobu-Hisa Kaneko, Chiharu Urano and Tohru Nakano have given us to drive this project.
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