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An acoustic gas thermometry system, which introduces a one-liter quasi-spherical resonator (QSR) made of oxygen-free copper, built at the National Metrology Institute of Japan (NMIJ/AIST), has been employed to measure thermodynamic temperatures from the triple point of water down to the triple point of mercury. The measurement adopted a thermostatic bath operated down to 263.15 K and a new one that can operate down to lower temperatures. The present acoustic gas thermometry system measured the speed of sound in argon on the isothermal curves of the triple point of water, 268.15 K, 263.15 K, 253.15 K, 243.15 K and at the triple point of mercury under the pressure range from 500 kPa down to around 60 kPa. Based on the measured speed of sound, the thermodynamic temperatures at the mentioned isotherms were determined relatively from the speed of sound at the triple point of water. Using the measured thermodynamic temperature T, the difference between T and the temperature T90, based on the International Temperature Scale of 1990 (ITS-90), (T − T90), along with the associated uncertainties, u(T − T90), were determined to be − 0.4 ± 1.1 mK for 268.15 K, − 1.0 ± 0.9 mK for 263.15 K, − 1.9 ± 0.9 mK for 253.15 K, − 2.5 ± 0.9 mK for 243.15 K and − 2.7 ± 0.9 mK for 234.3156 K. The present (T − T90) values were found to be consistent in all cases within the estimated uncertainty with the currently reported values existing in overlapping temperature range. [ABSTRACT FROM AUTHOR]

The measurements of the difference between the thermodynamic temperature T and the temperature T90 based on the International Temperature Scale of 1990 (ITS-90), (T-T90), by using an acoustic gas thermometer (AGT) working on neon gas is reported in this paper. The AGT introduced a one-liter quasi-spherical resonator made of oxygen-free-copper. The thermodynamic temperature was determined relatively from the speed of sound measurements at a desired temperature regarding to those at the triple point of water. At the present stage, the aimed temperature range was the gallium melting point. Based on the obtained measured speed of sound, the thermodynamic temperature T and the values of (T-T90) were determined, and the associated uncertainties were estimated. The (T-T90) obtained in the present work was found to agree within the estimated uncertainty with the currently reported values that exist in temperature range overlapping with the present work.

A new acoustic gas thermometry (AGT) system, which introduces a 1-L quasi-spherical resonator (QSR) made of oxygen-free copper, was built at the National Metrology Institute of Japan (NMIJ/AIST). The inner surface of the new QSR was machined using a diamond-turn tool. Improvement in the pressure measurement was introduced to allow direct measurement of the gas pressure in the resonator. The new AGT system was evaluated based on the measurement of the speed of sound in argon at the triple point of water and the melting point of gallium under the pressure range from 700 kPa down to 50 kPa. Based on these speed of sound measurements, the thermodynamic temperatures at the melting point of gallium were determined. The speed of sound measurements at isotherms of 283.15 K and 293.15 K were also conducted, and the related thermodynamic temperatures were determined. Based on the measured thermodynamic temperature T, the values of difference between T and the temperature T90 based on the International Temperature Scale of 1990 (ITS-90), (T − T90), along with the associated uncertainties, were calculated. The (T − T90) obtained in the present work were 1.3 ± 0.7 mK, 2.7 ± 0.8 mK, and 4.1 ± 0.8 mK for T90 of 283.15 K, 293.15 K, and 302.9146 K, respectively. These values were found to be in agreement within the estimated uncertainty with the currently reported values that exist in temperature range overlapping with the present work. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]