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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 article describes the composition and metrological characteristics of the State Primary Standard of temperature unit – kelvin – in the range from 0.3 K to 273.16 K GET 35-2021. GET 35-2021 allows reproducing and disseminating the temperature unit in accordance with the definition of kelvin, accepted at the 26th General Conference on Weights and Measures (26th CGPM) in 2018. GET 35-2021 includes three installations of acoustic gas thermometry developed in 2012–2019, which are the primary means of measuring temperature in the ranges of 79–273.16 K, 4.2–80K, 268.16–273.16 K. Equipment for reproducing the reference points of the International Temperature Scale ITS-90 has been upgraded in order to improve the accuracy. Based on the studies performed, the uncertainty of reproducing the thermodynamic temperature and temperature according to ITS-90 has been calculated. [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 difference was determined between temperature defined by International Temperature Scale (ITS-90) and thermodynamic temperature measured by relative acoustic gas thermometry at 79.0000 K and at 83.8058 K. Measurement were carried out using misaligned spherical acoustic resonator filled with helium at different pressures. The isotherms were fitted for four different acoustic modes simultaneously assuming the same thermal accommodation coefficient and third acoustic virial coefficient for all modes. Our measurements yield the following differences between the thermodynamic temperature T and ITS-90 temperature T 90 : T - T 90 = - 4.81 ± 1.02 mK at 83.9058 K and T - T 90 = - 4.47 ± 0.97 mK at 79.0000 K. [ABSTRACT FROM AUTHOR]

A relative acoustic gas thermometry unit for reproducing and transmitting the unit of thermodynamic temperature, i.e., kelvin, in the low temperature range 4.2–80 K was developed and tested. The proposed apparatus differs from previously developed relative acoustic gas thermometry devices in terms of the dimensions of the main resonator unit. Reducing the size of the resonator (cavity diameter) allowed the mass of the cooled parts of the installation to be diminished by several times compared to the mass of such parts of the previously used installation. This significantly reduces the quantities of liquid helium and nitrogen necessary for cooling the resonator, as well as decreases the time of temperature stabilization and, accordingly, the measurement time. The results of verifying the operation of the main systems of the developed installation are presented. [ABSTRACT FROM AUTHOR]

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]

Frequencies of acoustic and electromagnetic resonances were measured in a quasispherical acoustic resonator at the temperature of the triple point of water. The resonator was filled with helium He. Based on the analysis of experimental data, a value for the Boltzmann constant of k = 1.380651·10 J·K with uncertainty of 2.4 ppm was obtained. [ABSTRACT FROM AUTHOR]

Recent advances in primary acoustic gas thermometry (AGT) have revealed significant differences between temperature measurements using the International Temperature Scale of 1990, $$T_{90}$$ , and thermodynamic temperature, T. In 2015, we published estimates of the differences $$(T-T_{90})$$ from 118 K to 303 K, which showed interesting behavior in the region around the triple point of water, $$T_\mathrm{TPW}=273.16$$ K. In that work, the $$T_{90}$$ measurements below $$T_\mathrm{TPW}$$ used a different ensemble of capsule standard platinum resistance thermometers (SPRTs) than the $$T_{90}$$ measurements above $$T_\mathrm{TPW}$$ . In this work, we extend our earlier measurements using the same ensemble of SPRTs above and below $$T_\mathrm{TPW}$$ , enabling a deeper analysis of the slope $$\mathrm{d}(T-T_{90})/\mathrm{d}T$$ around $$T_\mathrm{TPW}$$ . In this article, we present the results of seven AGT isotherms in the temperature range 258 K to 323 K. The derived values of $$(T-T_{90})$$ have exceptionally low uncertainties and are in good agreement with our previous data and other AGT results. We present the values $$(T-T_{90})$$ alongside our previous estimates, with the resistance ratios W( T) from two SPRTs which have been used across the full range 118 K to 323 K. Additionally, our measurements show discontinuities in $$\mathrm{d}(T-T_{90})/\mathrm{d}T$$ at $$T_\mathrm{TPW}$$ which are consistent with the slope discontinuity in the SPRT deviation functions. Since this discontinuity is by definition non-unique, and can take a range of values including zero, we suggest that mathematical representations of $$(T-T_{90})$$ , such as those in the mise en pratique for the kelvin (Fellmuth et al. in Philos Trans R Soc A 374:20150037, 2016. doi:), should have continuity of $$\mathrm{d}(T-T_{90})/\mathrm{d}T$$ at $$T_\mathrm{TPW}$$ . [ABSTRACT FROM AUTHOR]

Acoustic gas thermometry relies on the fundamental relationship between the speed of sound in a monatomic gas and its thermodynamic temperature. The speed of sound is calculated from the resonance frequencies of a cavity whose dimensions or thermal expansivity must be measured with high accuracy. For quasi-spherical cavities, the use of microwave resonances is a successful and proven dimensional measurement technique. The simplicity and economy of cylindrical resonators makes them an attractive alternative to quasi-spherical resonators, particularly for high-temperature thermometry. This article summarizes the basic theory of cylindrical microwave resonators, and describes methods for obtaining cavity dimensions from the mode frequencies. The perturbing effects of cavity shape deformations, the wall to end-plate junction, coupling probes and non-conducting surface layers are discussed. The results of an experiment with a simple aluminum cavity are presented, which demonstrate the superior performance of the TE0 $$pq$$ modes over the more commonly used TM0 $$pq$$ modes. [ABSTRACT FROM AUTHOR]

This article describes the accurate determination of the molar mass M of a sample of argon gas used for the determination of the Boltzmann constant. The method of one of the authors (Moldover et al 1988 J. Res. Natl. Bur. Stand. 93 85-144) uses the ratio of the square speed of sound in the gas under analysis and in a reference sample of known molar mass. A sample of argon that was isotopically-enriched in 40 Ar was used as the reference, whose unreactive impurities had been independently measured. The results for three gas samples are in good agreement with determinations by gravimetric mass spectrometry; (〈 M acoustic / M mass-spec 〉 - 1) = (-0.31 ± 0.69) × 10 -6 , where the indicated uncertainty is one standard deviation that does not account for the uncertainties from the acoustic and mass-spectroscopy references.

Using exceptionally accurate measurements of the speed of sound in argon, we have made estimates of the difference between thermodynamic temperature, T, and the temperature estimated using the International Temperature Scale of 1990, T90, in the range 118 K to 303 K. Thermodynamic temperature was estimated using the technique of relative primary acoustic thermometry in the NPL-Cranfield combined microwave and acoustic resonator. Our values of (T-T90) agree well with most recent estimates, but because we have taken data at closely spaced temperature intervals, the data reveal previously unseen detail. Most strikingly, we see undulations in (T-T90) below 273.16 K, and the discontinuity in the slope of (T-T90) at 273.16 K appears to have the opposite sign to that previously reported., (© 2016 The Author(s).)