We review the principles, techniques and results from primary acoustic gas thermometry (AGT). Since the establishment of ITS-90, the International Temperature Scale of 1990, spherical and quasi-spherical cavity resonators have been used to realize primary AGT in the temperature range 7 K to 552 K. Throughout the sub-range 90 K < T < 384 K, at least two laboratories measured (T − T 90 ). (Here T is the thermodynamic temperature and T 90 is the temperature on ITS-90.) With a minor exception, the resulting values of (T − T 90 ) are mutually consistent within 3 × 10 −6 T . These consistent measurements were obtained using helium and argon as thermometric gases inside cavities that had radii ranging from 40 mm to 90 mm and that had walls made of copper or aluminium or stainless steel. The AGT values of (T − T 90 ) fall on a smooth curve that is outside ±u(T 90 ), the estimated uncertainty of T 90 . Thus, the AGT results imply that ITS-90 has errors that could be reduced in a future temperature scale. Recently developed techniques imply that low-uncertainty AGT can be realized at temperatures up to 1350 K or higher and also at temperatures in the liquid-helium range.
Piezoelectric ceramics mounted on the endplates of a cylindrical resonator were used as the source and detector for speed-of-sound measurements. The perturbations of the longitudinal gas modes of the cavity due to the compliance of the diaphragms (10 mm diameter, 0.3 mm thick) and the attached transducers were estimated from first-order perturbation theory. The fractional shift of the resonance frequencies in argon caused by the source and detector was 0.03 × 10 −6 at 0.1 MPa and 273.16 K. The high signal-to-noise ratio (up to 1 × 10 4 with a 6 s integration time) that was obtained with these transducers makes them suitable for acoustic thermometry. The heat dissipation in the source transducer was measured to be only 0.7 µW at the working voltage (7 V) and frequency (1 kHz).
The subrange inconsistency is a significant factor to uncertainty in the standard platinum resistance thermometer (SPRT) subranges of the International Temperature Scale of 1990 (ITS-90). This paper investigated the subrange inconsistency between the water-zinc and water-aluminum subranges. The calibration data for 60 SPRTs from four manufacturers were analyzed, and the result confirms that the coefficient c in the interpolation of ITS-90 is available to determine the subrange inconsistency in this temperature range again. The inconsistency, t, can be simply equal to 59.83c.
Relative primary acoustic gas thermometry determines the ratios of thermodynamic temperatures from measured ratios of acoustic and microwave resonance frequencies in a gas-filled metal cavity on isotherms of interest. When measured in a cavity with known dimensions, the frequencies of acoustic resonances in a gas determine the speed of sound, which is a known function of the thermodynamic temperature T. Changes in the dimensions of the cavity are measured using the frequencies of the cavity's microwave resonances. We explored techniques and materials for acoustic gas thermometry at high temperatures using a cylindrical cavity with remote acoustic transducers. We used gas-filled ducts as acoustic waveguides to transmit sound between the cavity at high temperatures and the acoustic transducers at room temperature. We measured non-degenerate acoustic modes in a cylindrical cavity in the range 295 K < T < 797 K. The fractional uncertainty of the measured acoustic frequencies increased from 2×10−6 at 295 K to 5×10−6 at 797 K. In addition, we measured the frequencies of several transverse magnetic (TM) microwave resonances up to 1000 K in order to track changes in the cavity's length L and radius R. The fractional standard deviation of the values of L deduced from three TM modes increased from 3×10−6 for T < 600 K to 57×10−6 at 1000 K. We observed similar inconsistencies in a previous study.
Currently, there exists only one set of experimental results at temperatures up to 680 K with the claimed relative standard uncertainty of (0.15-0.20)%. This paper reports new experimental results using the two-capillary viscometer in the temperature range from 298.15 K to 653.15 K with the claimed relative standard uncertainty of 0.062%. The new measurements agree with the existing high accuracy measurements and ab initio calculations in the overlapping temperature range within the extraordinary low relative differences of ±0.08%. The good agreement represents a robust proof of the potential models derived from the ab initio calculations, which play the powerful means in obtaining the thermophysical properties of dilute monoatomic gases over wide temperature ranges. In the experiments, the authors observed the occurrence of insufficient preheating existing with the two-capillary viscometer at high temperature.
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