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Acoustic Gas Thermometry



Michael R. Moldover, Roberto M. Gavioso, James B. Mehl, Laurent Pitre, Michael de Podesta, Jintao Zhang


Primary acoustic gas thermometry (AGT) exploits the simple relationship between the speed of sound in a dilute gas u and the thermodynamic temperature T of the gas. For monatomic gases such as argon and helium, u2 = (5/3) kT/m, in the limit of zero frequency and zero density, where k is the Boltzmann constant and m is the average molecular mass of the gas. The thermodynamic temperature is deduced from absolute measurements of the speed of sound on an isotherm (zero density is not experimentally accessible) that are traceable to the metre, the kilogram and the second. Isotopic impurities in commercially prepared helium are negligible; however, commercially prepared argon is a mixture of several isotopes. Determining the average molecular mass of an argon sample with a relative uncertainty near 10 6 is difficult; however, it can be accomplished using isotopic argon standards and analysis for chemical impurities, including other noble gases. Primary AGT has been conducted with estimated smallest relative uncertainties at the temperature of the triple point of water of order 10 6. However, the low uncertainties claimed for AGT have not yet been confirmed by independent measurements. Details are found in the appendix on AGT. Relative AGT determines the ratios of thermodynamic temperatures from measurements of the ratios of speeds-of-sound conducted on the isotherms of interest. Such ratio measurements are simpler than absolute measurements because they do not require traceability to either the metre or the second or the kilogram. The ratio measurements do require measuring ratios of lengths and frequencies. They also require that the average molecular mass of the thermometric gas does not change while the speed-of-sound ratios are measured. Thus, noble gas impurities in a noble working gas are acceptable for relative AGT conducted under conditions such that the gas mixture does not fractionate. Relative AGT has been conducted in the wide temperature range from a few kelv


acoustic thermometer, primary thermometry, speed of sound, thermodynamic temperature
Created January 16, 2014, Updated November 10, 2018