Cylindrical acoustic resonators developed at the National Institute of Standards and Technology (NIST) are routinely used to measure the speed of sound in gases with uncertainties of 0.01% or less. The pressure dependence of the data is fitted with model intermolecular potentials to obtain virial coefficients as well as gas densities p and heat capacities C p with uncertainties of 0.1%. The model intermolecular potentials are also used to estimate the viscosity and thermal conductivity of gases with cerrtainties of less than 10%. These techniques have been applied to numerous gases, and the results are tabulated. The gases include candidate replacement refrigerants, helium-xenon mixtures used in thermoacoustic machinery, and very reactive gases used in semiconductor processing. The viscosity can be measured directly with uncertainties of less than 1% using the Greenspan acoustic viscometer, a novel acoustic resonator developed at NIST. A second resonator is used to measure the Prandtl number (Pr C p / ) with uncertainties on the order of 2%. The thermal conductivity is determined by combining the Prandtl number with the acoustically determined density, viscosity, and heat capacity. Spherical acoustic resonators are used to measure the speed of sound with the highest possible accuracy. An argon-filled spherical resonator was used to redetermine the universal gas constant R with a fractional standard uncertainty of 1.7 10 -6. The same resonator was used to measure imperfections of the internationally accepted temperature scale (ITS-90) in the range 217 to 303 K. This work is being extended to 800 K. In effect, very accurate speed-of-sound measurements will be used to calibrate thermometers.
Citation: Modern Acoustical Techniques for the Measurement of Mechanical Properties - Experimental Methods in the Physical Sciences
Publisher Info: Academic Press, San Diego, CA
Pub Type: Books
Acoustic Resonator, Acoustics, Heat Capacity, Speed of Sound, Temperature Scale, Thermal Conductivity, Thermodynamic Properties, Transport Properties, Virial Coefficients