Acoustic Gas Resonators for Measurement of Thermophysical Properties and Thermometry

Abstract : Gas-filled cavity resonators can be used to measure the speed of sound c, the viscous diffusivity D(sub v), and the thermal diffusivity D(sub t). The relative sensitivity of a complex resonance frequency to these parameters depends on the shape of the resonator and choice of mode. Different choices are appropriate for different measurements. Spherical acoustic resonators are excellent for very high-precision measurements of the speed of sound in dilute gases. The experimental models of spherical resonators include the effects of the thermal and viscous boundary layers, and the elastic deformations of the solid parts of the resonator. Spherical resonators have been applied to the measurement of the gas constant R, to acoustic thermometry, and to high precision measurements of the speed of sound c(T,p) as a function of temperature and pressure for very pure gases. Precisions as high as a few parts in 10 million can be achieved. The ideal-gas specific heat and information about intermolecular interactions can then be extracted from c(T,p). Cylindrical acoustic resonators are easier to fabricate and are suitable for many purposes, although they cannot be modeled as completely as spherical resonators. More complex resonator shapes have been developed for determination of D(sub v) and D(sub t). The Greenspan acoustic viscometer, a resonator consisting of a cylindrical duct coupled at each end to large chambers, has a low-frequency mode in which the kinetic energy is localized in the duct, and the potential energy in the chambers. Its frequency response is strongly dependent upon D(sub v). As an absolute instrument, it is capable of determining D(sub v) to a precision well below 1%. A cylindrical resonator with a honeycomb structure interposed in the flow midway between the ends has been developed for measurements of the ratio of D(sub v) to D(sub t).