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Measuring collected gas with microwave and acoustic resonances



Keith A. Gillis, James W. Schmidt, Michael R. Moldover, James B. Mehl


With calibrations of large flow meters in mind, we established the feasibility of determining the mass M of argon gas contained within a 0.3 m3 commercially manufactured pressure vessel ("tank") with a relative standard uncertainty of ur(M) = 0.0016 at 0.6 MPa by combining the measured argon pressure and the measured microwave and acoustic resonance frequencies within the pressure vessel with an accurate equation of state for argon. This combination of measurements replaced the difficult problem of accurately measuring the average temperature of a large volume of gas with the easier problem of accurately measuring the average speed-of-sound in the gas. We heated the tank's top until it was 13 K warmer than its bottom. Remarkably, the temperature gradient changed the deduced mass by only (0.004 {plus or minus} 0.026) %. We conclude that resonance frequencies can be used to "weigh" the compressed gas in much larger tanks, which might be made from ferromagnetic steel and possibly at high pressures in un-thermostatted environments. Details of this work have appeared in two previous publications. In the first, we used microwaves to determine the tank's internal volume Vmicro with ur(Vmicro) = 0.0006 as well as the tank's thermal expansion (Moldover et al. 2015 Meas. Sci. Tech. 26 015304). In the second publication, we showed that the microwave results accurately predicted the wavenumbers kcalc of the four lowest-frequency acoustic modes of the gas. (Gillis et al. 2015 Metrologia 52 337-352) We used these modes to determine the mass of the argon in the tank. The largest component of ur(M) resulted from the poorly-understood interaction between the acoustic oscillations of the gas and the mechanical vibrations the tank's walls.
Proceedings Title
Proceedings of 9th International Symposium on Fluid Flow Measurement (ISFFM)
Conference Dates
April 12-17, 2015
Conference Location
Roslyn, VA
Conference Title
9th International Symposium on Fluid Flow Measurement


volume, pressure vessel, flow metrology, flow calibrations, acoustic resonances, microwave resonances, temperature


Gillis, K. , Schmidt, J. , Moldover, M. and Mehl, J. (2015), Measuring collected gas with microwave and acoustic resonances, Proceedings of 9th International Symposium on Fluid Flow Measurement (ISFFM), Roslyn, VA, [online], (Accessed May 22, 2024)


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Created April 23, 2015, Updated June 2, 2021