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Characterizing Gas-Collection Volumes with Acoustic and Microwave Resonances



Jodie Gail Pope, Keith A. Gillis, Michael R. Moldover, J B. Mehl, Eric Harman


We characterized a 1.8 m3, nearly-spherical, steel shell at pressures up to 7 MPa for use as a gas flow standard. For pressure, volume, temperature, and time measurements, the shell's cavity will collect gas; for blow-down measurements, the shell will be a gas source. We measured the cavity's microwave resonance frequencies fmicro to determine its pressure- and temperature- dependent volume: Vmicro(P, T) = 1.84740 m3 [1 + a(T295 K) + bP] with a fractional uncertainty of 0.011 % at a 68 % confidence level. The coefficients a and b were consistent with the dimensions and properties of the steel shell. The microwave-determined volume Vmicro was consistent, within combined uncertainties, with Vgas the volume determined by a gas expansion method: Vmicro/Vgas  1 = (0.00002±0.00014). When the shell was filled with gas, measurements of its acoustic resonance frequencies facoust and of the pressure quickly and accurately determined the mass of the gas in the shell, even when temperature gradients persisted. [K. A. Gillis et al. Metrologia, 52, 337 (2015)] After raising the nitrogen pressure in the shell from 0.1 MPa to 7.0 MPa in 45 minutes, the top of the shell was 20 C warmer than the bottom of the shell. Despite this large thermal gradient, the mass Macoust of gas determined from acoustic resonance frequencies settled to within 0.01 % of its final value after 5 hours. Following a smaller pressure change of 0.3 MPa, the top-to-bottom temperature difference was 1.5 C and Macoust settled to its final value in just 0.5 hours.
Proceedings Title
10th annual International symposium for fluid flow measurements, ISFFM
Conference Dates
March 21-23, 2018
Conference Location
Queretaro, MX


Acoustic thermometry, microwave resonance, primary gas flow


Pope, J. , Gillis, K. , Moldover, M. , Mehl, J. and Harman, E. (2018), Characterizing Gas-Collection Volumes with Acoustic and Microwave Resonances, 10th annual International symposium for fluid flow measurements, ISFFM, Queretaro, MX, [online], (Accessed June 16, 2024)


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Created March 27, 2018, Updated April 11, 2022