Determining the mass of gas in large collection volumes with acoustic and microwave resonances

Abstract

We characterized a 1.8 m3, quasi-spherical resonator, a pressure vessel informally known as the “big blue ball” or “BBB.” The BBB will be the collection volume of a NIST-designed gas-flow standard that will operate at pressures up to 7 MPa. Using microwave resonance frequencies, we determined the volume of the BBB filled with argon as a function of pressure (up to 7 MPa) and temperature (near ambient). The measured pressure- and temperature-dependences of the BBB’s volume are consistent with the published properties of carbon steels. We filled the BBB with 220 kg of argon in weighed increments of approximately 20 kg. We also determined the mass macoust of argon in the BBB by measuring the pressure, the acoustic resonance frequencies facoust of 3 modes, and using the known thermophysical properties of argon. The values of macoust were within ±0.03 % of the gravimetrically determined masses. In a typical application, the BBB will never be isothermal. Nevertheless, the acoustic resonance frequencies quickly and accurately determine the average temperature and mass of the gas in the BBB despite significant, long-lived thermal gradients that are created by pressure changes. A theoretical model for effect of temperature gradients on the acoustic resonance frequencies is presented.