Dynamic gas flow measurements using self-sustained acoustic resonance tracking

The National Institute of Standards and Technology is developing an acoustic, dynamic gas-flow standard. The standard consists of a large, unthermostated pressure vessel with known volume that we use as a gas source and as an acoustic resonator. We determine the mass flow dm/dt exiting the vessel by tracking the time-dependent pressure P(t) and the resonance frequency f_N(t) of an acoustic mode of the gas remaining in the vessel. We use P(t), f_N(t), and known values of the speed of sound w(T,P) to determine a mode-weighted average temperature <T>_φ of the gas in the vessel. As gas exits the vessel, the average temperature (and therefore f_N(t)) changes rapidly due to flow work. To track f_N(t), we sustain the gas oscillations using positive feedback; f_N responds to changes in <T>_φ within a time of order 1/f_N. We have validated this technique with calibrated critical flow venturis using radial modes of a spherical vessel (1.85 m³) and using longitudinal modes of a cylindrical vessel (0.3 m³) in flows ranging from 0.24 g/s to 12.4 g/s with an uncertainty of 0.51% (95 % confidence level). We have also measured leaks as small as 5 μg/s. Details of the acoustic tracking technique and requirements of the vessel/resonator will be discussed.