Dynamic measurement of gas flow using acoustic resonance tracking

The National Institute of Standards and Technology measured gas flows exiting large, unthermostated, gas-filled, pressure vessels by tracking the time-dependent pressure P(t) and resonance frequency f(N)(t) of an acoustic mode N of the gas remaining in each vessel. This is a proof-of-principle demonstration of a gas flow standard that uses P(t), f(N)(t), and known values of the gas's speed of sound w(p,T) to determine a mode-weighted average temperature < T >(phi) of the gas remaining in a pressure vessel while the vessel acts as a calibrated source of gas flow. To track f(N)(t) while flow work rapidly changed the gas's temperature, we sustained the gas's oscillations using positive feedback. Feedback oscillations tracked < T >(phi) with a response time of order 1/f(N). In contrast, driving the gas's oscillations with an external frequency generator yielded much slower response times of order Q/f(N). (For our pressure vessels, Q similar to 10(3)-10(4), where Q is the ratio of the energy stored to the energy lost in one cycle of oscillation.) We tracked f(N)(t) of radial modes in a spherical vessel (1.85 m(3)) and of longitudinal modes of a cylindrical vessel (0.3 m(3)) during gas flows ranging from 0.24 to 12.4 g/s to determine the mass flows with an uncertainty of 0.51 % (95 % confidence level). We discuss the challenges in tracking f(N)(t) and ways to reduce the uncertainties.