Resonant acoustic measurement of the effective diffusion coefficient of gases through soil

Abstract

Recent interest has developed within the environmental community in measuring the effective diffusion coefficient (De) of gases through soil. An electroacoustic system will be described that uses a plane-wave resonator and a phase-locked-loop automatic resonance frequency tracker. The gas analyzer is simple and can be built for about $100 from readily available plumbing parts and inexpensive transducers. A soil core is fitted into the top of a valved sample chamber and the resonator below is charged with a pure ‘‘tracer’’ gas (e.g., He, SF6), then the valve opened. The rate at which the tracer gas is replaced by air within the resonator is controlled by the soil’s De. The resonant frequency is maintained by a phase-locked-loop that compares the signal from an electret microphone with a square wave reference generated by the same monolithic function generator (XR 2206) that excited an electrodynamic loudspeaker with a sinusoidal current. The resonator temperature is monitored using a monolithic IC temperature sensor (AD 592) providing an output current of 1 μA/K. The mean molecular weight of the gas mixture in the resonator is directly determined in real-time from the ratio of the absolute temperature to the square of the fundamental acoustic resonance frequency. [Work supported by ONR.]

Sonic gas analyzer for hydrogen and methane

“…Most sonic gas analyzers that have been constructed in my laboratory have used a phase-locked loop (PLL) to track the resonance frequency as the gas mixture concentration changes [14,18,19,21]. A PLL system, operating around 5 kHz, is used to track the resonance frequency in the system reported here.…”

“…Using an analyzer with a single resonator, Schuhmann used acoustical gas analysis to make non-invasive determination of gaseous diffusion rates through porous media [21,22]. This technique, with its higher accuracy and precision, provided new insights into the determination of tortuosity factors and generated corrections to the standard chromatographic analysis which produced results that were previously confounded by advection and non-equimolar effects [23].…”