Preliminary Results for an Ultrasonic Gas Monitoring System 2005-01-2876

A critical function of life support systems on board manned spacecraft is to continuously revitalize the ambient air and to maintain its proper composition throughout the duration of the mission. This involves gaseous processes that need continuous monitoring. Sound traveling through a gas propagates at different speeds and attenuates to different degrees depending upon the composition of the gas. We previously justified conceptually the operation of a real-time gaseous process control monitor for gas mixtures based on acoustic attenuation and phase velocity. In the present study, on the theory front, we have refined a non-empirical model of acoustic propagation in polyatomic gases based on quantum mechanics and the kinetic theory of gases. To check the validity of the model against experimental values, we have obtained data in a custom-built test cell which allows precise gas mixing, controlled pressurization of the mixture from 0.6 to 30 atm, four operating frequencies (92, 149.1, 215, and 1000 kHz), and variable transmitter-receiver separation paths (0.2 to 7.5 in, with increments of 0.1 in). The theoretical predictions agree quite well with experimental data for mixtures of nitrogen, methane, and carbon dioxide. As part of the hardware development effort, we have built a prototype flow-through acoustic sensor. The sensor incorporates two pitch-catch acoustic paths of 30 and 60 mm, separated vertically by 2 cm. Four piezoelectric transducers operating at 215 kHz are used. Upon cross-calibration of the two sensing paths, the measured attenuation agrees to within 1% of the test cell value.