Pitching for gas concentration

Penn State University researchers have developed a prototype sonic gas analyser that automatically and continuously tracks the concentration of a gas in an air/gas mixture based on changes in pitch.

"The system automatically cancels out the background and flow noise and can detect changes in gas concentration as low as 0.003%, plenty sensitive enough, for example, to let you know if you've got an explosive mixture," said Miguel Horta, a doctoral candidate in acoustics who currently is working on the sonic gas analyser as part of his dissertation.

The Penn State researchers are developing the current prototype to continuously track concentrations of hydrogen produced by bacteria in microbial fuel cells (MFC). In MFCs, bacteria feed on the organic matter in wastewater and produce hydrogen for use as fuel while simultaneously cleaning the water.

However, the researchers said their system also could be adapted for tracking toxic or flammable gases in mines, sewers or landfills, for hydrogen detectors in battery compartments of boats and electric cars, or in industries where gases are consumed as feedstocks.

In the Penn State sonic gas analyser, a miniature speaker produces a tone that sounds like a whistle and is barely audible outside the resonator. Two tiny microphones capture the tone, called a resonance frequency, and the two-microphones outputs then are subtracted to double the signal of interest and cancel any extraneous noise before being fed to an electronic tracking system, called a phase-locked loop. This tracking system determines the changes in the resonance frequency caused by the changes in gas concentration, while simultaneously correcting for any changes in the gas temperature.

"If the concentration of a gas in the gas/air mixture passing through the system changes, the new concentration will affect the sound's speed which will, in turn, change the resonance frequency. That change in resonance frequency or pitch, as detected by the microphones and tracking system, tells us what the change in gas concentration is at every instant without disturbing the system or requiring extraction of gas samples," said Horta.

Sonic gas analysers can be used in the same applications as thermal conductivity analysers, the researchers point out. However, since the sonic gas analyser only introduces sound, it doesn't change the temperature of the gas mixture, as do thermal conductivity analysers.