Sonic gas analyzer

Penn State

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

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

The Penn State researchers are developing the current prototype to track continuously the 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 say their new 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 analyzer, a miniature speaker produces a tone barely audible outside the resonator that sounds like a whistle. Two tiny microphones capture the tone, called a resonance frequency, and the two-microphone outputs are then 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 the researchers what the change in gas concentration is at every instant without disturbing the system or requiring extraction of gas samples.

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

Keeping the temperature unaltered by external devices is crucial to obtaining detailed information about the gas mixture in a microbial fuel cell without affecting the bacteria.

Components of the new Penn State sonic gas analyzer. The sample chamber (bottle) is connected to the analyzer cell (resonator) by a sealed fan (pump) that circulates the gas mixture through the system.