Audible sensing of low-ppm concentration gases

Reliable and consistent measurement of various gas molecules at low parts per million (low-ppm) concentration is an essential intermediary step towards low-cost versatile gas analysers. Here we demonstrate physical measurements of various low-ppm molecules by means of an acoustic standing wave-based resonating device with a speaker and a microelectromechanical microphone, using either the sound pressure level (SPL) at a fixed frequency, or alternatively, tracking the resonance frequency as gas molecules enter the acoustic device. Low-ppm gas molecules are measurable at frequencies near standing wave resonances, as slight shifts in the speed of sound result in a shift of the acoustic frequency response function. Since a chemical receptor layer on the sensing component is not required for the measurements, absolute physical quantities of the molecules are measurable, which coincide well theoretically with expected values for pure chemicals. Various conditions are tested using different low-ppm molecules to demonstrate the limit of detection (LOD), using a low-cost and portable acoustic setup. An LOD is determined to be in the order of several ppm for most molecules. Gas analysers based on a physical approach present new opportunities for mobile gas sensing in practical applications.