Acoustic gas sensor with ppm resolution

Acoustic sensors based on the well-known relation between sound velocity and mean molecular mass are suggested for the determination of small concentrations of "pollutants", such as CO2, in air. The theoretical basis for high resolution is outlined, and a basic design is presented, together with experimental results. Sound velocity is measured continuously at high resolution in an oscillator controlled by the acoustic transit time between a transmitter receiver pair operating in the 40kHz range. The static error band is better than + -3 per cent of full scale, response time less than two minutes, and short-term resolutions of 0.3 ppm rms, and 3 ppm rms have been obtained in terms of frequency and CO2 concentration, respectively.