Measurement of oxygen concentration in atmospheric air using ultrasound time of flight with humidity compensation

An absolute gas concentration can be measured from the variation in the speed of sound between two gases in principle. Owing to the small difference in the speed of sound between the atmospheric air and oxygen (O-2) gas, measuring O-2 concentration with high accuracy in the humid atmospheric air using ultrasound needs careful investigation. The authors show successfully a method to measure the absolute concentration of O-2 gas in humid atmospheric air using ultrasound. It was possible to measure O-2 concentration in the atmospheric air with accuracy by compensating for the influence of temperature and humidity by calculation. The O-2 concentration was calculated from the conventional speed of sound equation by utilizing small mass variation for the change in moisture as well as temperature. This method using ultrasound enabled us to measure the O-2 concentration in the atmospheric air as 21.0%, which is in agreement with the standard atmospheric dry air. The measurement error values after the humidity compensation are about 0.4% or less. Furthermore, this method takes only about a few ms for measuring O-2 concentration and, thus, can be used as a high-speed portable O-2 sensor for industrial, environmental, and biomedical instruments.