Ultrasonic gas sensing technology is based on the principle of sound velocity difference in ultrasonic propagation (also known as ultrasonic time difference method) to calculate the gas concentration and flow rate. The ultrasound wave propagation speed in the downstream direction will increase, and the backward direction will decrease. By a pair of ultrasonic transducers to alternately (or simultaneously) transmitand receive ultrasonic waves. By observing the difference in the propagation time between the upstream and the backward flow of the ultrasonic in the medium(gas or liquid), the fluid velocity is measured indirectly, and then calculate the flow data based on fluid velocity. With the advantages of non-contact measurement and wide range application, ultrasonic gas sensing technology is increasingly used in the fields of medical and health, industrial processes andenergy measurement.