Ultrasonic gas sensing 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) transmit and 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.