The transit-time ultrasonic flowmeter utilizes the propagation time of the ultrasonic signal in the fluid.

A pair of transducers are installed on the outer surface of the pipe as shown in the diagram. Each transducer works alternatively as both transmitter and receiver of ultrasonic signals.

When the ultrasonic signal is transmitted toward the upstream side against the flow direction, more propagation time is required (T1). On the other hand, when it is transmitted toward the downstream side with the flow direction, the propagation time is less (T2). That is, the signal is delayed or speeded up by the moving fluid. The difference in time between "T1" and "T2" is proportional to the flow velocity, and the flow volume can be calculated by multiplying it by the cross-sectional area, which is obtained by using the pipe diameter and wall thickness.