Depending on the measurement technology and transducer type, ultrasonic flowmeters come in several different types, such as clamp-on, portable, insertion-wetted, inline / spool-piece type, dedicated / fixed-installation flowmeter, etc.

The most widely used ultrasonic flowmeters are based on the transit-time principle.

As shown in the figure, a typical transit-time ultrasonic flowmeter system utilizes two transducers(upstream and downstream) that function as both ultrasonic transmitter and receiver. The flowmeter operates by alternately transmitting and receiving a burst of sound energy between the two transducers and measuring the transit time that it takes for sound to travel between the two transducers.

The difference in the transit time measured is directly related to the velocity of the liquid in the pipe.