Flow sensors are designed to measure the flow rate of a fluid. There are several methods utilizing ultrasonics to measure flow.

The most common method relies on the Doppler effect. The Doppler effect is the shifting of frequency with motion of the device, an object being tracked, or the medium through which the sound propagates. In most instances two (or more) transducers are used in a pitch-catch mode. The first transducer emits ultrasound of a known frequency and the second receives the signal. Any change in frequency is attributable to the flow rate of the fluid being measured. For more precision, several transducer pairs are often used at a variety of angles and positions to get the average flow. This method can minimize the effects of turbulent flow, wall effects, and the like.

A variation on this technique utilizes phase shifts in a standing ultrasonic wave rather than frequency changes. A continuous ultrasonic beam will have a reliable and repeatable phase shift with velocity of the medium.

Another common method of determining flow rate is to simply measure time of flight differences. This method requires a very uniform fluid but in many process industry applications this is not a problem. If the time of flight is faster the medium is moving more quickly and vice versa.

Another method of utilizing piezoelectrics to measure flow is based on the creation of vortices within the flow. A protrusion into the flow creates vortices which in turn create vibrations in the protrusion. These vibrations change in a predictable way with the velocity of the fluid. By using a piezoelectric material attached to the protrusion it is possible to determine flow. This is a very useful technique when the fluid conditions are tough on the piezo element – high temperatures or corrosive chemicals are good examples.