A Doppler flow meter is a type of ultrasonic flow meter. These devices use a scientific principle called the Doppler effect to measure the volumetric flow through a pipe. This works in such a way that the Doppler flow meter is clamped onto the outside of the pipe, so that it does not disrupt flow, as is required for other types of flow meter.
Doppler flow meters use the frequency of an ultrasonic beam to determine the flow rate of a liquid. This measurement is possible because of the Doppler effect, discovered by the Austrian scientist Christian Johann Doppler in the early 1800s. Essentially, the Doppler effect states that the frequency of sound waves varies depending on the movement of the receiver of the sound, or the source of the sound relative to the medium that the sound travels in. In this case, the medium that the wave travels in is the liquid flowing through the pipe.
For a Doppler flow meter to work properly, the liquid must contain particles or bubbles, so one works well for dirty liquids but is not very effective for clean water. The flow meter transmits an ultrasonic signal, and the signal is reflected by the particles and bubbles, which causes the frequency of the signal to shift. This frequency shift is what is measured by the flow meter.