A device that measures the movement of liquid in a pipe. "Invasive" flow meters are coupled into the pipe and derive the flow rate from the rotation of a turbine or paddle wheel that is moved by the liquid.

"Non-invasive" meters use two ultrasonic transducers that clamp on the outside of the pipe at a prescribed distance from each other based on the pipe's composition and diameter. They derive the flow rate based on the transit time of ultrasonic pulses sent from transmitter to receiver. In liquids with high solid or gaseous content, rather than measuring transit time, the amount of frequency shift is detected (see Doppler effect).