Device that measures the velocity of a gas or liquid .
It has applications in medicine as well as in chemical engineering, aeronautics, and meteorology. Examples include venturi tubes , and rotameters (tapered graduated tubes with a float inside that is supported by the flowing fluid at a level that depends on the flow rate). Ultrasonic flow meters, in which reflecting ultrasound off a flowing liquid leads to a Doppler shift (see {{link=Doppler effect">Doppler effect ) that is calibrated to provide the flow rate of the liquid, have important industrial applications and have also been used to measure arterial blood flow.