A type of non-intrusive flow meter used to measure the velocity of a fluid flowing in pipes and open channels. It uses the principle of ultrasound in which an ultrasonic signal is passed through the fluid, which is detected by another sensor located a short distance away. The time is measured between the pulses of emitted and detected ultrasound. The delay between sending and receiving the signal is related to the velocity of the fluid. While inexpensive to operate and with no moving parts, ultrasonic flow meters are sensitive to changes in fluid density and to distortions of flow profile.