be used to measure the rate of blood flow, by passing an ultrasonic beam through the tissues, bouncing it off a reflective plate, then reversing the direction of the beam and repeating the measurement, the volume of blood flow can be estimated. The frequency of the transmitted beam is affected by the movement of blood in the vessel and by comparing the frequency of the upstream beam versus downstream the flow of blood through the vessel can be measured. The difference between the two frequencies is a measure of true volume flow. A wide-beam sensor can also be used to measure flow independent of the cross-sectional area of the blood vessel. Open channel flow meters In this case, the ultrasonic element is actually measuring the height of the water in the open channel; based on the geometry of the channel, the flow can be determined from the height. The ultrasonic sensor usually also has a temperature sensor with it because the speed of sound in air is affected by the temperature. See also Flow measurement Magnetic flow meter Turbine flow meter Advantages of Ultrasonic Flowmeters References Lipták, Béla G.: Process Measurement and Analysis, Volume 1. CRC Press (2003), (v. 1) Ultrasonic Acoustic Sensing Brown University Lynnworth, L.C.: Ultrasonic Measurements for Process Control. Academic Press, Inc. San Diego. External links Doppler Shift for Sound and Light at MathPages The Doppler Effect and Sonic Booms (D.A. Russell, Kettering University) Ultrasound Flow meters