A moving object causes changes in sound frequency. The Doppler flowmeter uses the principle of the Doppler frequency shift caused by sound scattered or reflected fromt he moving particles in the path of flow. It transmits a continuous sonic wave inclined at some angle to the flow. Small inclusions of bubbles, solids, or eddies in the flow would reflect or scatter the sound back to a receiver. If there is any movement of the inclusions, a Doppler-shift would appear in the frequency of the returned signal. The difference the transmitted and received frequencies is proportional to the motion of the flow.

Governing equation:

V_davg is unknown here and by measuring f_d, V_davg can be calculated.

There are three types of Ultrasonic Flowmeters. The principle of operation of each type is dependent upon the speed at which high frequency sound propagates. One type of ultrasonic meter is based on the Doppler-shift or the phenomenon in which the wavelength of an approaching sound source is shorter than the wavelength of that same source as it is moving away. The transducer emits a sonic beam into the process and entrained particles or bubbles reflect the beam back to the transducer. The measured difference in the wavelengths (transmitted verses reflected) is proportional to the process' velocity. These meters are used on closed full pipes and the transducer may be mounted either on the outside of the pipe or inserted (hot tapped) into the pipe.

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