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Doppler Ultrasonic Flowmeters rely on the Doppler effect to relate the frequency shifts of acoustic waves to the flow velocity. It usually requires some particles in the flow to reflect the signals. The rule of thumb is 25 PPM suspended solid or bubbles with diameters of 30 micron or larger for 1 MHz or higher transducers. Lower frequency transducers may require "dirtier" fluid conditions.

The Doppler formula for a sound or light source moving toward the observer at a velocity V is

Since the input signal from the transducer forms an angle with the flow direction, the velocity V should be replaced by the projected velocity V cos. The acoustic waves traveling upstream and downstream will have the observed frequencies

The difference in frequency is

since the flow velocity V is much smaller than the speed of sound c in the fluid.

By re-arranging the above equation, the flow velocity can be written as

Common specifications for commercially available Doppler ultrasonic flowmeterss are listed below:

		Score Phase Condition  Gas   Dirty   Liquid   Corrosive      Dirty     Open Channel   Gas   Clean   Liquid   Clean     Viscous  : Recommended : Limited applicability
	Line Size:	Inline model: 10 ~ 1200 mm (0.4 ~ 48 inch) Clamped-on model: 75 mm (3 in) and up
	Turndown Ratio:	100 : 1