eFunda: Introduction to Doppler Ultrasonic Flowmeters
 Doppler Ultrasonic Flowmeter
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 Overview 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

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

Pros and Cons

 • Pros: - No obstruction in the flow path, no pressure drop - No moving parts, low maintenance cost - Can be used in corrosive or slurry fluid flow - Portable models available for field analysis and diagnosis • Cons: - Higher initial set up cost