How the Doppler effect acoustic frequency shift principle is used in flow measurement instrumentation.
The frequency and wavelength of an acoustic wave remains constant if the source and receiver are fixed relatively to each other, and the medium in which the wave propagates has not changed.
If the acoustic source is moving away from the receiver, then the wavelength will become stretched and the frequency lowered relative to the receiver.
Conversely, if the acoustic source is moving towards the receiver, then the wavelength will become compressed and the frequency raised relative to the receiver.
In the case of measuring flow, the doppler effect is applied by bouncing acoustic waves off particles and bubbles in the flowing fluid. Although the acoustic source is static relative to the receiver, it will become a moving one when it bounces off a moving particle or bubble.
A doppler flowmeter can be used to measure liquids such as waste water and other liquids which contain solid particles or gas bubbles.
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