Sounding Out the Flow: How Ultrasonic Flowmeters Work

In 1842, Christian Doppler discovered that a stationary observer perceives a sound to have shorter wavelengths as its source approaches, longer wavelengths as its source recedes. The Doppler effect explains why one hears rising pitch in the blowing horn of an approaching car. When the car zooms away, the pitch seems to drop. Ultrasonic Doppler flowmeters put this frequency shift to work in so-called dirty liquids containing acoustical discontinuities— suspended particles, entrained gas bubbles or turbulence vortexes.

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