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.

When transmitted into a pipe that contains flowing liquid with such discontinuities, an
ultrasonic pulse or beam reflects from them with a change in frequency that is directly
proportional to the liquid’s flow rate. Thus, the ultrasonic Doppler flowmeter calculates flow
rate from the velocity of the discontinuities, rather than from the velocity of the liquid. It suits
liquids such as certain wastewaters, slurries, sludges, crude oils, phosphates and pulp
stocks. Although the Doppler flowmeter generally works well with mining slurries, high-density
polyethylene (HDPE) pipes may cause inaccuracies because their flexure changes the
diameter of the measurement area. If flexure is great enough, it may break the coupling
between an exterior-mounted transducer and the pipe’s outside surface.