![]() ![]() Ultrasonic sound is transmitted into a pipe with flowing liquids, and the discontinuities reflect the ultrasonic wave with a slightly different frequency that is directly proportional to the rate of flow of the liquid. This metering technique utilizes the physical phenomenon of a sound wave that changes frequency when it is reflected by moving discontinuities in a flowing liquid. The basic principle of operation employs the frequency shift (Doppler Effect) of an ultrasonic signal when it is reflected by suspended particles or gas bubbles (discontinuities) in motion. ![]() Thus, the ultrasonic Doppler flow meter calculates liquid flow rate from the velocity of the discontinuities, rather than from the velocity of the liquid. 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. Consideration must be given to the lower limits for concentrations and sizes of solids or bubbles, and the liquid must flow at a rate high enough to keep the solids suspended. The Doppler ultrasonic flow meter must have particles or bubbles to reflect the ultrasonic signals. The two types of ultrasonic flow meters, Doppler and transit time, each function by way of two different technologies. Portable ultrasonic flow meters are available to aid in industrial application. They are clamp-on devices that attach to the exterior of the pipe (and fit a variety of pipe sizes) and enable measurement of corrosive liquids without damage to the ultrasonic sensor. Ultrasonic flow meter technology is a non-contact means of measuring the velocity of a fluid. Ultrasonic Doppler flow meters put this frequency shift to work in so-called dirty liquids containing acoustical discontinuities – or suspended particles, entrained gas bubbles, or turbulence vortexes. When the car zooms away, the pitch seems to drop. This became known as the Doppler effect and explains why one hears rising pitch in the blowing horn of an approaching car. In 1842, Christian Doppler discovered that a stationary observer perceives a sound to have shorter wavelengths as its source approaches, and longer wavelengths as its source recedes. ![]()
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