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What is a Doppler Flow Meter?

By H.R. Childress
Updated Jan 24, 2024
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A Doppler flow meter is a type of ultrasonic flow meter. These devices use a scientific principle called the Doppler effect to measure the volumetric flow through a pipe. This works in such a way that the Doppler flow meter is clamped onto the outside of the pipe, so that it does not disrupt flow, as is required for other types of flow meter.

Doppler flow meters use the frequency of an ultrasonic beam to determine the flow rate of a liquid. This measurement is possible because of the Doppler effect, discovered by the Austrian scientist Christian Johann Doppler in the early 1800s. Essentially, the Doppler effect states that the frequency of sound waves varies depending on the movement of the receiver of the sound, or the source of the sound relative to the medium that the sound travels in. In this case, the medium that the wave travels in is the liquid flowing through the pipe.

For a Doppler flow meter to work properly, the liquid must contain particles or bubbles, so one works well for dirty liquids but is not very effective for clean water. The flow meter transmits an ultrasonic signal, and the signal is reflected by the particles and bubbles, which causes the frequency of the signal to shift. This frequency shift is what is measured by the flow meter.

A Doppler flow meter converts the measured frequency shift to a volumetric flow rate. The velocity of the flow in the pipe is calculated using the frequency shift, the original frequency of the ultrasonic signal, the speed of sound through the transmitter material, and the sine of the angle at which the signal entered the liquid. Once the velocity is known, the volumetric flow rate can easily be calculated by multiplying the velocity by the area of the pipe.

Doppler flow meters may have either one or two sensors. Both types contain both a transmitter and a receiver, but in a one-sensor Doppler flow meter, they are contained in one sensor. Reflective particles in the liquid reflect the transmitted signal back to the receiver. In a two-sensor flow meter, the transmitter and receiver are clamped to opposite sides of the pipe.

Doppler flow meters can be a good monitoring option because they do not disturb the flow and require little maintenance. In addition, they can make quick and accurate flow measurements on many types of liquids under a variety of temperature and flow conditions. On the other hand, the liquid must contain bubbles or particles for the meter to operate properly, and lined or stainless steel pipes can interfere with the transmitter signal, decreasing the accuracy of the measurements.

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