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Doppler ultrasound measures liquid flow by reflecting sound waves. Christian Doppler discovered the Doppler effect in 1840, which was later confirmed using sound. Ultrasonic Doppler velocity measurement is used in medical diagnostics and industrial applications to measure liquid flow velocity. The first applications were in the medical field, and later in industrial applications. A full tube is required for proper measurement.
Doppler ultrasound is a technique used to measure the flow of liquids by reflecting sound waves. Astronomer Christian Doppler first suggested the Doppler effect in 1840, when he discovered that some stars had different colors than expected. He proposed that the color differences were due to stars moving towards or away from the observer, changing their visible colors. Although Doppler was studying starlight, scientists believed the effect also occurred with sound.
A later experiment using musicians on a moving train and observers standing on a train platform confirmed Doppler’s theory using sound. When an object moves towards a stationary or motionless observer, the sound waves are compressed slightly, resulting in a higher pitch than the actual sound. After the object reaches the observer and moves away, the apparent sound becomes lower in pitch, because the sound waves are slightly stretched.
Longer sound waves have a lower pitch and the result is a sound that appears lower than it actually is. The experiment is easy to reproduce by listening to a vehicle approach an observer, then pass and move away. If the vehicle honks, the oncoming vehicle’s horn has a higher pitch, which will change to a lower pitch as the vehicle passes and drives away.
This Doppler effect can be used in an ultrasonic flow measurement device. Ultrasonic sounds are very high frequencies above the range of human hearing. They can pass through many human liquids and tissues before being absorbed, making them useful in medical diagnostics and industrial applications. An ultrasonic Doppler velocity measurement takes advantage of the frequency shift when sound waves are reflected off moving liquids.
The best results are obtained when an ultrasound Doppler unit measures a liquid containing bubbles or solid particles. Ultrasonic sounds do not reflect well from clear or very thick liquids because a reflection of the sound back to a receiver is needed to measure the liquid flow velocity. The device emits short pulses of high-frequency sound and compares the return signal with the output signal. Any difference in frequency can be calculated to obtain the velocity of the liquid.
The first applications of Doppler ultrasound measurement were in the medical field, where sound measurements were used to control blood flow in arteries and veins without having to perform surgery. Applications have also been developed to monitor a fetus during pregnancy by observing its heartbeat and blood vessels. Devices developed in the late 20th century could show visible movement of heart valves to diagnose defects and blockages.
In an industrial application, the ultrasonic Doppler measurement works best if the sound is sent into the liquid at an angle other than 90°. Particles or bubbles in the liquid must move towards or away from the device in order for it to accurately measure velocity. A full tube is required for proper measurement, because a partially filled system will not return a usable audible signal for measuring speed.
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