Sound is very useful for measuring the flow rate of liquid flows, particularly ultrasound, which has a frequency beyond the frequency range that the human ear can hear. There are a few ultrasonic flow measurement principles that exist. As such, ultrasonic flow meters are very versatile. We can distinguish ultrasonic flow meters that use ultrasonic wave technology and the more conventional ultrasonic flow measuring principles like the Doppler effect and Transit-Time principle. However, not all these measurement principles are suitable for low flows of pure liquids.
In the medical world, ultrasound imaging is a way of looking inside the human body to make organs visible. As a derivative of ultrasound imaging, the flow velocity of blood in arteries or veins can be measured this way. This which is useful to detect constrictions in these blood vessels. Conveniently, the same conventional Doppler effect can be of use to measure the flow rate of low flow liquids.
The Doppler effect, also known as Doppler shift, is a well-known phenomenon in everyday life. For example, you can experience the Doppler effect when you hear an ambulance with blaring sirens passing by. You may notice that the tone of the siren appears higher as the ambulance approaches you (higher sound frequency).
Suddenly, the sound of the siren becomes lower as the ambulance passes by and moves away from you (lower frequency). This is due to the fact that sound waves compress to some extent when the ‘emitter’ moves towards you at a certain speed. The resulting sound is a higher frequency and therefore a higher tone. Similarly, sound waves expand when the emitter moves away, giving a lower tone.
Something comparable occurs when measuring blood flow velocity in ultrasound imaging. The ultrasound wave frequency will change when moving particles, like red blood cells in the blood vessel, reflect these waves. Since change in frequency directly links to the velocity of the moving (and reflecting) particles, this frequency shift is a measure for the flow velocity of the reflecting (and moving) particles. Thus, it reflects the velocity of the fluid containing these particles. However, this shows the limitation of the Doppler effect for liquid flow rate measurement. The liquid needs to contain particles, either solid particles or entrained air bubbles, that reflect the ultrasound waves.
Another conventional way to use ultrasound for flow measurement, which does not rely on particles in the flowing liquid, is by transit time. This technology relies on positioning an ultrasound emitter on one side of a fluid tube and a sensor diagonally across the tube.
With a liquid flowing through the tube, the difference in transit time of the ultrasonic wave from emitter to sensor in upstream and downstream direction is a direct measure of the liquid flow velocity. When combined with the known cross section of the tube, the volumetric flow rate is calculated.
Flow measurement using transit times works best for large diameter pipes and high flow ranges with practically measurable transit time differences. Therefore, it’s not ideal for small diameter tubes and low flows. Sound traveling from the emitter to the receiver in a small-diameter tube will result in a very tiny time band. And this becomes more difficult when the sensor tube has a small diameter.
Although this shows that ultrasound can measure flow rates, principles applying the Doppler effect or conventional transit times are not suitable for pure fluids or low flow rates. To this end, another solution is available: ultrasonic wave technology.
So, how would users conduct flow measurement for pure (as well as non-pure) liquids with a low flow rate down to 0.4 liters per minute? For this purpose, a technique based on the propagation of ultrasound waves inside a very small, straight sensor tube without obstructions or dead spaces is suitable, allowing for low flows.
In practice, this works as follows. The fluid flows through the sensor tube. Then, on the outer surface of the sensor tube, multiple transducer rings are positioned radially around along the tube. These transducers create ultrasonic sound waves by oscillation. Since every transducer can emit and receive, all upstream and downstream combinations are recorded and processed.
Allowing the mutual spacing between transducers to be big enough, the transit time differences between the recordings are sufficiently large enough (in the nanosecond range) to calculate a reliable flow velocity of the fluid. Further enhancement of this effect comes from filtering out disturbing sound waves in a smart way.
Ultrasonic flow meters of the ES-FLOW series, from Bronkhorst, can measure and control volumetric liquid flow rates from 0.4 up to 1,500 ml/min using ultrasound. They effectively determine the flow velocity. And when flow velocity is multiplied by the known tube cross-section inside the device, this results in volumetric liquid flow rates. To learn more, check out our article about Mass Flow vs Volume Flow.
The robust and versatile ES-FLOW ultrasonic flow meter series is insensitive to external vibrations. In principle, they can work with liquids that contain small particles and gas dissolved in liquid or with pure liquids. Furthermore, ultrasonic wave technology can automatically use the actual measured speed of sound. This means that the technology is liquid independent and calibration per fluid is not necessary.
The straight sensor tube with no obstructions or dead spaces and self-drainability meets the sanitary standards for food and beverage processing. As such, this is one of the application fields in which the ES-FLOW ultrasonic flow meter is of use. These instruments are used to measure the amount of coloring, flavoring agents, and acid that are supplied as additives to a candy production process. Using highly repeatable and accurate dosing improves process quality control.
The ES-FLOW ultrasonic flow meter series is ideal for HVOF spraying solutions. This coatings process improves wear, corrosion, and temperature resistance. In this application, the ES-FLOW enables better control of the coating quality and thickness by controlling a pump to obtain the necessary process pressure.
In mixing systems for mRNA vaccine production, ES-FLOW ultrasonic flow meters are ideal for CIP (clean-in-place) purposes. They are able to flush process lines with liquid cleaning agents between different batches.
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