Ultrasonic flow meters: how do they work?

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. A few ultrasonic flow measurement principles do exist, and as such an ultrasonic flow meter is very versatile. We can distinguish ultrasonic flow meters based on ultrasonic wave technology and the more conventional ultrasonic flow measuring principles like the Doppler effect and Transit time principle. Not all these measurement principles are suitable for low flows of pure liquids.

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1. Conventional ultrasonic flow measuring principles

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, which is useful to detect constrictions in these blood vessels. The conventional Doppler effect is applied here to measure the flow rate.

Doppler effect principle
Ultrasonic flow meter working principle - Doppler effect

Ultrasonic flow meter working principle - Doppler effect

This Doppler effect, also known as Doppler shift, is a well-known phenomenon in everyday life that you can experience when you hear an ambulance with blaring sirens passing by. You may have noticed that the tone of the siren appears higher when the ambulance approaches you (higher sound frequency), suddenly becoming lower as the ambulance passes by and moves away from you (lower frequency). This is explained by the fact that sound waves are compressed to some extent when the ‘emitter’ moves towards you at a certain speed, resulting in 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 is directly linked 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, and hence of the fluid containing these particles. This shows the limitation of the Doppler effect for liquid flow rate measuring: the liquid needs to contain particles - solid particles or entrained air bubbles - that reflect the ultrasound waves. This technique is therefore not useful for liquids with particles. 

Transit time principle
Ultrasonic flow meter working principle - Transit-time Principle

Ultrasonic flow meter working principle - Transit-time Principle

Another conventional way to use ultrasound for flow measurement, which does not rely on particles in the flowing liquid, is by 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. Combined with the known cross section of the tube, the volumetric flow rate is calculated. 

Flow measurement based on transit times works best for large-diameter pipes and high flow ranges with practically measurable transit time differences, so not for small-diameter tubes and low flows. Sound travelling from the emitter to the receiver in a small-diameter tube will result in a very tiny time band; this becomes more difficult because the sensor tube has a smaller diameter.

Although this shows that (ultra)sound can be used for flow measurement, principles applying the Doppler effect or conventional transit times are not suitable for pure fluids or at low flows. To this end, another solution is available: ultrasonic wave technology.

Ultrasonic sensor ultrasonic wave principle
Ultrasonic wave technology

2. Ultrasonic wave technology

How to conduct flow measurement for pure (as well as non-pure) liquids with a low flow rate down to 0.4 litres 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. On the outer surface of the sensor tube, multiple transducer rings are positioned radially around along the tube, which create ultrasonic sound waves by oscillation. Every transducer can emit and receive - so 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 (in the nanosecond range) to calculate a reliable flow velocity of the fluid. This effect is further enhanced by filtering out disturbing sound waves in a smart way. 

3. Bronkhorst flow meters based on ultrasonic flow measurement

Bronkhorst ultrasonic flow meters of the ES-FLOW series can measure and control volumetric liquid flow rates in the range of 0.4 up to 1500 ml/min using ultrasound. They effectively determine the flow velocity which, when multiplied by the known tube cross-section inside the device, results in volumetric liquid flow rates. To learn more about Mass Flow vs Volume Flow, check our knowledge base.

The robust and versatile ES-FLOW ultrasonic flow meters are insensitive to external vibrations and in principle can work with liquids that contain small particles and gas dissolved in liquid. Furthermore, ultrasonic wave technology as used in this device can automatically use the actual measured speed of sound, meaning that the technology is liquid independent and calibration per fluid is not necessary.


How can you benefit of the ES-FLOW ultrasonic flow meter?

  1. A versatile flow meter for all liquids: Do you have changing process conditions or do you make use of various liquids like additives or solvents? The ES-FLOW technology is fluid independent, therefore recalibration is not needed when the liquid changes. Even non-conductive liquids as demi water or oil can be measured.
  2. Compact intrinsic hygienic design: Cleaning processes can be pretty time consuming. But due to the straight sensor tube design with no dead volume and small internal volume, particles have reduced chance of clogging the instrument. This design results in a easy and quick cleaning procedure, consequently the process down time is limited.
  3. Advanced signal processing: The on-board PID controller is the perfect choice for driving any control valve or pump. This enables you to establish a complete, compact control loop with fast response time. ES-FLOW can also operate as a stand-alone dosing unit.

Would you like to learn more about utrasonic flow meter for low flow?

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ES-FLOW in thermal spraying process

4. Ultrasonic flow meters in customer processes

In the confectionery industry; candy production

The contained straight sensor tube with no obstructions or dead spaces and self-drainability meets the sanitary standards required for food and beverage processing. As such, this is one of the application fields in which the ES-FLOW ultrasonic flow meter is applied. These instruments are used to measure the amount of colouring & flavouring agent and acid that are supplied as additives to a candy production process, improving the process quality control. 

Ultrasonic flow meters in HVOF spraying process

The ES-FLOW ultrasonic flow meter series is used in HVOF spraying solutions to apply coatings that improve 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 desired process pressure. 

ES-FLOW ultrasonic flow meter in mRNA vaccine production

In mixing systems for mRNA vaccine production, ES-FLOW ultrasonic flow meters are used for CIP (clean-in-place) purposes, to be able to flush the process lines with liquid cleaning agents between different batches. 

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