Ultrasonic flow meters: how do they work?

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. 

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.

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. 

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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