The operation of Bronkhorst ES-FLOW flow meters is based on the propagation of ultrasound waves inside a very small, straight sensor tube, without obstructions or dead spaces. At the outer surface of the sensor tube multiple transducer discs are located, which create ultrasonic sound waves by radial oscillation.
Every transducer can send and receive, therefore all up- and down-stream combinations are recorded and processed. By accurately measuring the time difference between the recordings (nanosecond range) the flow velocity and speed of sound is calculated. Knowing these parameters and the exact tube cross-section, the ES-FLOW is able to measure liquid volume flows in the range of 2 up to 1500 ml/min.
The distinctive character of this flow meter is that it’s capable to measure the actual speed of sound, meaning that the technology is liquid independent and calibration per fluid is not necessary.
Source of below information: Wikipedia.
Please note, these techniques are completely different to the technology that is applied in the ES-FLOW series and has been provided for comparison and education only.
Transit Time Principle
Ultrasonic flow meters measure the difference of the transit time of ultrasonic pulses propagating in a flowing fluid. This time difference is a measure for the average velocity of the fluid along the path of the ultrasonic beam. By using the absolute transit times both the averaged fluid velocity and the speed of sound can be calculated. This measurement principle is only possibly for large bore tubes, for higher flow ranges than the scope of Bronkhorst.
Doppler Effect Principle
Another method in ultrasonic flow metering, also not well suited for very low flow rates, is the use of the Doppler shift that results from the reflection of an ultrasonic beam off sonically reflective materials, such as solid particles or entrained air bubbles in a flowing fluid, or the turbulence of the fluid itself, if the liquid is clean.
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