Ultrasonic flow meter

A ultrasonic flow meter measures the velocity of a liquid or gas through a pipe using acoustic sensors. This has some advantages over other measurement techniques. The results are slightly affected by temperature, density or conductivity. Maintenance is inexpensive because there are no moving parts.

Ultrasonic flow meters come in three different types:

• Transmission (contrapropagating transit time) flowmeters
• Reflection (Doppler) flowmeters
• Open-channel flowmeters

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Transit Time flowmeters

The most commonly used is the transit time flowmeter which is applied for clean or nearly clean fluids, as well as natural gas pipe systems.

Transit time flowmeters work by measuring the time difference between an ultrasonic pulse sent in the flow direction and an ultrasound pulse sent opposite the flow direction. This time difference is a measure for the speed of the fluid in the path of the ultrasound beam in terms of the speed of sound, c, in the fluid. By using the absolute transit time and the length between the ultrasound transducers, the current speed of sound is easily found.This can be adversely affected by many things including conductivity and solid content.

By using at least 3 transducers, an "ultrasonic anemometer" measures wind speed and direction in open air, with no moving parts.

Doppler flowmeters

Doppler flowmeters are used for slurries, liquids with bubbles, gases with sound reflecting particles, or single phase turbulent clean liquids.

Measurement of the doppler shift resulting in reflecting an ultrasonic beam off either small particles in the fluid, air bubbles in the fluid, or the flowing fluid's turbulence is another recent, accurate innovation made possible by electronics.

Open-channel flowmeters

Open channel flow measurement is based on upstream levels in front of flumes or weirs although in-channel devices using cross-correlation and doppler are also used. Ultrasonic open channel flow meters are widely used to measure flows in streams, rivers and where hydraulic head is an issue. Many examples of open channel flow meter exist in the world today with varying degrees of accuracy.

References

• Lipták, Béla G. Process Measurement and Analysis Volume 1. CRC Press (2003), ISBN 0-8493-1083-0 (v. 1)
• Ultrasonic Acoustic Sensing Brown University
• Electronics-Manufacturers.com
• Ultrasonic Flaw Detection for Technicians, Chapter 2, 3rd ed., 2004 by J. C. Drury (~5 pages)