Ultrasonic Flow Meter - Some Basic Principles

By: Jim Furness

Ultrasonic flow meter technology has much improved over the last few years and now forms a viable flow measurement technique compared to other methods such as turbine and electromagnetic type systems. The first Doppler ultrasonic flow meter units became available in the early 70's. To begin with, as with all new technologies they were not very accurate, difficult to install and costly. However, the newest Time of Flight ultrasonic flow meter and Doppler types are proving to be both reliable and accurate. Accuracy has improved significantly with most ultrasonic flow meter manufacturers claiming 0.5% accuracy if the meters are correctly installed.

With these two technologies, which ultrasonic flow meter should you use?

Although Doppler ultrasonic flow meters use an older technology, they are ideal for liquids with air bubbles or slurries with significant solid particles. They determine the flow using the Doppler shift method, by measuring the change in frequency created by an object moving towards, or away from the measurement point. Normally liquids with a minimum of 100ppm concentration at 100microns particle size or bigger would be suitable for a Doppler ultrasonic flow meter. One stumbling block of most Doppler ultrasonic flow meter systems is that they are less accurate in low flow conditions.

A transit time ultrasonic flow meter is ideal for clean fluids such as water and oil. These meters are more advanced than Doppler ultrasonic flow meter types due to the advanced calculations used to determine the flow rate. Two transducers are used, the first sends an acoustic signal which is bounced off of the bottom of the pipe, the second transducer receives the signal. The ultrasonic flow meter is then able to determine flow by calculating the time it takes to receive the signal.

Portable ultrasonic flow meter kits are now available for plant wide surveys with minimal fuss and no down time. The first portable ultrasonic flow meter units were heavy and quite large, basically a luggable design. As ultrasonic flow meter design has improved over time, the instruments have become much smaller, lighter and more power efficient and are truly hand portable. Also many ultrasonic flow meter models now have integral data loggers, allowing the user to leave the meter on site so it can record average flow rates and flow totals.

An ultrasonic flow meter will be most accurate when fitted on a long, straight piece of pipe, free from obstructions such as elbows and valves. Also, any rusty pipes should be cleaned before using an ultrasonic flow meter. Ultrasonic jelly should also be smeared under the sensor to ensure a solid contact on the pipe.

The biggest advantage to using an ultrasonic flow meter is maintenance. Older insertion meters usually require the plant to be shut down while installing. However an ultrasonic flow meter can be installed without stopping the process. Also as an ultrasonic flow meter has no moving parts it cannot wear out.

Both Doppler and transit time portable ultrasonic flow meter kits are available from Omni Instruments for hire or purchase.

By Richard Burgess
OmniInstruments.net

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