The Limitations of Ultrasonic Flow Measurement Techniques

No liquid flowmeter is perfect, and even expensive Coriolis flow devices suffer from some drawbacks. This article outlines the current challenges for this exciting flow measurement technology.

The Limitations of Ultrasonic Flow Measurement Techniques

Image Credit: Image Credit: Titan Enterprises Ltd

Attenuating Fluids

Ultrasound will struggle to travel through some fluids, regardless of how good an ultrasonic flowmeter is. It is possible to increase gain in an ultrasound system, but an already weak signal will mean that gain also increases background noise, ultimately reducing the flow range of the flowmeter.

One option is to move the sensors closer together in the design of the flowmeter, but because the distance between the crystals is directly linked to the flow induced phase shift, this will result in a reduction in the device’s dynamic range.

In some cases this may be feasible because the underlying technology already offers a wide flow range, and restricting this somewhat would not be a problem. This is not an option for standard products, however, meaning that an ultrasonic flowmeter would not be appropriate when working with particularly badly attenuating fluids.

Suspended Air Bubbles

Suspended air bubbles generally have a similar effect to an attenuating fluid, with the bubbles effectively acting as tiny resonators to absorb any ultrasonic vibrations. The air bubble’s size will dictate the resonant frequency, so if these are exactly the wrong size there will be little or no signal.

Small amounts of a viscous recirculating fluid can contain gases in suspension, so any recirculation system must utilize enough volume of fluid to allow for sufficient de-gasing. A good starting point involves the separation of inlet and outlet to ensure gases are not introduced through “splashing”. Large air pockets will completely eradicate the signal, so an ultrasonic flowmeter is an almost perfect air detector.

Pulsating Flow

Pulsating flow poses problems with most types of flowmeter, and it is also relevant to ultrasonic flowmeters. An electronic flowmeter will always possess an operating cycle, and if the frequencies of measurement periods happen to coincide, or this takes place in an unsuitable period, flow pulsation aliasing may occur.

This can result in a highly inaccurate signal (flow reading); for example, a cycle time of 100 Hz coupled with an actual measurement period of 5 ms (half the time) faced with pulsations at the same frequency and peak of the flow would result in the measurement period of the reported flow being too large.

On the other hand, should the flow pulsation be exactly at minimum flow, then the reported flow will be too low. Erroneous readings will also be caused by in-between and multiples of this example.

There are means of removing pulsations available, which can eliminate this problem, but Titan Enterprises is actually working on an approach to measuring accurately despite the prevalence of these pulsations.

Limited Range of Speed of Sound in the Fluid

Ultrasonic flowmeter manufacturers will generally stipulate the maximum and minimum speed of sound in the fluid, over which their specific flowmeter will function. As such, fluids that have a particularly high or low velocity may pose problems.

For example, water is generally quoted as 1482 m/s, while glycerine is around 1920 m/s and many other refrigerants are around 600 m/s. Designing an ultrasonic flowmeter capable of operating over this whole range with no user input is especially difficult.

Thankfully, Titan Enterprises has successfully developed an ultrasonic flow system that can operate over a wide range to accommodate most fluids.

This information has been sourced, reviewed and adapted from materials provided by Titan Enterprises Ltd.

For more information on this source, please visit Titan Enterprises Ltd.

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