Flow Measurement Will SAW–Technology Become the Perfect Wave for Flow Measurement?

Author / Editor: DOMINIK STEPHAN* / Dominik Stephan

New flow measurement technology – More often than not it is about inner values, but sometimes, it is also about what is lacking inside. That is the case with ultrapure water, and with a new flow measurement technology which works without any built-in sensors. How does it all come together? With the perfect wave, of course.

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(Pictures: Bürkert)

Sometimes the presence of valuable substances and trace elements is important — and sometimes the absence of any foreign contaminants is important, as with ultrapure water. Highly purified and virtually free of foreign matter, it is used for medicines, injections and as a process fluid in chemical analysis. Certain difficulties arise, however, when the question is to find out whether ultrapure water flows through a pipe or not — and if so, in what quantity. For the fluid, which is subject to the most exact demands and specifications, must not to be made impure.

An Overview of Flowmeters

A flow meter typically has a sensor in the pipe. Regardless of the principle on which the process works, measuring elements will come into contact with the process liquid at some point. There are exceptions to every rule, of course: Alternatives, such as magnetic inductive measurement (MID) or Coriolis mass flow meters, exist, but these too have their limitations.

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MID measuring instruments or “mag meters” require a given minimum conductivity and a minimum liquid flow rate. They are just as ill equipped to cope with non-flowing liquids and high temperatures as they are with iron deposits. The low conductivity of ultra-pure water alone makes mag meters unsuitable for these applications.

That just leaves the Coriolis method — but these flow meters are extremely complex, large and expensive and therefore difficult to justify for certain applications. But wait — who says it is essential for a flow meter to have a sensor in the pipe?

From Lab to Production Line

“We were trying to find a gap in the market,” said Volker Erbe, product manager for sensors at Bürkert, “something which would send a clear message to customers that this would give them value added”. The specialists in fluid processes from Ingelfingen, Germany, embarked upon a radical rethink of the principle of flow measurement. On their journey they hit upon a process which works without any inbuilt fittings at all. Its secret? Surface acoustic waves — or SAW for short. These structure-borne sound waves spread out over a surface without penetrating very far into the material.

Research into SAW–waves was already underway at a number of institutes before Bürkert got wind of the subject. Could it also be possible to use the technology for flow measurement? Initial laboratory experiments gave some indication of the potential of the process but the real development work was only just beginning.

What to Keep in Mind When Developing Field Devices

“Success in the laboratory is a far cry from success in the field,” said Erbe, “because in order to work, a process needs to be robust and feasible for mass production. This added to our task in that we had to achieve the necessary reproducibility.” The development work was in full flow from about 2009 onwards, both at Bürkert’s French site in Triembach au Val and at the parent company in Ingelfingen. “Even today the work is still ongoing to fully optimise the development for production,” added the product manager.

Further, the sensitive electronics call for extra care in production — which is why the colleagues in Triembach insist on robots for the welding work because of the supreme precision and reproducibility which they offer.

SAW Development Makes Waves in Flow Measurement

The time to unveil the Flowave range came for Bürkert at the Hannover Messe 2014 trade fair — the first flow meter to use microacoustic surface waves to determine flow rate and temperature. The surface acoustic waves are induced by an interdigital piezoelectric transducer on a pipe surface. These waves are sent through the medium on a zigzag path in a similar way as with optical waveguides.

A pulse is sent from both sides of the measured section at the same time, allowing the delay time to be calculated and the flow speed to be deduced. The development certainly made waves, causing a stir in professional circles and beyond: the members of the jury at the world’s biggest industrial fair were so impressed that they nominated Flowave for the coveted Hannover Messe Hermes Award.

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The Rayleigh angle at which the sound waves are released into the fluid differs for every propagation velocity. In connection with the cycles and the received signal characteristics, and depending on whether the signals have flowed through the medium once or more than once, this generates an “acoustic fingerprint”.

This unique signature can be used to calculate the flow rate, density and temperature. The fanning out of the surface waves into the medium and the multiple reception of the signals make the measurement independent of the flow profile and resistant to fault.

From Flow Measurement to Multi–Parameter Sensors

There are even more possibilities, however, with the delay time measurement method. “If you use other measuring instruments,” continued Erbe, “something has to flow. We take a different approach. If the fluid is not moving in the pipe, we can measure it. Then the delay delta is just zero.”

The technology has potential for further growth due to the fact that this method also identifies the temperature and density of the medium, almost as a “by-product” as it were. Given that mass flow rate can be easily calculated from these data, there is nothing standing in the way of its development into a multiple parameter solution. But that is not all. “We are also thinking in terms of viscosity and concentration measurement,” confirmed Erbe, “and that really is high-end!”

Just What Customers Demand

Until then Flowave has to persuade the users of its benefits, starting with hygienic processes or water of low conductivity (e.g. ultra-pure water). Other options, which so far could only be measured with Coriolis instruments, shall follow at a later date. “We asked ourselves what was uppermost priority for our customers,” explained Erbe, “in view of the fact that they need to spend an exorbitant amount of money on something which they do not even need at this junction.”

He conceded that the Coriolis measuring technique does have its justification, but in many cases the accuracy of a mag meter would be entirely sufficient. If it would work, that is — which cannot easily be guaranteed with media of poor conductivity or non-flowing liquids. “There have been no viable alternatives yet because they cannot supply the necessary capacity for cleanliness, hygiene or accuracy.”

Ultrapure Water for Starters

In contrast to common setups, the SAW technology developed by Bürkert is a simple stainless steel pipe on the process side with the exact same surface quality as the rest of the tube — all thanks to the “perfect wave” that is SAW.

Pressure drops and contamination are things of the past, and the medium can no longer interfere with the measuring elements. There is simply nothing in the pipe that could be a cause of concern for the process engineers. Less is sometimes more, after all — and not just in the case of ultrapure water.