How SAW technology can improve liquid flow measurement

Video: Engineers at Bürkert have developed a new range of flow measurement devices based on surface acoustic wave (SAW) technology that enable the highly accurate measurement of liquid flows. Rachael Morling visited the company at its facility in France to learn about the technology built into the new FLOWave sensor range. Watch the video here!

There are a wide variety of technologies available on the market for measuring flow, all of which have their strengths and weaknesses depending on which application they will be used in. As a result, numerous factors need to be considered to ensure you select a technology which fulfils the demands of the application as closely as possible. But what if I was to say that research and development by one company has resulted in a ‘breakthrough in flowmeter technology’?

This was the claim by Bürkert Fluid Control Systems when it held its press conference at its headquarters in the Alsatian municipality of Triembach-au-Val in France. Established in the beautiful Vosges mountains in 1965, the facility focuses on the development and production of industrial sensors for the measurement of flow rates, analytical values like pH and conductivity, level, pressure and temperature.

Research and development is of the utmost importance here, with 8% of the site’s annual turnover regularly invested in R&D, resulting in the facility becoming a competence centre for industrial sensors. In fact in the engineering department, 38 employees focus on optimisation of the company’s portfolio and the development of new products.

So, when it came to developing a new flow measurement solution, the team understood that some existing technologies have limits in use and potential weaknesses. They also realised that there are many flowmeters and brands that are already well established in the marketplace, and that developing a product just like these would not bring any new advantages to potential customers. The team therefore compiled a list of the features that they believed would make an ‘ideal flowmeter’ and it was at this point that they decided that no product covered everything on that list. They therefore began developing a product that would encompass as many of the ‘ideal’ features as they could, while avoiding the compromises of other designs.

The next step for Bürkert was to look for a technology that would form the basis of the new product, one that would be capable of fulfilling all the requirements while offering even more benefits to users – and surface acoustic wave (SAW) technology proved to be the solution.

A local university team had already started to research the principle and partnered with Bürkert for development and production of the design.

The result was FLOWave, a compact and lightweight solution for liquid flow measurement. This uses new technology based on SAW to deliver unrestricted, highly accurate measurement – even in turbulent or laminar flow.

What is SAW technology

SAW technology uses ‘wave propagation’, as in seismic activities. In the FLOWave sensors, the surface acoustic waves are excited on a pipe surface by a piezo-electric interdigital transducer specially developed by Bürkert for this purpose. Similar to an optical waveguide, waves are sent on a zigzag path through the measured medium – in this case the fluid – in acoustic waveguide technology.

The Rayleigh angle, below which the sound waves are decoupled from the surface of the tube and emitted out into the fluid, differs individually for each propagation velocity of the waves. In combination with the temporal curves, the reception signal characteristics and, depending on whether the signals have passed through the medium singly or multiply, an ‘acoustic fingerprint’ of the medium is created. From this, the volume flow rate, density and temperature, and so mass flow rate, can be determined, as well as additional information about the medium itself.

The radiation of the surface waves into the medium and the multiple reception of the signals produce a measurement that is independent of the flow profile and resistant to interference. Since the procedure requires no elements within the pipe, the diameter and therefore the flow resistance remain unchanged.

One major benefit of inline flow measurement in fluids by means of SAW technology is that there are no internal components or constrictions on the measuring tube, explains the company. The inside of the tube can be manufactured with the same surface quality as the rest of the pipeline and is identical to any other straight pipe section with respect to hygiene, cleaning and flow conditions. Furthermore, there are no pressure drops and no effect of fluids on the sensor elements. The only required specifications are the sensor size and the process conditions.

FLOWave uses at least four of the interdigital transducers, and these are located on the outside of the measuring tube, ensuring they have no direct contact with the liquid. Each transducer can act as a transmitter or receiver.

The absolute time taken for the wave to travel from the transmitter to the receiver depends mainly on the tube diameter and type of liquid. The difference between the wave propagation time in the forward and backward direction is proportional

to the flow. The analysis of all the signals and comparisons based on different criteria such as amplitude, frequency and runtimes, allows evaluation of the quality of the measurement, the existence of gas bubbles or solids, plus the kind of liquid.

A new development

The benefits offered by SAW technology have resulted in the development of what the company claims is a ‘revolutionary’ new flowmeter. It is, for example, able to measure volume flow as well as other process values; it offers a very large measuring range as well as measurement at high pressures; it is highly accurate and reliable; and the sensing element is not in touch with the liquid – enabling use in hygiene-sensitive applications. It can also be installed in any position.

FLOWave is compact in size, with both the housing and the tube manufactured entirely from stainless steel. It is currently available in DN15, DN25, DN40 and DN50 pipe diameters, with clamp connections in conformity with various standards. Nominal pressures up to PN40 initially cover the majority of pressure stages that occur in applications.

The SAW interdigital transducer sensors are fixed components of the measuring tube and are connected to the transmitter, which contains both the user interface and the necessary output signals. The transmitter is based on Bürkert’s new electronics platform – the Efficient Device Integration Platform (EDIP) – which was developed by the company to ‘open the door to industry 4.0’ for its products.

Of additional benefit, the display is a separate module, so the meter can be used with or without this. If used, the display can be placed beside or above the transmitter and can be rotated in 90° steps. It requires a 24 VDC power supply.

The electrical connections on the FLOWave transmitter comprise two M20 cable glands and a third gland with an M12 plug. The output signals include one analogue output (AO, 4-20mA) and one digital output (DO); while a third signal can be switched between AO and DO through parameterisation.

Applications and future developments

The FLOWave measuring system currently provides very accurate flow and temperature data, the company explains. In the future, it will allow density measurement to be used to determine the mass flow rate.

Thanks to its design, the flowmeter can work with a stagnant liquid, meaning reliable flow figures are available for even the smallest flow volumes. The technology also enables it to recognise quick flow changes reliably, making it suitable for fast filling processes.

Of additional benefit, explains Bürkert, factors such as system vibration in the plant, magnetic and electrical effects, and the conductivity of the liquid, have no effect on the accuracy or reliability of the flow measurements carried out by FLOWave.

Following its initial market introduction, FLOWave will be suitable for hygienic applications and for the measurement or monitoring of water and similar liquids. The intention is to extend the scope over a short time to include a much wider range of applications. As an example, low conductivity water is often used within the pharmaceutical industry. So, Bürkert examined what this industry required.

It turned out that customers needed a product that offered a range of features, including flexibility, cleanability, compact size, light weight, easy installation, and one that would also be compliant with numerous standards. There are, for example, general requirements for flow measurement in ‘clean utilities’ – the material needs to be to ASME BPE 2012 part MM, as well as FDA approved; the surface finish needs to be according to ASME BPE SF 1; it has to be drainable and cleanable according to ASME BPE SD; installation must be according to cGMP; there must be no dead legs, etc.

During future development, the technology will be extended to many more relevant applications.

There will, in fact, be continued development of the line, with the ultimate goal of it being a multi-parameter unit with high-end functionality and performance with a range of functions to support applications in a range of industries.

Applications in the coming years are expected to continue to be within the hygienic industries, in addition to dairy related food production, breweries, chocolate and jam production, and also for liquid differentiation.

In the future, therefore, more industries and applications will be able to benefit from the use of flow measurement carried out using SAW technology.