Surface accoustic wave (SAW) technology offers flexibility and can lower costs.
by Ian Webster

Coriolis flow meters first came into commercial use in the 1970s, and have been developed and refined to meet a growing number of applications across a wide range of industries ever since. The demand for more advanced, non-contact measurement technology brought about the rise in ultrasonic, electro-magnetic and Coriolis technology. However, where fluid density is not required, there is now an alternative that offers increased accuracy and clean-ability.

SAW flow meterImage 1. Surface acoustic wave (SAW) technology can be used to deliver precision flow measurements from a smooth, uninterrupted flow-through pipe section. (Images courtesy of Burkert)

The development of new flow meter designs based around surface acoustic wave (SAW) technology delivers precision flow measurement from a smooth, uninterrupted flow-through pipe section, meeting all the requirements of a hygienic application.

Challenging the Constraints of Flow Measurement

Coriolis flow meters are regarded as the top specification flow meter due to their versatile capabilities for both fluids and gases. The initial cost of such devices can be high, and the design requires the flow direction to be determined before installation. Once installed there will be a pressure loss across the flow meter, depending on the specification.

In addition, the designer may have to choose between drainability and accuracy. The more accurate bent-tube Coriolis flow meter can be difficult to drain and may present a contamination issue. In such cases it may be necessary to specify the straight-tube Coriolis device to ensure hygiene standards are met, but this can be at the expense of accuracy.

In terms of liquid flow measurement, there is a need for a compact, non-contact device that is accurate irrespective of media characteristics, flow direction and flow conditions. In answer to that need, engineers have developed SAW technology to create a compact device that has the same hygienic properties and clean-ability as the rest of the process pipework.

Simplicity Through Advanced Design

The main principle of this flow measurement device is based on the wave propagation forms similar to seismic waves, which start from an initial point of excitation and spread along the surface of a solid material. SAW technology uses at least four interdigital transducers that are located on the outside of the measuring tube and therefore have no direct contact with the fluid. Each transducer acts both as a transmitter and as a receiver.

The signals that are received are processed by the on-board electronics and software, which were developed in conjunction with a university studying SAW technology and its applications. The analysis of all the signals and comparisons based on criteria such as amplitude, frequency and run times allows evaluation of the measurement, the existence of gas bubbles or solids, as well as the kind of liquid.

The internal surface of the flow measurement tube can be manufactured to the same specification as the rest of the production pipeline, which means that hygienic cleaning processes, including clean-in-process (CIP) and sterilization-in-place (SIP), can be maintained to the highest standard.

Accuracy Without Compromise

Until now, it has been difficult to match the accuracy levels of existing technology without introducing other weaknesses to the application. Magmeters rely on the process fluid being conductive, while other designs require components to be in direct contact with the fluid, making it difficult to meet the necessary hygiene standards.

Stainless steel pumping meterImage 2. The meter using SAW technology meets all the requirements of a hygienic application.

Accuracy and repeatability often rely on laminar flow, which can require certain lengths of straight pipe upstream of the flow meter. Continued testing and development of the SAW technology has minimized this requirement. In addition, specifically designed, on-board software introduces an automatic calibration process that can maximize accuracy, even in applications that require short pipe runs.

Another issue that dominates discussions around flow meter selection is weight, especially in process skids, where it needs to be minimized in order to retain the portability of the skid.

In some cases, a DN25 unit can weigh over 15 kilograms, compared to just 2.4 kilograms for products that have been created using SAW technology.

In situations where the flow meter needs to be removed from the installation for any reason, such as cleaning, the implications for manual handling need to be considered.

It may require additional mechanical assistance, which will add to the overall time for maintenance or cleaning.

Finally, any discussion regarding flow meters would not be complete without some mention of the initial cost.

Due to the technology and the scale of these more traditional flow meters, even the most basic specification can represent a significant investment. Furthermore, the energy consumption of the two oscillators is considerably higher than flow meters that use more energy efficient technology.

Improving Technology to Reduce Costs

SAW technology offers a way to measure the flow of a liquid and its temperature without installing a large, expensive, and energy-draining piece of equipment. As well as being a leap forward in terms of size and energy use, it also enables the measuring tube to be free from parts. This cuts the risk of contaminating the medium to zero while also eliminating pressure loss, leakage and dead zones.

Furthermore, SAW technology works regardless of flow direction and conductivity of the medium, and it will be developed further to measure multiple parameters.

From the outset, the aims of this development project were to resolve the traditional issues associated with the Coriolis flow meter, such as clean-ability, pressure drop, weight, cost and energy consumption.

At the same time, the designers were also keen to launch a product that could be improved over time, using more reference data to refine the measurement algorithms for different types of media, which is very much the case with this emerging technology.

See other Flow Meters articles here.