Surface Acoustic Waves (SAW)
Bürkert customers use SAW technology for inline flow measurement of fluids with the innovative and tried-and-tested Type 8098 FLOWave flowmeter. In this glossary article, you’ll learn more about the basics of SAW technology, the measuring principle, the uses and benefits, and examples where this technology has been used successfully for years.
Basics
The abbreviation “SAW” stands for “surface acoustic waves”. It therefore means waves that propagate along surfaces. The type of propagation is comparable with acoustic wave propagation in seismic activities, such as an earthquake.
These surface waves are artificially generated in miniaturised form by means of SAW technology. In the case of the FLOWave flowmeter, these surface waves now propagate along the sensor measurement tube, and these acoustic surface waves can then be used to measure the flow of fluids. Bürkert has had the use of SAWs for flow measurement patented.
Declaration of the SAW measurement principle
The surface waves are generated by interdigital transducers (IDTs for short). These are placed at set areas of the FLOWave measurement tube. An electrical impulse emitted with a frequency within the 1 MHz range excites the IDTs, generating SAWs.
A wave front propagates along the tube surface, as well as through the medium, from the initial centre of excitement that arises. The wave front is also “reflected” by the measurement tube and shifts through the medium in the pipe several times.
Detailed description of the process shown in the video
1st step
Interdigital transducer 1 initiates a surface acoustic wave (SAW)
2nd step
- The SAW propagates over the measurement tube surface, but also couples into the fluid
- The extraction angle is also known as a Rayleigh angle. As this depends on the fluid, it enables the density of the medium to be measured to be determined
3rd step
- Arriving on the other side of the measurement tube, part of the wave front uncouples from the fluid again and with the measurement tube surface as a SAW, and then moves to the next interdigital transducer.
4th. step
- Another part of the wave is uncoupled again (“reflected” by the pipe) and shifts to the other side of the measurement tube in the shape of a wave front, where the same effect occurs again and the IDT receives the wave again on this side of the pipe.
- The excitation of each IDT then leads to a sequence of received signals at two other IDTs. The exact sequence and number of “wave reflections” depends on factors such as the nominal diameter of the flowmeter, the length of the sensor measurement tube and the size and placement of the IDTs.
5th step
- During the measurement process, the interdigital transducers then act as senders and receivers during the measuring procedure. First, an IDT transmits in a forward flow direction. The other then goes against the flow direction.
Determination of measured values
The time difference of the propagation period of the measurement signal in the forward direction and backward direction of the flowing fluid is proportional to the flow velocity. The volume flow can be calculated based on these measured values. The temperature is measured by an integrated digital temperature sensor.
Based on the parameters of temperature, density, flow velocity and volume flow that are determined this way, it is possible to determine the mass flow rate as well.
Further special functions derived from the process values (differentiation factor, acoustic transmission factor, concentration of certain mixtures) offer additional information on the fluid in question, such as the detection of gas bubbles or solid parts as well as recognition of media changes.
Advantages
Based on the use of SAW technology, various advantages are created for the industrial flow measurement of fluids:
Only one pipe (316L / 1.4435 stainless steel)
- No additional parts that come into contact with media
- Contactless & hygienic measurement
- No moving parts – 100% wear-free
- No narrowing, meaning no pressure drop / loss in pressure
Multi-parameter measurement
- A single device for determining several parameters
- Less effort for device / asset management
- Seamless monitoring
- Special functions for recognising gas bubbles, solid parts or media changes
Robust & industrial-suited
- Interference from plant vibrations is eliminated by the high excitation frequency of 1 MHz.
- No influence on measurement by magnetic or electrical effects
- The measurement is not dependent on the conductivity of the medium and is possible even if this is low or not existent
Compact & light
- Reduced space requirements allows for the downsizing of machines and systems
- Simple installation and commissioning
- No additional brackets or reinforcements required
- Minimal maintenance required, e.g. at prescribed maintenance intervals
Application examples
SAW technology is used particularly in hygiene-sensitive environments, in the pharma & biotech and food & beverage sectors. Hundreds of customers have trusted innovative FLOWave flow measurement for years - whether it is for ensuring the quality of pharmaceuticals, the detection of media changes, for flow measurement in WFI circuits or for the dosing of edible oil.
Further information as well as examples of successfully implemented FLOWave projects for leading, innovative companies in the pharmaceutical and food industries can be found on the following website.