A Perfect Match
9 May 2019
Optimizing clamp-on flow measurement with Lamb-wave sensors
Clamp-on ultrasonic flowmeters are a cost-efficient, low-maintenance way to measure volumetric flow through a pipe. these ben-efits stem primarily from the external sen-sors that characterize clamp-on technology. unlike traditional insertion sensors, their clamp-on counterparts do not require any pipe alterations or process downtime for in-stallation; instead, the sensors are mounted on the exterior of the pipe and measure flow by transmitting acoustic signals through the pipe wall and into the fluid.
Despite never making contact with the me-dium, clamp-on sensors can demonstrate very high levels of accuracy and repeatabil-ity on par with or better than other flow mea-surement technologies. the key to achieving optimal performance is properly matching the sensors to the pipe being measured. As clamp-on technology can operate on pipes of varying diameters, materials and wall thicknesses, it is important for suppliers to offer a suitably broad selection of sensors with differing frequencies. this will help users to achieve the best possible match between the sensors and the resonant fre-quency of the pipe wall, resulting in more accurate flow measurement.
Hear Mode vs. Lamb Wave
Clamp-on ultrasonic flowmeters can be equipped with two different types of sensors – shear mode or lamb wave (also known as wide beam). shear-mode sensors inject an acoustic signal into the pipe wall, effectively forcing the signal through the pipe and into the opposing sensor (Figure 1).
Shear mode performs reasonably well with-in a limited range of process conditions. However, although easier and less costly to employ than lamb-wave sensors, shear-mode sensors are sensitive to suspended solids and bubbles exceeding a few percent by volume – which can lead to poor perfor-mance or even complete loss of the receive signal. these sensors are best suited for flow surveys or check metering applications that require a portable flowmeter kit with limited capacity.