Shear Waves and Lamb Waves

Basically, the principle of ultrasonic flow measurement of gases is the same as for liquids (more).

Classically, clamp-on ultrasonic flow measurement in liquids is made with shear wave transducers: The propagation of shear waves can be described in a similar way as the propagation of light beams. When a sonic transducer located on the pipe wall emits a signal into the pipe, a part of the signal crosses the interface between the pipe wall and the fluid and is transmitted into the fluid, the rest being reflected back into the pipe.

When working with shear waves in gases, only a small part of the ultrasonic signal sent by the emitter is transmitted through the gas to a receiver on the other side of the pipe. This is due to the high density difference between pipe wall and gas causing a high proportion of reflection at the pipe wall/gas interface. The damping of the signal is also much higher in gases than in liquids. The signal that reaches the receiver is thus only a very small fraction of the emitted signal. Sophisticated signal processing algorithms and lots of experience are thus needed for extracting desired signals and rejecting unwanted noise signals.

Alternatively, it is possible to work with another type of sonic waves: Lamb waves. Lamb waves are plate waves travelling parallel to the pipe wall. They travel quite far while continuously transmitting energy into the gas so that all in all, more energy is transferred into the gas than when using shear waves. This can be seen as the pipe wall itself being used as sonic transducer. With the Lamb wave transducers, the wave length of the signal sent into the pipe needs to be within a certain range in relation to the pipe wall thickness. Thus, a transducer of a specific frequency is limited to a certain range in wall thickness.

For shear wave transducers, there is no upper limitation in the wall thickness. They can be used without problems on pipes with very thick walls, for example in high pressure applications. At low pressures, the Lamb wave transducers are preferred as the shear wave transducers face a very low signal amplitude as a consequence of the low gas density.

Each transducer type thus has its specific application range. All FLEXIM transmitters can handle both wave types, thus allowing for an optimal adaption of the measuring system to the specific measuring task.