Ultrasonic flow meters based on the Doppler Effect, have been around for decades and they provide a great way to measure full pipeline flow without affecting the fluid passing the measurement point. Like all flow meter measurement methods, they have their limitation of possible applications. Doppler Ultrasonic flow meters suffer from not being effective at low flows where the pipeline velocity is very low. They also have problems with clean water or pure liquid flows in that they measure the speed of particles in the flow. At the other end of the scale, they also have problems with high particulate sludge or slurry flows in that the ultrasonic pulse cannot penetrate the flow medium.
Sierra Instruments' Innovasonic flow meter
In recent times, the transit-time ultrasonic flow meter has been replacing the traditional Doppler-Effect flow meters and now they are beginning to supplant electromagnetic flow meters for municipal water and waste water flows. They are normally strap-on units to the pipeline and are thus quite unobtrusive. This is in stark contrast to my early experiences of overseeing the construction of a large waste water treatment plant that I had designed. I remember being surprised at the bulk (and the expense) of an 18” diameter in-line electromagnetic flow meter being installed.
Well how does it work? A digital transit-time ultrasonic flow meter is a pair of transducer sensors are mounted at opposite sides of the acoustically-conductive pipe so that the direct path between them is at some 45 degrees to the flow direction. A 1 MHz ultrasonic sine-wave signal is fed to each transducer sensor and is received by the sensor in the other position. The signal flowing upstream goes slower than the one going downstream and when the two digitised sine-wave output signals are compared by the on-board transmitter software, the phase shift gives the velocity flow.