Flow meters based on the ultrasonic principle can be excellent solutions to flow measurement problems. Our CEO Charles Wemyss exposes five secrets to consider when selecting the best Ultrasonic flow meter.
In traditional clamp-on ultrasonic flow meters, the transducers are clamped to the outside of the pipe and an acoustic gel is essential to maintain a smooth conduction of the sound waves into the liquid. This conductive path is the key to good measurement and bubbles of air trapped in the gel attenuate the signal.
There is cross talk where the sound waves travel through the wall of the pipe between the transducers. Usually, this pipe wall is very acoustically conductive and the crosstalk has to be cancelled out electronically. Cross talk is also stronger as the viscosity increases.
Doppler ultrasonic flow meters can be used on fluids that have enough discontinuities to reflect the ultrasonic energy. For example, particles, bubbles, and eddies. Transit time ultrasonic flow meters should not be used in these circumstances and require no bubbles particles or swirls.
Clamp-on or invasive?
Critically, applications use fixed in place invasive devices with a number of separate beams to cover the whole of the flow profile. These high-end versions cost tens of thousands of pounds and require installation between flanges. A 4-path 600mm unit is a metre long, weighs over 1.25 tonnes and requires 10 metres of straight length. Promised accuracy might be ±0.2% but does not handle flow velocities less than 0.6 m/s as well.
The contrast with clamp-on transit time is stark and set up in a few minutes. No pipe changes required therefore no process shutdown. Weight is around 1.25 kg, but straight length requirement is also 10 metres for a 600mm pipe. Accuracy drops to ±0.5 or ±0.75% and repeatability also drops off. The clamp-on ultrasonic flow meter can be moved to other pipes and other pipe diameters. As two sensors are clamped to the outside of pipes with jubilee clips there is a chance that the sensors can be mischievously misaligned.
Careful with that viscosity
Transit time U/S devices are just not suitable for high viscosity applications i.e., over 140cP. Attenuation of the ultrasonic signal is proportional to density, speed of sound in the fluid and viscosity. For Reynolds numbers over 1,000 the laminar flow becomes unstable as turbulent plugs start to form. This holds up to Reynolds numbers of 5000 (above that the flow is fully turbulent).
Flow velocity only
Strictly speaking, these are not volumetric nor mass flow measurement devices. In a single beam device, the phase shift is interpreted to provide point velocity only. A value for the area of the tube is the second-most important variable to determine volumetric flow rate.
You can speak to our team today about your flow meter queries and questions or for a quotation on a specific product. If you are unsure on what you need to meet your requirements, our specialists will be sure to find the right flow meters solution.