What it does

An ultrasonic flow meter uses ultrasound to measure the velocity of fluids for many applications. You’ll find it most often in water and other liquids in pipes. And a clamp-on ultrasonic achieves high accuracy in low and high flows, installs easily, and isn’t affected by external noise. But how does it work?

How it works

Ultrasonics use sound waves at a frequency beyond the range of human hearing, typically 0.5, 1, or 4 megahertz. The device sends its signal with a insertion transducer, making contact with the liquid, or an external transducer, sending the signal through the pipe wall. Clamp-ons allow users to measure flow in a pipe from the outside, which makes installation and maintenance easier.

A typical transit-time ultrasonic flow meter has two transducers that function as both transmitter and receiver. Thus, the device alternately sends and receives a signal between the two, then measures the time it takes to travel between them in both directions. The difference in the transit time, or ∆ time, is directly proportional to the flow speed.

This image shows a common setup, the V or two-pass mount with clamp-on transducers. Here, the signal goes from the first transducer, through the pipe wall and the liquid, to the back wall of the pipe. It reflects from that wall, then travels back to the second transducer.

The same process then repeats in reverse as the second transducer sends its signal. The difference between the times of flight equals the ∆ time. When the liquid in the pipe is still, the ∆ time equals zero.

And now for the math. You need to convert the raw ∆ time into the flow velocity. You can find the angle of the ultrasound path if you know the speed of sound in the pipe and the liquid. This angle can help you convert the path into a straight line in the pipe. This will show you the flow speed.

Once you have this number, you convert it into a flow rate by multiplying it times the cross section of the pipe, as with any velocity-based flow meter.

Why we use it

Ease of installation: A clamp-on, while still needing straight pipe upstream from the measuring site, won’t require a system shutdown to install.

Low maintenance: With no blades or bearings to replace or electrodes that can foul over time, you can practically “set and forget” an ultrasonic.

Original content here.

Author: John, Chief Engineer @ Sierra Instruments

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