Installation & Wiring
What the pipe wall takes away from a clamp-on ultrasonic reading
A clamp-on meter is sold as non-intrusive, which is true and easy to misread. Nothing touches the liquid, but the sound has to cross the pipe wall twice, and the wall is part of the measurement.
The couplant matters more than the electronics. A dry or gapped transducer loses most of its signal, and the meter compensates by raising gain, which increases noise rather than accuracy. Renew the couplant on schedule, and refit the transducers with the pipe surface cleaned back to bare metal.
Then there is the geometry. Two diameters upstream is not enough for the velocity profile to settle after an elbow; five is the working minimum, and ten is better. A sensor mounted on the top or bottom of a horizontal pipe collects gas or solids; the 3 and 9 o’clock positions avoid both.
Finally, wall thickness and lining: a scaled cast-iron main attenuates the beam, and a loose cement lining scatters it. If the signal strength is low with a clean wall and fresh couplant, the pipe is telling you to cut in a spool piece.