By timing sound through the fluid.

Pulses with the flow arrive sooner than those against it.

That time difference converts to velocity.

Velocity and pipe area give volume.

Nothing is placed in the stream.

There is no restriction to pump against.

Stability over a very long service life.

No impeller wears and no bearing seizes.

Mechanical meters drift gradually as they age.

That drift is invisible without recalibration.

It also costs no pumping energy.

Over decades both matter.

When the system cannot be cut into.

Surveys and temporary measurement suit it.

Retrofit where draining down is impractical.

It installs on a live pipe.

In-line units are more accurate.

Billing generally uses in-line.

Anything that stops sound crossing cleanly.

Entrained air is the commonest cause.

Heavy aeration or particulate degrades it too.

A poorly vented system produces odd readings.

Vent properly before blaming the meter.

Clamp-on units also need sound pipe wall condition.

Yes, exactly as any flow meter does.

The technology removes the obstruction, not the physics.

A disturbed velocity profile gives a wrong number.

Manufacturers publish upstream and downstream lengths.

Clamp-on installation makes this easy to ignore.

Ignoring it produces confident wrong readings.

Acoustic coupling to the pipe.

Coupling paste or pads transmit the signal.

Surface preparation matters considerably.

Internal scale attenuates the signal.

Pipe material and wall thickness must be entered correctly.

Errors there scale the whole reading.

In-line certified versions, yes.

Legal metrology approval is required.

Clamp-on is usually not accepted for billing.

Requirements vary by jurisdiction.

Confirm before specifying for charging.

Recertification intervals apply as well.