A clamp-on meter on a half-full pipe reports roughly double the real flow, with no error flag and a number that looks entirely plausible. If your line is not reliably full, this is the instrument you actually need.
The sensors go in the channel, not on the outside of the pipe. They are in this catalog because they solve the one problem no clamp-on meter can: a pipe that is not running full.
A clamp-on ultrasonic meter computes volumetric flow as Q = v × A. It measures velocity. It does not measure area — it takes the area from the pipe dimensions you typed in at setup. And that arithmetic silently assumes the pipe is completely full of liquid.
Run that meter on a gravity sewer that is half full and one of two things happens.
The acoustic path passes through air, the signal dies, and the meter drops out. That is the good outcome, because at least you know something is wrong.
Or — the transducers happen to sit low enough that the path stays submerged. The meter measures a perfectly real velocity, multiplies it by the full pipe area, and hands you a flow rate that is roughly double reality. No error flag. No warning. A number that looks entirely plausible and that everyone downstream will believe.
That is not an instrument defect. It is the instrument doing exactly what you told it, on a pipe it was never designed for.
It measures both terms of the equation, continuously.
Combine measured depth with the known geometry of the pipe or channel and you get the actual wetted cross-sectional area at this instant. Multiply by measured velocity, and you have real volumetric flow — whether the pipe is 15% full or 95% full. An integrated temperature sensor corrects the velocity measurement.
The traditional way to measure open channel flow is to build a hydraulic structure — a weir or a flume — and read the head over it. It works, and it means pouring concrete in a live sewer: civil engineering, a contractor, a permit, and a flow diversion.
Area-velocity determines flow from velocity, depth, and temperature directly. It goes in, and it starts logging.
You are putting a sensor in the flow. Anything in a sewer will eventually collect rag — that is not pessimism, it is the operating environment. Both instruments below use velocity-profiled sensors designed to shed material rather than snag it, but you are accepting a maintenance interval.
You accept it because the alternative is not measuring the flow at all.
Permanent · expandable to 16 sensorsPermanent area-velocity meter for partially full pipes and open channels — raw sewage, industrial effluent, storm water. ±2.5% FS velocity, ±0.2% FS depth, 4–20 mA and Modbus. IP68.
Specifications
Portable · MCERTSBattery-powered, MCERTS-accredited portable area-velocity monitor for regulated discharge, sewer networks, and storm overflow. Five-year ATEX battery, GSM/GPRS telemetry, IP68 / NEMA 6P.
SpecificationsPermanent monitoring, multiple points, into SCADA → ORAKEL. One control unit expands to 16 sensors, which turns a point instrument into a network monitoring system. For inflow-and-infiltration studies and catchment modelling, instrumenting many points from one head end is the difference between a project and a purchase order.
Regulated discharge, and the number has to satisfy an inspector → MSFM. It is MCERTS accredited, which means the conversation is about the data rather than about whether the instrument that produced it was adequate. Five-year battery life makes remote and unpowered sites viable, and the ATEX-rated pack is not decoration — a sewer generates methane and hydrogen sulfide and should be treated as a potentially explosive atmosphere.
If your pipe is running full, do not buy either of these. Buy a clamp-on meter — it is a better tool for that job, it has nothing in the flow, and it costs you no maintenance interval.
It is the first question, and it is the one that decides everything else. Tell us about the line and we will tell you which category of instrument you are actually shopping for.
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