How flow is measured — clamp-on ultrasonic, and inline meters — and what affects accuracy.
Most flowmeters measure the velocity of the water and multiply by the pipe’s inside area to give flow rate (for example L/min, m³/h, or US gpm). Adding up flow rate over time gives the totalized volume. Because the area comes from the pipe size, an error in the inside diameter becomes a larger error in flow — a 1 % error in diameter is about a 2 % error in flow.
- Two transducers clamp to the outside of the pipe, one upstream of the other. Each sends ultrasonic pulses through the pipe wall and water to the other.
- Transit time. A pulse travelling with the flow arrives slightly sooner than one travelling against it. The difference in travel time is proportional to the average velocity along the sound path.
- Nothing inside the pipe. No cutting, no pressure drop, and no wetted parts, so it can be fitted without shutting the system down.
- Needs clean, full pipe. Transit-time meters need the sound to cross the water, so they work best on clean water with little air or solids. The pipe material, wall thickness, and any lining must be entered correctly.
- Doppler ultrasonic meters are a different design that bounces sound off particles or bubbles; they suit dirty water, not clean water.
- A magnetic field crosses the pipe. Water moving through it generates a small voltage (Faraday’s law), picked up by two electrodes in the pipe wall. The voltage is proportional to velocity.
- Needs conductive liquid — ordinary water is fine, but very pure water (commonly below about 5 µS/cm, depending on the meter) is not.
- No moving parts and no obstruction in the bore, so little pressure drop and good tolerance of solids.
¶ Paddle wheel and turbine meters
- A rotor (a paddle wheel sticking into the flow, or a turbine filling the bore) spins at a speed proportional to velocity. A magnet or Hall sensor counts the revolutions and gives pulses or a frequency.
- Inexpensive and simple, common on make-up, bleed, and chemical feed lines.
- Moving parts wear, and they are sensitive to debris, scale, and fibres wrapping on the rotor. They lose accuracy at low flow, where the rotor slips or stalls.
See Meters and Switches for pulse-output water meters and the K-factor.
- Insufficient straight pipe. Elbows, tees, valves, pumps, and reducers distort and swirl the flow. A meter too close to them reads wrongly — often by several percent or more. This is the most common installation error.
- Entrained air and a partly full pipe. Air bubbles scatter ultrasonic signals and add volume to a mag or rotor meter’s reading. A pipe that is not full gives large errors for every meter type.
- Pulsation from metering and positive-displacement pumps makes the reading swing and can upset averaging.
- Wrong pipe data on clamp-on meters — outside diameter, wall thickness, material, and lining. Old pipe with scale inside has a smaller bore than its nominal size.
- Pipe condition for clamp-on. Heavy internal scale, lined pipe, corrosion, and poor acoustic coupling (dried or missing couplant, paint, rust) weaken or distort the signal.
- Coating on mag meter electrodes from grease, scale, or iron gives drift or noise.
- Debris and scale on rotors slow or jam paddle and turbine meters.
- Operating outside the meter’s range, especially very low flow.
- Electrical noise. VFDs and poor grounding upset mag meters in particular; they depend on correct grounding to the water.
- Straight run. Give the meter straight pipe upstream and downstream — typically at least 10 pipe diameters (10D) upstream and 5D downstream, and more after a pump, a control valve, or two elbows in different planes. Always follow the manufacturer’s figure for the meter and the fitting upstream of it.
- Keep the pipe full. Mount in a rising pipe or in a low section of horizontal pipe. Avoid the highest point of a line (air collects) and a pipe that drains to an open outlet just after the meter.
- Flow direction. Install inline meters with the arrow pointing the direction of flow.
- Clamp-on transducers on the side of horizontal pipe (commonly within about ±45° of horizontal) — not on top, where air gathers, or bottom, where sediment sits. Clean the pipe surface to bare metal or smooth plastic, use the couplant supplied, and space the transducers exactly as the meter calculates.
- Mag meters need grounding to the water per the manufacturer, using grounding rings on plastic or lined pipe.
- Paddle wheels usually mount on the side of a horizontal pipe, at the insertion depth specified; not at the bottom, where debris collects.
- Control valves downstream, not upstream, of the meter where possible, so the meter sees steady, full flow.
- Isolation and bypass valves allow an inline meter to be removed without shutting down the system.
Cutting in or removing an inline meter means opening a pressurized line. Isolate, drain, and depressurize first — see Isolation and Pressurized Systems.
¶ Maintenance
- Clamp-on: check the transducers are tight and correctly spaced, and renew the couplant as the manufacturer directs (gel couplant dries out; pads last longer). Check the signal strength or quality reading if the meter shows one.
- Mag meters: clean the electrodes if the reading drifts. Many meters have a built-in verification function.
- Paddle and turbine meters: inspect and clean the rotor, check the bearings and shaft for wear, and replace worn rotors. Intervals depend on water quality.
- Verify, do not assume. Compare the totalizer against a second measurement — a calibrated meter, a timed tank fill, or a pump drawdown — rather than trusting a single meter indefinitely.
- Recheck settings after any change: pipe size, K-factor, units, and totalizer scaling.
Models documented in this wiki: