Measuring the flow in a pipe usually means cutting the pipe and fitting a meter in line, which requires a shutdown and adds a permanent pressure drop. A clamp-on ultrasonic flow meter avoids both. Its transducers strap to the outside of the pipe, so it can be fitted to a running system in minutes, used for spot checks and surveys, or left in place for monitoring.
How Transit-Time Measurement Works
Two transducers are mounted on the pipe a set distance apart. Each acts as both transmitter and receiver. The meter sends a burst of ultrasound from the upstream transducer to the downstream one, then in the reverse direction, and times both trips. Sound travelling with the flow arrives slightly sooner than sound travelling against it. The difference between the two transit times is proportional to the average velocity of the liquid along the sound path. From that velocity and the pipe's internal cross-section, the meter calculates volume flow and keeps running totals.
Because the method relies on sound crossing the liquid, it works best on clean or moderately dirty liquids that carry sound well — water, sea water, wastewater, oils, chemicals, beverages. Large amounts of entrained air or solids scatter the signal. MEWOI's supplier data gives a turbidity limit of 10,000 ppm with few bubbles.
V, W and Z Mounting
- V method: both transducers on the same side; the sound crosses the pipe twice, bouncing off the far wall. This is the usual choice for small and medium pipes.
- W method: both on the same side with the sound crossing four times. The longer path improves resolution on small pipes.
- Z method: transducers on opposite sides; the sound crosses once. This gives the strongest signal and suits large pipes or difficult liquids.
The meter calculates the correct transducer spacing from the pipe's outside diameter, wall thickness, pipe material, any lining and the liquid type. Entering these accurately matters as much as fitting the transducers carefully.
Installation Details That Decide Accuracy
- Straight pipe: bends, valves and pumps distort the velocity profile. MEWOI recommends at least 10 pipe diameters of straight pipe upstream, 5 downstream and 30 from a pump outlet.
- Full pipe: mount where the pipe is always full, avoiding the top of horizontal pipes where air collects; mounting at the side of the pipe is common.
- Coupling: clean the pipe surface back to sound material and use couplant under each transducer.
- Pipe data: use measured wall thickness where possible. An ultrasonic thickness gauge is useful on old or corroded pipe.
- Signal quality: check the meter's signal strength and quality indicators before trusting the reading.
Portable vs Fixed Meters
MEWOI's ultrasonic flow meter range covers both. The handheld KY3000J/KZ3000J and UDS200H/HDS200H series run from built-in rechargeable batteries for surveys and checks. The wall-mounted BDS200H, KDS200H and LDS200H types are for permanent installation, with 4–20 mA, pulse and relay outputs, RS485 Modbus and SD card logging on the LDS200H. Transducer sets cover pipe sizes from DN15 up to DN6000 depending on model, with clip-on or bracket mounting, and standard (to 90°C) or high-temperature (to 160°C) versions. The supplier specifies accuracy of ±1% of reading, which can be improved to ±0.5% by on-site calibration on the handheld models.
Frequently Asked Questions
Does it work on plastic pipe? Yes. Steel, stainless steel, cast iron, copper, PVC, aluminium and fibreglass pipes are all listed by the supplier, and lined pipes can be measured if the lining is entered in the set-up.
Can it measure gas? No. These meters are designed for liquids. Gas flow needs a different type of ultrasonic meter.