Clamp-On Ultrasonic Flow Meter

A clamp-on meter is fitted to the outside of a pipe that is already full and running. Two transducers are strapped to the pipe wall, and the transmitter times an ultrasonic pulse sent with the flow against one sent against it; the difference between the two transit times is proportional to the velocity of the liquid. Nothing is cut, welded or drained, and nothing sits in the flow.

Three transducer sizes cover the bore range. The small S2 pair fits DN15 to DN100 and is rated for liquid from 0 to 160 °C; the M2 pair covers DN50 to DN700; the L2 pair covers DN300 to DN6000. Where a clamp-on reading is not suitable, wetted insertion transducers are available for pipes above DN80. The transducers are sealed to IP68 and work submerged to 3 m, which suits a meter chamber that floods.

The transmitter is a wall-mounted IP65 enclosure, 156 × 150 × 85 mm, with a backlit two-line display and a keypad for setup. It runs from 85 to 264 V AC or an isolated 8 to 36 V DC supply and draws less than 1.5 W. It has an isolated 4–20 mA or 0–20 mA current output, an open-collector pulse output, an isolated relay and an isolated RS485 port. Two three-wire Pt100 inputs let the same transmitter calculate heat across a flow-and-return pair, and three 4–20 mA inputs are optional.

Accuracy is better than ±1%, with a repeatability of 0.2% and a measurement period of 0.5 s. That assumes a settled flow profile: the meter needs at least ten diameters of straight pipe upstream and five downstream, and thirty upstream where a pump is close. It reads through metal, most plastic and glass-fibre pipes carrying water, sewage with a small particle content, oils, chemicals, effluent, beverages, ultra-pure liquids and solvents.