An open channel flow meter measures how much liquid runs through a channel, drain or flume that is open to the air. An ultrasonic transmitter mounted above the surface times the echo off the water to find its level, and the Nivelco controller converts that level into flow using the known shape of a Parshall flume or weir.

Nothing touches the effluent, so there are no wetted parts to foul or wear — a good fit for the outlet channel of an ETP or STP, where you need a reliable record of the volume discharged.