Open Channel Ultrasonic Flow Meter

An open channel has no pipe to fit a meter into, so its flow is measured through its level. A weir plate or flume of known shape is built into the channel, and the depth of water passing over or through it fixes the flow rate. This meter measures that depth from above with an ultrasonic sensor that never touches the water, and converts it to flow for the structure it has been set up for.

Because the flow is calculated from the level, the level has to be right. The sensor resolves distance to 1 mm with a precision of 0.3% under standard conditions, over a depth range of 0 to 6 m, and has a blind zone of 0.25 to 0.5 m directly below its face, which sets how high above the top water level it must be mounted. Temperature compensation is built in, an external sensor can be added for dynamic compensation, and false echoes are recognised and rejected automatically. The resulting flow precision is 1% to 5%, depending on the weir or flume.

Flow is measured from 0.1 L/s to 9,999 m³/s and totalised to 99,999,999 m³ on the local LCD. It is sent out as a 4–20 mA signal and over RS485 with Modbus or a customised protocol, and the logged record can be downloaded over USB. The data is held in EEPROM, so it survives a power failure.

The electronics sit in a die-cast aluminium housing sealed to IP65, mounted either in one piece with the sensor or split, with the transmitter at a distance. The integral unit stands 254 mm tall on a 60 mm sensor. It runs from 100 to 230 V AC or 24 V DC, every incoming and outgoing line is protected against lightning and short circuit, and it works in ambient temperatures from −30 to +70 °C. It is fixed over the channel on an L-shaped bracket, or on a flange, with the sensor face kept parallel to the water surface.