Open Channel Flows
For any open channel that is free flowing, there is a specific relationship between depth of water and flow rate.  Whenever a given depth occurs, there will always be the same flow.  Therefore, if the flow rate is known for each depth, a depth-to-flow relationship can be constructed.  The Open Channel Flow Monitor is pre-programmed with depth-to-flow relationship tables.  When the user digitally selects the correct depth-to-flow relationship for an application, the Open Channel Flow Monitor's LCD screen automatically displays accurate flow rate measurements for any depth that is reached.  This water flow is averaged over time to display total flow.

When using a primary device to measure flow, such as a measuring flume or weir, there is a mathematical relationship between depth and flow.  When measuring flow in a channel that does not have a primary device, the depth-to-flow table can be constructed by measuring the flow at several different depths, using Global Water's Flow Probe.

Depth Measurement
Global Water's Open Channel Flow Monitor measures flow depth with the highly-accurate WL400 Water Level Sensor, which is a fully submersible water level sensor constructed of stainless steel.  The standard level range is from 0 to 3' of water, which provides for accurate flow measurements even with a depth change as small as a fraction of an inch.  The pressure transducer is mounted slightly below the "zero" flow depth of the channel, upstream from the throat of the flume or weir.  For open channels with no primary device, the sensor is mounted below the lowest expected water depth.  The water level sensor has a 2-wire 4-20 mA signal, so the flow computer can be mounted several hundred feet away, if required.  The sensor and computer can be connected with standard, twisted-pair signal wire.

Global Water also offers an ultrasonic water level sensor that allows the Open Channel Flow Monitor to measure semi-solid flows or flows that are not suitable for a standard submersible pressure transducer.  Global Water recommends that the ultrasonic sensor option only be used in areas where AC power is available.  You should strongly consider using a submersible pressure transducer for applications that are in areas without AC power.

Display and Outputs
The Open Channel Flow Monitor displays flow rate on a large 6-digit LCD screen.  The totalized output is shown on a second LCD display.  The display reads in user-defined units of flow, including mgd, csf, gpm, m³s.

The Flow Monitor also produces an analog 4-20 mA output signal that is proportional to flow.  This can be used for chart recorders, dataloggers, or as an input to telemetry or process control systems.  In addition, the meter has a flow-proportional output signal for triggering water samplers.

Digital Flow Computer
The Global Water Open Channel Flow Monitor has powerful internal electronic circuitry that calculates and totalizes flow.  Desired engineering units and depth-flow relationships (from over 20 pre-defined tables) can easily be selected using the unit's four front panel push buttons.  User programming does not require the unit's enclosure to be opened.  The unit's manual includes programming information, including depth-to-flow tables for all standard flumes and weirs.

Enclosure
The enclosure is made of weatherproof injection molded plastic.  It is suitable for use in a wide variety of environments.  The enclosure can be wall-mounted and fitted with conduit for sensor and power connections, if required.

Open Channel Flow Monitor Specifications

Rate Display: 6 digit + decimal place, LCD
Totalizer Display: 6 digit + decimal place, LCD
Accuracy: Pressure transducer: +0.2% FS + Flow Monitor: +0.01% + the depth-flow-table error
Flow Units: CFS, GPM, m3s, MGD
Totalizer: Related to Flow Units
Enclosure: 4 3/4" W x 8" H x 3" D, NEMA 4X (IP67)
Power: 120 VAC to 240 VAC, optional: Battery powered (Must be specified at time of ordering)
Current Draw: 50mA Typical, 16mA in sleep mode, and 120mA when the output relays are on

Pre-defined Tables
Parshall: 1", 2", 3", 6", 9", 12"
Palmer-Bowlus (4D): 4", 6", 8", 10", 12", 15"
Weir: 45° V notch, 90° V notch, 1' rectangular, 2' rectangular
H: 0.4HS, 0.6HS, 0.5H, 0.75H, 1.0H, 1.5H, 2.0H
Trapezoidal: 60°