Open Channel Flow Meters
RS Hydro are the UK market leaders for open channel flow meters. Unlike many other flow monitoring systems, applications requiring open channel flow meters are far more challenging. There is no universal technology applicable to every open channel flow measurement application; it is therefore imperative that open channel flow meter installations should be based on extensive application experience: something that RS Hydro has a lot of, covering every application field in the UK and overseas.
The majority of open channel flow measurement applications have insignificant head for anything other than a flume or small weir installation. Other applications can use v-notch or rectangular weir arrangements but they all typically create a hydraulic backup and incorporate a non-contact open channel flow meter measuring level with either an ultrasonic transducer, pressure sensor or shaft encoder. Alternatively it is possible to install flow sensors in the channel or stream itself. These are know as area velocity flowmeters and measure level and velocity together to compute flows. The ISCO 2150 is probably the most widely used area velocity flow meter worldwide. The ISCO LaserFlow is a non-contact laser-based open channel flow meter which is able to user laser to profile at multiple depths in the flow with very high accuracy. Many of these technologies are required to continuously monitor flows for licensing, monitoring, MCERTS, process measurements and EPR permits. RS Hydro has extensive experience of all of these.
RS Hydro have an extensive range of small open channel flowmeters for both temporary and permanent installations. Measurement technologies include doppler sensor, submerged probe, bubbler, area velocity and laser-based flow measurement. Whatever the requirements of your small channel or wastewater application, RS Hydro can supply the flow monitoring equipment and expertise to provide the total open channel flow meter solution.
RS Hydro supply a range of profiling ADCP flow meters (Accoustic Doppler Current Profiler) to measure larger channels or rivers. ADCP can either bd- or side-mounted and can measure up to 100m. For shallow applications it is possible to use multiple area velocity flow meters in series to provide flow monitoring for one larger cross-sectional area e.g for braided river or low-flow applications.
V-notch, weir and flume applications are widespread and typically employ a non-contact ultrasonic level controller with integrated rating equations to provide level-to-flow calculations. RS Hydro supply the full range of Siemens MCERTS-certified open channel flow meters including the HydroRanger 200, Multi-Ranger and OCM3. We can also supply standard level sensors and connect to dataloggers or wireless telemetry systems and then utilize rating equations to convert level to flow.
Regularly maintained non-contact open channel flow meters, properly installed in an ideal location, will give inaccurate discharge flow measurements if the operator uses poor measuring techniques. Head measurement is very important. With frequent occurrence, flow monitoring installations are often not compatible with the relationship that actually exists between the known head and discharge relationship. As such, operators should make sure that calibration curves or tables match the flow measurement devices that are being used.
Although chart recorders, staff gauges and floats are often used to provide an instantaneous head and hence flow measurement, today's technology allows continuous online flow measurement to be made with optimum accuracy. There are many flow meter manufacturers that produce flow and level measurement devices that broadly fit into the following categories:
- non-contact ultrasonic;
- non-contact laser;
Ultrasonic Non-Contact Flow Meter
Non-contact ultrasonic sensors emit acoustic pulses and receive an echo from the material being measured. The sensor is connected to a transceiver that calculates level by processing the time between the original signal and its echo. For most level measurement applications in the environmental industries, ultrasonic technology is the best choice. Because it’s non-contacting, it avoids the corrosion, material build-up, and general wear and tear associated with contacting devices. Higher maintenance costs and repairs associated with contacting devices can easily exceed any initial savings.
Most industries now pay for the volume of water consumed or materials discharged into the environment. In many jurisdictions, environmental regulations such as EPR or MCERTS now require strict reporting of all discharges, with accompanying fines or penalties. When plants implement steps to reduce pollution and discharges, they need an effective way to measure their progress. At one time, the only alternative was magmeters which are expensive and not always practical for many open channel installations.
Today, accurate measurement of river and stream flow monitoring is simple and cost-effective with an ultrasonic open channel flowmeter. Combined with an ultrasonic transducer, it monitors flow and provides data you can use for rainfall/storm water studies, inflow/ infiltration studies and sewer system evaluations.
Good examples of open channel flowmeters are the LUT400 and HydroRanger Series from Siemens Milltronics and the LaserFlowfrom ISCO which provide good accuracy and reliability.
Open Channel Flow Meter: Laser
To most the LaserFlow would seem to be technology from another planet. In part this is almost true but the origins of this technology emanate from deep within Teledyne ISCO where laser has been used for other applications. The Laserflow is unlike other non-contact technologies as it does penetrate and profile the flow below. The LaserFlow is set to revolutionize open channel flow. Technologies such as non-contact radar only act to measure the surface velocity and therefore cannot offer the same accuracy as the LaserFlow. Due to the shorter wavelengths of ultrasound, radar technology has not been used in the same way as other non-contact flowmeters. The LaserFlow flowmeter provides the user with flow measurements under a wide range of flows and site conditions such as:
- Shallow flow
- High velocities
- Flows with high solid contents
- High temperature flows
- Caustic flows
- Large sewers
Many technologies such as the wetted area velocity sensors require a wetted sensor to profile the flow and are obviously susceptible to significant wear and tear. The LaserFlow is non contact and as such has almost no physical wear and tear.
- Personnel have no contact with the flow
- Easy installation
- One-time entry into confined space for installation/removal
- Maintenance caused by immersed sensor is eliminated
- No site calibration required
- Field replaceable
Hydrostatic Sensors / Pressure Sensors
A third way of determining head is by utilizing low-range hydrostatic or water level sensors, in stilling wells, to establish the upstream head. However, such devices ordinarily produce a 4-20mA output of level not flow. This level has then to be interpreted by a further device or plotted on a calibrated stage-discharge curve to obtain a true flow. It should be stated that pressure transmitters are prone to drift, movement and blocking. Other devices include bubbler systems, or angular transducers sensing shaft rotation on a float-driven system. These types of system are perhaps best suited to the transmission of data via satellite or microwave, providing a central control location with current water stage information, as they utilize small amounts of power.. Remote sites are very well suited to being powered by batteries which are charged through small solar cells. However, vandalism of the solar panels can be a problem because they must be exposed. Concealing solar panels in some way (such as in the top of a tree) can help.