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.

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;
• hydrostatic.

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.

Advantages

• 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.