Open Channel Flowmeters

We provide reliable MCERTS-certified flow measurement systems for clean water, wastewater and environmental applications. Our systems cover all technologies including ultrasonic, hydrostatic and laser and supported by extensive field experience.

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 with a constant drive to innovate; something that RS Hydro know all about.

What are the options?

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 & Signature are probably the most widely used area velocity flow meters worldwide. The ISCO LaserFlow is a non-contact laser-based open channel flow meter (MCERTS-certified) which is able to use a 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.

Fit & Forget?

Never, but we can get very very close! You can never truly fit and forget any instrument. Unlike some flow measurement applications such as magflo meters and drinking water, open channel flow measurement can be a complex application. Even regularly maintained level-based ultrasonic or radar flow meters, properly installed in an ideal location, will give inaccurate discharge flow measurements if the operator uses poor measuring techniques. All aspects of design and measurement technology need to be considered when designing an open channel flow measurement system. Fundamentally, level-based technologies relying on primary flumes or weirs will presume that the primary structure is perfectly installed and maintained. This is easier said than done but when you are happy that this is all perfect then you must also make sure that your head measurement is very accurate. In the UK, a network of MCERTS Inspectors insures that all water and industrial companies have certified discharge flow monitoring systems. Althout MCERTS audits occur every 5 years, companies have to prove that they have an ISO-documented system for maintaining their flow meter and data records.

There are many flow meter manufacturers that produce flow and level measurement devices that broadly fit into the following categories:

  • non contact laser;
  • wetted area-velocity;
  • Weirs & Flumes.


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.

Weirs & Flumes

A third way of measuring of measuring open channel flow applications, and historically the most common, is the use of weirs and flumes. All manner of primary structures have been used the world over including v-notches, rectangular weirs, Parshall flumes, H-flumes, crump weirs, flat-v and many other hybrid versions. All of these require a level sensor to measure the hydrostatic head to to effect the rating equation within the flow meter to produce a flow rate. For most level to flow applications, 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. However, weirs and flumes do not last forever and require routine maintenance. A word of caution, repair and downtime costs of medium to large structures can be enormous. Hence the shift to retro-fit non contact flow meters is becoming the new standard.

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 LaserFlow from ISCO which provide good accuracy and reliability.

Weirs and Flumes >>