Open Channel Flow Meters

How Open Channel Flow Meters Work

Volumetric flow rate expresses the volume of fluid passing through a given cross-section per unit of time and can be expressed, for example, in m³/h, m³/s or l/s. In a fully filled pipe, the cross-section through which the liquid flows is defined by the internal diameter. In an open channel or a partially filled pipe, on the other hand, there is a free surface and the wetted section varies according to the level.

In open channel flow measurements, it is therefore necessary to relate the geometric characteristics of the section to the behaviour of the fluid. In area-velocity systems, the fluid velocity is combined with the area of the wetted section: the level makes it possible to reconstruct the area actually occupied by the liquid on the basis of the configured geometry. In other systems, the level is measured at a hydraulic section with known characteristics and converted into a flow-rate value.

The geometry of the channel is an integral part of the measurement. A circular, rectangular or irregular section requires different parameters, just as installation conditions change in the presence of non-straight sections, obstacles, vortices or variations in the section. The measuring principle, sensor position and configuration must therefore be consistent with the point at which monitoring is carried out.

Technologies for Measuring Flow in Open Channels

For measurement in open channels, we offer flow meters based on different technologies: non-contact radar systems, ultrasonic instruments using the area-velocity principle, and hydraulic flumes that exploit a known relationship between level and geometry. The choice varies according to the channel, the fluid and the installation conditions, but the objective remains the same: to convert level and velocity, or the hydraulic behaviour of the section, into a flow-rate value that can be used for process control.

Non-Contact Radar Flow Meters

In radar systems, the sensor is installed above the flow and can carry out non-contact measurement of the liquid. The RIF970 combines radar measurement of surface velocity, based on the Doppler effect, with level detection; this data, combined with the geometry of the section, makes it possible to calculate the flow rate of the channel.

Non-contact measurement is particularly useful when installing a submerged sensor is impractical, or when debris, sediment or material carried by the flow could interfere with a device directly exposed to the liquid.

Ultrasonic Meters and Area-Velocity Technology

Area-velocity technology determines flow rate by combining fluid velocity with the area of the section through which it passes. The level is used to calculate the wetted section based on the configured geometry, while velocity is detected using the Doppler principle.

RIF940 uses an ultrasonic sensor installed on the channel bed and measures the velocity and level of the liquid. AVFM6.1 uses a submerged ultrasonic sensor for continuous monitoring of these variables and can also be configured with submerged velocity plus a non-contact ultrasonic level sensor for applications involving heavily aerated fluid or a high concentration of suspended solids.

The presence of a submerged sensor makes it particularly important to assess the conditions of the fluid and the installation point. Sediment, deposits, turbulence and the characteristics of the section can, in fact, affect measurement quality and the long-term management of the instrument.

Venturi and Palmer-Bowlus Flumes

In systems based on hydraulic flumes, the flow rate is derived from a known geometry. The narrowing of the section changes the liquid level according to a relationship defined by the characteristics of the flume; the level measurement can therefore be converted into a flow-rate value.

Among the available configurations, Palmer-Bowlus is designed for partially filled pipes with a circular section, while the Venturi system is designed for applications with a rectangular section.

How to Choose an Open Channel Flow Meter

An open channel flow meter must be selected on the basis of the actual conditions of the measuring section. Channel geometry, flow and liquid characteristics, integration with control systems and installation method are the main factors to consider. You can rely on Riels technical consulting to find the configuration best suited to your needs.

> Channel type and geometry: open channels, watercourses and partially filled pipes can have circular, rectangular or irregular sections. Dimensions, shape and the presence of any hydraulic flumes all affect the configuration of the measuring system.

> Flow conditions: level range, minimum and maximum velocity, flow-rate variations, turbulence, vortices, obstacles and sediment are all factors to consider when correctly defining the measuring point.

> Liquid characteristics: the measurement may involve clean water, wastewater, industrial liquids and flows containing suspended solids, sludge or debris. Depending on the conditions, it may be worth considering submerged-sensor solutions or non-contact technologies.

> Integration with control systems: flow meters can be connected to PLCs, SCADA systems, dataloggers or data-acquisition platforms. RIF940 has analogue and digital outputs; AVFM6.1 offers 4-20 mA outputs, relays and optional Modbus RTU; RIF970 communicates via RS-232/RS-485 with the Modbus RTU protocol. Palmer-Bowlus and Venturi configurations are also available with Modbus, pulse output, analogue output and flow-rate logging.

> Maintenance and diagnostics: for submerged systems, it is important to check for deposits, scaling or blockages, while non-contact solutions reduce direct exposure to the fluid and can simplify management of the measuring point.

Applications of Open Channel Flow Meters

Open channel flow meters are used in numerous contexts related to flow management and monitoring, from water networks and hydraulic infrastructure to water treatment and wastewater treatment plants. They are also used for industrial discharge, partially filled sewers, irrigation channels and water distribution systems, as well as for monitoring rivers, watercourses and drainage networks. The various available technologies make it possible to adapt the measurement to variable sections, the presence of solids or sediment, and different flow conditions.

Benefits and Accuracy of Riels Open Channel Flow Meters

Riels open channel flow meters combine precision, versatility and reliability, with solutions based on different measuring principles and contact or non-contact configurations. The range makes it possible to meet different application needs and, depending on the model, to monitor level, velocity and flow rate, integrating the data into supervision, logging and process control systems.

Riels has been operating since 1967 in instrumentation for process control and industrial automation, complementing its range of instruments with support, calibration, testing and commissioning services. Our quality system is compliant with UNI EN ISO 9001:2015.

Discover Riels open channel flow meters and, with the support of our technical team, find the solution best suited to your application.

FAQ

What is the difference between an open channel and a partially filled pipe?

Both feature a free liquid surface. In an open channel, the flow section is exposed to the atmosphere, whereas a partially filled pipe is a pipeline in which the liquid does not occupy the entire internal cross-section.

Which technologies can be used to measure flow in an open channel?

Available technologies include non-contact radar, ultrasonic sensors using the area-velocity principle, and systems based on hydraulic flumes such as Venturi and Palmer-Bowlus. The choice depends on the geometry, fluid characteristics and installation conditions.

When should an ultrasonic or area-velocity meter be chosen?

These systems determine flow rate by combining fluid velocity with the area of the wetted section, derived from the level and the configured geometry. Their suitability must be assessed in relation to the channel, the liquid and the installation point.

Can the flow rate of wastewater or liquids with suspended solids be measured?

Yes. Various technologies can be used on wastewater and fluids containing suspended solids, but the quantity and characteristics of solids, sediment, sludge or debris affect the choice of measuring principle and the type of installation.

Is it possible to monitor data remotely?

Some systems can be connected to data-acquisition instruments, dataloggers and control networks. Communication protocols and monitoring functions, however, depend on the model and configuration chosen.

What factors can affect measurement accuracy?

Key factors include channel geometry, section configuration, turbulence, vortices, obstacles, sediment, flow variations and correct sensor positioning.

What maintenance do they require?

Maintenance varies depending on the technology. Submerged sensors may require checks for deposits, scaling or blockages, while non-contact systems reduce direct exposure to the fluid. Periodic checks of the measuring point, configuration and installation conditions remain important.