Flow Determination using Level Measurement

Flow is measured using a level measurement and hydraulic structures such as weirs or Venturi flumes or by using two parallel level measurements. The actual flow calculation is based on known channel shapes and particular hydraulic findings considering certain standards (e.g. DIN 19559 Part 2 for Venturi flumes or DWA A111 for weirs).

Hydraulic methods take a Q - h relation into account to calculate flow Q from the level h.

Q = k • f(h)

Defined Q - h relations can be found on hydraulic structures such as weirs, Venturi flumes etc.

Vertical Weirs

Depending on the flow rate different types of weirs (such as overflow weirs, triangular weirs and many more) are used. The overflow height is a measure for the flow rate. NIVUS provides mains-powered systems as well as self-sufficient systems operated with standard batteries or rechargeable batteries featuring automatic data transmission.

Different types of weirs

NIVUS products for flow measurement on weirs are NivuMaster level meters for applications with mains supply. For self-sufficient flow measurements there are i-Series level sensors in combination with NivuLog Easy data loggers available.

Movable overflown weir flaps allow optimum utilisation of available storage capacities. In such cases, there are two possibilities to determine flow. On one hand flow can be determined solely by measuring the inclination of the weir flap. This can be achieved by using an inclinometer.

On the other hand it is also possible to install an extra level measurement within the storage space (i.e. i-Series sensors) in addition to the inclinometer and to determine flow by using a dedicated transmitter such as Hydraulic Calculator Plus.

At weirs with tangential onflow an inclined water surface can be found frequently. To accurately measure discharge it is necessary to install two parallel level measurements at both end points of the weir. The HydraulicCalculator then computes the correct overflow volume from both level values.

On weirs with formation of backwater the overflow coefficient required for flow measurement or overflow determination is decreasing. Using parallel level measurements in headwater and tailwater the current backwater level on the weir can be determined. This allows determination of the current overflow coefficient and by using an appropriate transmitter such as HydraulicCalculator Plus it is possible to determine flow or discharge rates correctly.

Venturi measurements are flow measurements in flow channels with special shapes. Within the constriction of such a channel the flow changes from streaming to shooting condition.
Impounding level and flow relate to each other exponentially. This is why the flow level can be used to calculate the flow.

NIVUS offer venturi half shells in addition with NivuMaster level readers.

The measurement method for flow rate detection in part filled pipes and channels uses two parallel level readings. The levels are measured in a known distance, e.g. by measuring in two consecutive shafts within a sewer system.
The NIVUS exclusive metering system is putting geometrical conditions (such as slope, diameter and width of a channel etc.) and latest hydraulic flow models in relation to
each other.

Benefits

• Contactless measurement
• Low maintenance
• Accurate method (calibration using cross correlation; <2% deviation)
• Independent from mains power
• Detection of backwater and free discharge

NIVUS product: NivuSmart Q