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A Weir Box incorporates a thin-plate weir (V-notch or Cipolletti) into a pre-engineered structure.  Weir Boxes are generally used to measure flows from 3.990 – 493.5 gpm [0.2518 – 31.14 l/s] - although larger flow rate Weir Boxes are available.

Openchannelflow Weir Boxes conform to ISO/DIS 1438 Hydrometry – Open Channel Flow Measurement Using Thin-Plate Weirs, are extremely versatile, and can be configured for either above ground or shallow burial installations and are commonly used to measure dam seepage and treated industrial discharges.

Flows entering / exiting the Weir Box can be piped or it can be free spilling.  The inherent design flexibility of the Weir Box design means that multiple inlet / outlet flow streams can be accommodated as well as dual (or triple) flow streams measured independently.

Each Openchannelflow Weir Box consists of:

  • Box body (usually aluminum, fiberglass, or stainless steel) 
  • Fixed inlet baffle
  • Fixed weir plate carrier
  • Interchangeable weir plate (aluminum or stainless steel)

The Weir Box body is specifically sized to generate the proper weir pool for the intended flow rate – something lacking in most competing weir boxes.

To help secure the Weir Box, each unit is provided with stainless steel anchor feet on the outside of the box to allow it to be mounted to a concrete slab.

Standard Weir Box Sizes

Openchannelflow offers three standard weir box sizes.  For each weir box size, four different V-notch weir plates are available.  


22 1/2°






3.990 - 14.31 gpm

5.427 - 19.45 gpm

8.309 - 24.99 gpm

11.58 - 29.59 gpm

20.07 - 35.06 gpm


3.990 - 39.44 gpm

5.427 - 53.63 gpm

8.309 - 82.11 gpm

11.58 - 114.5 gpm

20.07 0 110.6 gpm


3.990 - 98.12 gpm

5.427 - 113.5 gpm

8.309 - 204.3 gpm

11.58 - 284.9 gpm

20.07 - 493.5 gpm

Should the above combinations not match a site's flow needs, custom weir boxes (and weir plates) are offered.

Weir Box Materials

Openchannelflow manufactures Weir Boxes in a number of different materials to suit your specific application needs:

  • Aluminum
  • Fiberglass (FRP) - with stainless steel weir plate
  • Galvanizes steel
  • Stainless steel

Of the materials, stainless steel is the most common material for dam seepage / demanding industrial discharge monitoring, while fiberglass is used to measure treated industrial flows.

Weir Box Accessories

To help customize our Weir Boxes to your specific application needs, Openchannelflow has developed a variety of accessories to cover almost every need, including:

  • Piping / end connections
  • Flow condition options
  • Flow meter mounts
  • Sampler / parameter mounts
  • Custom configurations (i.e. dual / triple flow stream monitoring)

Weir Box Limitations

As with any open channel primary device, Weir Boxes should not be used on pressurized conduits unless the flow can be slowed and conditioned to allow accurate flow measurement. Energy absorbing, baffling, and flow straightening may all be necessary to properly condition piped flow before it enters the weir pool of the main box structure.

Weirs Boxes should be used with caution on flows with floating trash, debris, or high solids contents as sedimentation will occur upstream of the weir (raising the weir pool depth) and debris may cling to the crest of the weir (affecting the development of the nappe over the crest).  

However, in dam seepage applications, this feature of trapping solids upstream is useful in monitoring for breakthrough of the dam structure.

Finally, researchers have recommended that weirs (and by extension Weir Boxes) only be used to measure flows at ordinary temperatures (39 to 86ºF) [3.9 to 30º C].  However, if power is available at the site, an immersion heater can be placed in the weir pool to heat the flow stream - which may allow for lower temperature operations.

More About Weir Boxes

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