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Portable Weir Plates are a simple way to measure the flow of water in natural channels.  With interchangeable plates, Openchannelflow weir sets allow you to select the weir that best fits each site's flow rates.  Precision manufacturing ensure the weir plates are interchangeable and field replaceable.

The weirs of each Portable Weir Plate set conform to ISO/DIS 1438 Hydrometry – Open Channel Flow Measurement Using Thin-Plate Weirs for their dimensions and flow rates.

Weir Flow Rates

Each Portable Weir Plate set comes with four interchangeable weir plates:  22-1/2º, 45º, 90º, and 1-foot Cipolletti.  

Weir Angle

GPM

L/S

22 1/2º

3.990 - 67.53

0.25148 - 4.318

45°

8.309 - 39.44

0.5039 - 9.051

90º 20.07 - 346.5 1.217 - 21.86

1-foot Cipolletti

135.1 - 534.2

8.196 - 32.40

 

In addition to these standard weirs, additional custom weirs (and weir sets) are also available.

Weir Materials

All standard portable weir plates sets are construction from T-304 stainless steel.  Stainless steel is rugged, holds an edge (important for a properly formed weir crest), and can be laser or water jet cut for extreme precision.  Other materials are prone to loose dimensional tolerances, swelling, and are susceptible to crest damage.

How a Weir Operates

A weir is a fixed obstruction across a channel through over which flow passes.  As the flow passes over the crest and drops into the downstream channel it is accelerated.  As with a flume, this acceleration creates a know relationship between upstream level and flow rate.  

The point of measurement of a thin-plate weir should be at least 3-5 times the maximum anticipated head (Hmax) upstream of the weir plate.  This far upstream and the water level is outside of the drawdown zone near the weir.  Like a long-throated flume a weir does not have a fixed point of measurement – anywhere upstream of the drawdown zone is acceptable.  

One drawback to the use of a weir is that the zero elevation from which the level is determined is the lowest point of the weir crest downstream of the point of measurement.  

It is usually simplest to verify / calibrate the level reading of a weir when flow has been stopped.  The water upstream of the weir crest should just touch the crest itself (but not be lower than the crest elevation).

Weir Construction

In order to allow the nappe (body of water flowing over the weir crest) to spring clear of the weir crest (flow edge), the crest should be no thicker than 1-2 mm [0.03 – 0.0-inches] thick.  Thicker than this the nappe may collapse against the downstream face of the weir due to insufficient aeration (causing the weir to under-indicate the actual flow rate).  Thinner than this and the knife-edge created will be too difficult to maintain.

The crest itself must be sharp and burr-free.  Rounded edges, like overly thick crests, tend to allow the nappe to collapse.  Flow should always spring clear of the crest.

Weir Limitations

As with any open channel primary device, weirs are not suitable for use on pressurized conduits.  Pipes entering a weir pool / box may run full, but should not be pressurized.  Piped flow should be well baffled and the approaching velocity profile well distributed before flow reaches the point of measurement of the Portable Weir Plate set.

Weirs 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).  It should be noted, however, that for dam seepage, the ability to trap solids upstream is useful in monitoring for breakthrough of the dam structure.

Finally, researchers have recommended that weirs only be used to measure flows at ordinary temperatures (39 to 86ºF) [3.9 to 30º C].  At the lower temperatures, ice may begin to form on the crest – greatly affecting the flow readings.

More About Portable Weir Plates

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