How to Find the Weir Point of Measurement

Weirs are used much like flumes to measure flow rate in open channel systems. While they both offer generally the same outcome, there are quite a few essential differences that set weirs and flumes apart from each other. One of those differences is how you take the measurements.

You won’t get much use out of a weir if you’re not taking accurate measurements, even if the weir itself is functioning properly. Learn all about finding the weir point of measurement and ensure that your weir is giving you the accurate readings necessary.

Where Is the Point of Measurement?

If you’re familiar with flume systems, you may know that the point of measurement is conveniently within the body of the flume. Many assume this to be the case with weirs as well, but the point of measurement is actually not within the weir itself.

The point of measurement for a weir is upstream of the weir’s crest. This is because the water’s surface suffers drawdown as it flows over the crest, which, if measured directly, will completely throw off the results. To avoid the drawdown effect, you need to take the measurement upstream from the weir crest at about three to five times the maximum head.

Why Not Measure at the Face?

If you’ve opted for a resold or amateur-crafted weir, you may find an engraved measurement gauge at the face of the weir. This makes it very easy and convenient to read, but the measurement presented isn’t very accurate. This is due to the weir’s nappe and its effect on the flow.

As water approaches the nappe of a weir, its velocity will increase. When velocity increases, the surface level of the flow decreases. Measuring at the face is measuring the greatest extent of this surface-level decrease, so even accounting for the +/- 2% accuracy accepted for most weirs, your measurement will be significantly off. 

How Should a Weir Measurement Be Gauged?

The first step in properly measuring the flow rate in a weir is finding gauge zero. This is the point in which the measurement shows a zero flow rate. One may think that zero would indicate an absence of water entirely as it does it flumes since the zero-point is the floor of the flume, but weir pools always have a bit of water within them. 

Considering the nature of a weir pool, you’ll have to calculate the gauge zero-point beforehand. You do this by using the crest of the weir as the zero gauge. Of course, measuring directly from the crest can be difficult, so one can instead measure from the floor of the weir to the surface of the water, then deduct the elevation of the crest.

Find a Quality Weir

With an understanding of the proper weir point of measurement, you can make use of a weir of your own. For a wide selection, look no further than Tracom. All our various weir styles and options are customizable, so you can find something that uniquely fits your individual needs. Contact us today to get started.

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