weir box

How to Select a Weir

If you’re looking to get a weir for flow measurement, there are several considerations to keep in mind. Finding the right kind of weir means examining the conditions of your flow channel and considering the different types of weirs and what they can offer. Learn all about how to select a weir, and find out which particular design and application would work best for your unique needs.

Triangular

A triangular weir is also known as a V-notch weir, and it’s specifically designed to measure low flows. The notch angles here measure anywhere from 20 to 120 degrees, and they can be used for flows up to 10 cubic feet per second. Their biggest advantage, however, is that they provide the best profile for any discharges that are less than 1 cfs.

Rectangular

Rectangular weirs are designed to be an alternative to their triangular counterparts when you need to measure higher flows. In fact, channels with high flows that are suitably fit for weir boxes almost always employ the rectangular design. Just keep in mind that the discharge equation is a bit more complicated than what you’d find with other weir types.

It’s important to remember that rectangular weirs come in two different forms. You can either get a contracted or suppressed rectangular weir. Contracted weirs are the ones with difficult discharge equations, while suppressed weirs are designed to remedy that. The downside of a suppressed weir is that the nappe has to be properly aerated and the channel must be able to support using the weir’s sidewalls as the weir ends.

Cipolletti

A Cipolletti weir is similar to a contracted rectangular weir with one major difference. The notch ends angle out with a 4:1 ratio that simplifies the discharge equation. They’re not quite as accurate as rectangular weirs or triangular weirs, but Cipolletti weirs can be useful as part of a larger weir set, as there’s an ambiguous discharge between weir sections here.

Circular

Circular weirs are unique in that they’re specifically designed to be mounted within pipes or conduits. There really aren’t many other applications for them, but if you need one in a pipe, there is no better option. Just keep in mind that flow rate is not determined by pipe size. Instead, you’ll need to measure the open weir area.

Alternatives to Weirs

In some instances, a weir may not be appropriate at all, regardless of what kind of weir design you opt for. Several disadvantages of weirs could compromise your flow measurement efforts entirely. For example, weirs suffer from high head loss, and there’s little resistance to submergence. You may not have enough space for the upstream weir pool requirements, and you’ll have to perform periodic maintenance on the weir pool when you do have enough space to install one properly.

Weirs From Tracom

Now that you know how to select a weir, you just need a manufacturer. That’s where Tracom can help. Our team will work with you to create a custom design perfectly suited for your flow channel conditions to ensure accurate measurements and reliable installation. Contact us today to get started!