How Thin Plate Weirs Work

weir box

When you’re looking for an accurate way to measure open channel flows, one of the best options is a thin plate weir. These handy devices can offer incredibly accurate measurements, but they only work under certain conditions. You’ll need to make sure that you’re getting the most out of your weir, so it’s important that you understand how they function. Learn how thin-plate weirs work, and discover how you can prevent the loss of accuracy over time.

Flow Conditions

One of the most important factors for any weir is the flow condition. They’re only going to work under free-flow conditions, which are also known as free spilling conditions. That means that the water going over the crest of the weir, or the nappe, must be aerated well enough. Additionally, the downstream surface must not be elevated enough to overtake the crest of the weir. If it does, your weir is no longer free flowing but submerged instead. Under submerged conditions, you won’t be able to get accurate measurements.

The Point of Measurement

The point of measurement in a weir is a bit different compared to a flume. In a flume, the point of measurement is typically within the device itself, but a weir creates conditions in which the point of measurement would actually be upstream of the weir, typically at a distance of three or four times the maximum head. Measuring anywhere closer would be compromised by the drawdown of the flow’s surface that occurs as it approaches the weir crest.

Flow Rate Limitations

While weirs are certainly adept at providing accurate measurements, they are quite limited by the rates of flows they can actually measure. When working with a weir, it’s important to consider what your minimum flow rate is. The 22-½ degree V-notch weir has the lowest minimum flow rate out of all weir configurations, but even that figure is still relatively high when compared to a lot of flumes like the 1-inch Parshall flume. Be sure to consider what minimum flow rates you’ll be dealing with before opting for a weir.

Head Limitations

Much like flow rate, you’ll find head limitations in weirs as well. In this case, you’ll need to consider the maximum head you’ll be dealing with in your flow channel. For the most part, the maximum head limit for weirs is about two feet, but that’s not always the case. There are some weirs and data sets that can be useful for higher heads, but the accuracy of any measurements tends to falter quite quickly after two feet.

Clearing Sediment

To get the most out of your weir, you’ll have to perform regular maintenance. Fortunately, this maintenance is fairly standard and relatively simple. One of the most important things to do is clear the sediment from the weir pool that will inevitably build up over time. With enough buildup, the dimensions of your weir pool will change, and that means the flow won’t be acting the way you need it to. It’ll speed up, causing all your measurements to underrepresent the actual flow rate.

Weirs from Tracom

Now that you know how thin-plate weirs work, it’s time to get one of your own. That’s where Tracom can help. Contact our team today to design a custom weir box for your unique flow channel conditions.

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