A weir can be an incredibly helpful device for measuring flow rate, but those measurements can only be taken accurately when certain conditions are met. If the installation is flawed, your measurements aren’t going to be accurate, defeating the entire purpose of the weir in the first place. Here are the most common weir flow errors that can throw off flow rate measurements.
One of the most common problems in flow measurements comes from a weir box being installed incorrectly. To ensure the flow rate is properly measured using the standard equations, a weir box must be installed in a particular manner. That means that the crest of the weir must be located at the center of the channel. Additionally, it must be level from side to side and plumb, or vertically level, from top to bottom.
For a weir to provide accurate measurements, the flow must take a particular approach before reaching the crest. This approach can vary based on the size of the weir and the intensity of the flow, so be sure to keep those variables in mind before determining what the appropriate approach conditions are.
Generally speaking, the flow approaching the weir should be traveling around 0.5 feet per second in a straight direction. You also need to consider the maximum anticipated head or the depth of the water as it flows through the weir. Ensure that there’s a uniform cross-section among the approaching flow equal to 20 times as much as the maximum head anticipated. Anything less, and the accuracy of any measurements can be compromised.
The crest of a weir is the point in which the measurement is taken. Because of this, the crest itself needs to be maintained to ensure accurate measurements. There are three primary problems that can arise regarding the crest, and you’ll have to be prepared to deal with them all. First, the crest can wear down over time. As flow passes over through long periods of time, the crest can receive nicks and abrasions altering its shape.
Worn crests are just the beginning, as particularly dirty flows can leave behind debris and trash. Even rust and biological growth can form on the crest, which will throw off measurements. Finally, the crest can get rounded overtime, which throws off the calculations. While you don’t have to keep it as sharp as a knife, having a relatively sharp crest is essential for accurate measurements.
The downstream conditions of your weir box can also have a significant impact on your measurements. While the measurement will be correct considering the crest and flow approach, poor downstream conditions can alter the actual flow rate making the measured flow rate useless.
Poor downstream conditions are often expressed by weir submergence. This is when the nappe, or the water flowing through the crest, is submerged. While there are ways to account for the differences due to submergence, it’s always easier to simply remove the submergence factor entirely. To do that, you can raise the weir crest, or clean the approach channel to remove any debris or sediment.
These common weir flow errors can be a hassle to deal with, but Tracom makes it easy. With our experts at your disposal, you can get a weir that’s specially designed to fit your unique flow conditions and installation you can count on. Contact Tracom today to get started!