A weir box can be one of the best tools for measuring flow rates under certain conditions, but they only work properly if they’ve been installed correctly. Unfortunately, there are some mistakes that tend to arise during the installation process that can cause some serious trouble. Take a look at four common weir installation errors.
Poor Approach Conditions
The approach channel is one of the most essential aspects of the flow rate measurement setup with a weir box. Several requirements must be met before it can be considered viable for measurement. Approaching flow should be no more than 0.5 fps, and the channel itself needs to be at least as wide as twice the anticipated max head on either side of the crest’s max head level.
When flow approaches the weir, it needs to be completely straight and uniform for at least 20 times the maximum anticipated head. That’s the only way to create a velocity profile that works for flow rate measurement. Any deviation from those requirements, and your measurements won’t be reliable.
Poor Downstream Conditions
The downstream is just as important as the upstream when it comes to proper weir functionality. Downstream condition requirements are a bit more straightforward, however, as they typically center around the level of the water after it passes through the weir. If the water level is too high, it could become higher than the nappe, which results in submergence.
In flumes, submergence isn’t always an insurmountable obstacle. It will make the flow rate indicate higher than it actually is, but it can be adjusted for in some cases. With a weir, however, submergence can affect the flow rate itself, effectively rendering the weir as a device that’s altering the flow rather than measuring it.
Not Leveled
For proper measurements, you’re going to need a clean installation. While the actual process can be difficult and labor-intensive, the requirements are relatively straightforward. The crest will have to be centered in the channel and level in all directions. Additionally, it must be vertically plumb. Anything less will throw off your measurements.
Poor Crest Condition
The crest of the weir is arguably the most important part of the entire device. Because of this, it’s important that it’s maintained as well as possible. For example, the crest needs to be in good condition without any abrasions or nicks that could affect its ability to function. Even minor damage can sometimes make quite a serious impact. Additionally, the crest needs to be sharp, though not quite as sharp as a blade. Just prevent rounding. Finally, it needs to be regularly cleaned to prevent the buildup of debris.
Weirs From Tracom
Knowing these four common weir installation errors to avoid means you’re ready to get a weir of your own. At Tracom, our team is happy to work with you to ensure you get the accurate measurements you need and deserve. Contact
