If you’re looking to install a weir and want to handle it yourself, you can virtually guarantee a successful installation that will lead to accurate volumetric flow rate measurements. All you need to do is follow the appropriate steps. Here are the most important tips for weir installation you need to keep in mind when setting up your weir.
Optimizing the Approach
To get accurate measurements, you’ll need to optimize the approach flow toward the weir. The weir can only provide accuracy if the flow approach meets certain conditions. While all flows are different, these conditions must match the type of weir you’re looking to install for every aspect to have uniform functionality.
The first step is ensuring that the incoming flow is straight and uniform for at least 15 to 20 times the maximum anticipated water depth. Then, the incoming flow must be traveling at a subcritical velocity and distributed evenly among the channel. Any turbulence or imbalance can create errors when it comes to flow measurement. Finally, the velocity pattern must be consistent throughout the approach.
Setting Up the Weir
When it comes to installing the weir itself, placement is the most important aspect of the process. First, the weir must be centered in the flow stream. If it’s to the side, the water may not pass over it evenly or at a point in which the typical equations can determine the flow rate. The center is determined by the location of the crest, or the edges in which the water actually flows over.
The crest itself also has several requirements to function properly. The lowest point of the crest needs to be at least two to three times the maximum anticipated water depth above the floor of the channel. Additionally, the walls of the weir need to extend at least twice the maximum anticipated water depth on each side.
As the flow exits the crest, certain conditions need to be met to ensure accurate measurements. First, you must determine where your downstream pool will actually be located. A general rule of thumb is that the water level in the downstream pool should be no less than 2 inches from the bottom of the crest. This will allow you to capture an accurate measurement by measuring the point of maximum head about three to five times that distance before the crest.
It’s essential that you take your measurements at the maximum head of the flow. Any closer to the crest itself, and your measurements will be affected by drawdown. Drawdown is the decrease in head as the water approaches the crest. When your downstream pool is too low or too high, the exact point of measurement is thrown off, so you can’t use the three to five times the maximum head distance from the crest to measure accurately.
Professional Weir Installation
With these tips for weir installation, you can tackle the challenge yourself, but Tracom is always happy to make the job easier for you. Our team will work with you to craft a custom weir suitable for your unique flow conditions and have it properly installed and offering accurate measurements in no time. Contact us today to learn more about everything we can do for you.