A weir is one of the best ways to practice flow rate measurement, though you’ll need to make sure it’s installed properly to get the accurate readings you need. Unfortunately, there’s quite a bit that can go wrong in the installation process that you’ll need to look out for. Learn how to install a weir, and discover how you can make sure your weir is working properly.
Maintain a Good Approach Channel
The approach channel is an important aspect of any flow rate measurement device, but weirs have special requirements you won’t want to ignore. They need to be straight and have a uniform cross section for about 15 to 20 times the maximum anticipated head or water depth upstream of the weir. This can be a tall order for some flow channels, so make sure you have enough upstream space before installation.
Ensure a Subcritical Approach Flow
Weirs are able to offer accurate flow rate measurements because the flow reaches criticality as it passes through the weir. To reach criticality at the right point, the approach flow needs to be subcritical because it reaches criticality via acceleration. Additionally, the subcritical flow will need to be balanced and uniformly distributed to ensure that criticality is reached at the proper moment.
Center the Weir
When installing a weir, you must make sure that it’s centered in the flow channel. That’s the only way you’re going to get accurate measurements. Anything that deviates from the center is going to throw off your measurements and could lead to a variety of other problems you’ll have to deal with. Additionally, the weir must be level from front to back and side to side.
Set the Crest Correctly
The crest is one of the most important parts of the weir, and making sure it’s centered in the flow channel isn’t enough to ensure that it functions properly. You’ll also need to make sure that the lowest point of the crest is the right distance from the floor. The right distance is defined as 2 to 3 times the length of the maximum anticipated head. Additionally, this crest must have its lowest point at least 2 inches above the downstream water level.
Measure the Sidewalls Properly
The sidewalls of a weir have some specific requirements in order to work properly. The distance between the sidewall and the edge of the crest must be at least 2 times the maximum anticipated head of the flow. Anything less than that will lead to flawed measurements and other problems for your weir.
Set the Right Point of Measurement
Measuring at the right point is your only hope for getting the accurate results you need. The measurement point is going to be upstream of the weir about 3 to 4 times the maximum anticipated head. If you measure closer, you’ll find the surface affected by drawdown as it approaches the crest.
Weirs From Tracom
Now that you know how to install a weir, it’s time to get one of your own. At Tracom, you can work with our design team to craft a brand new weir uniquely suited for your flow channel conditions. Contact us today to get started!