When you’re implementing a weir for flow measurement, there are several factors that go into taking accurate measurements. The weir needs to be properly installed, and the flow needs to be effectively regulated. Even the measuring itself has to be done properly if you want to reach that +/- 2% margin of error most weirs can promise.
To get the most out of your weir measurement, there are a few problems you have to look out for. By knowing what to avoid, you’ll be in a better position for success whether you’re looking to get a new weir structure or perform maintenance on what you already have. Here are the most common weir flow problems to look out for.
Far too many weir flow problems begin with the installation itself. The installation has to be done well or you won’t come anywhere close to getting accurate measurements. Without being properly installed, the flow could bypass it entirely and compromise your entire flow measurement efforts.
To ensure proper installation, have it mounted with the crest in the center of the channel, vertically plumb, and level from side to side. That’s the only way to guarantee flow accuracy, even if you manage to prevent the water from bypassing your weir entirely.
After installing the crest and the weir correctly, the crest itself must be properly maintained. Failing to do so will lead to inaccurate measurements, as it doesn’t properly allow for consistent flow that’s reliably measurable. That means making sure the crest is free of any nicks or other damage. If it becomes too rounded or accumulates trash, that’s going to throw off your measurements as well, so maintenance is absolutely essential.
Poor Velocity Profile on Approach
In order for a weir to provide proper flow measurements, it must receive a satisfactory flow velocity on approach. For the most part, it should be moving around 0.5 feet per second. You’ll find a wide variety of available options when it comes to managing approach flow rate like that.
Additionally, the weir has to be the proper size for the flow approach velocity. The crest level needs to be at the maximum anticipated head with a channel that’s two times as big. This will allow consistent and measurable flow through the weir for optimum performance. Any deviation from those guidelines will lead to inaccurate measurements.
Monitoring the approach of the flow is certainly important, but you must monitor the downstream conditions as well. One of the most common problems in that regard is having a submerged nappe. If the weir is oriented in such a way that the water level after passing through the weir is enough to submerge the nappe, or the water flowing over the crest, then it will significantly impede the flow of water. That, of course, will lead to widely inaccurate measurements that defeat the entire purpose of a weir.
Finding a Quality Weir
Knowing what weir flow problems to look out for puts you in the perfect position to get a weir of your own. At Tracom, you’ll find an extensive selection of types and sizes available for tackling a wide variety of flow channel setups. Contact us today to take the first step into getting a customized and long-lasting weir.