Selecting the Right Types of Weirs

Weir boxes are vital to directing open channel flow for measurements. They direct open channel flows for measurement purposes and are essential to these sorts of operations. They have a simple design, are easy to use and provide consistently reliable measurements. They are especially good for low-flow operations.

These boxes have various common elements, but picking the right one for your operation’s needs is vital to getting the right measurements and effective flow monitoring. Check out some advice about how to select the right types of weirs for your operation.

Types of Weirs

There are five major types of fiberglass weirs. Each is good for different operations. These include:

V-Notch: These are popular for low-flow operations of less than 1 cubic foot per second.
Rectangular: These come in either contracted or suppressed forms and are for high flows with complex discharge equations.
Proportional: These are ideal for man-made channels and are easy to install and use.
Circular: These are ideal for mounting inside pipes.
Cipolletti: These are uncommon and require more specialized knowledge and maintenance.

It’s important to know what type of weir you need and how it functions to get the right readings for your operation.

What to Keep in Mind About Types of Weirs

You have a wide range of types of weirs from which to choose for your operation. The major aspects of the device you need to keep in mind when selecting yours are the upstream freeboard, the channel width and configuration, and the weir pool size.

Upstream Freeboard

If you’re looking at a rectangular weir without any end contractions, you must select one with channel banks that allow the installation of breather tubes at each bank. The tubes are essential to permit air circulation between the downstream plate and the underside of the water passing over the weir’s crest (the nappe). Without these tubes, the discharge can overwhelm the device and the nappe can collapse against the weir plate. This leads to erratic and error-prone flow readings.

The smaller the notch opening, the higher the head upstream has to be to pass the necessary flow rate. Check to be sure that the flow won’t rise higher than the plate and the upstream banks.

Channel Width and Configuration

Channel width and configuration are a concern if you choose a weir that isn’t rectangular and without end contractions. If you choose a width that is too narrow, the flow will reach a higher velocity as it passes through than that on either side, which will create erroneous readings. Ideally, you want your channel width configured to be at least twice the maximum head over the weir.

Weir Pool Size

A weir’s pool size depends on the maximum head the device can handle, unless you have additional conditioning for the flow. The weir pool should always be straight and of constant dimensions. It should stretch a minimum of 20 times the maximum head over the weir to function properly.

Find Your Perfect Weir at Tracom

If you need more information or advice on which weir to use in your operation, Tracom is ready to help. Check out our complete range of weir box options we offer and get in touch with us to place your order or answer your questions today.