January 07, 2011

Dear Control Engineering: Do you have any information about measuring flow in an open channel? This is to measure flow over a dam. Are there companies that make such devices?

Regards, Bhumika Yagnik

Measuring flow through an open stream is probably among the oldest instrumentation applications you can imagine. One of the simplest approaches is to cut a notch in the dam, either a square cut or a V shape, and calculate your flow based on the height of the water (or whatever liquid it is) flowing through the cut. Using a V gives you a greater turndown ratio. This is called a weir, and there are formulas that allow you to make a pretty good approximation of the volumetric flow.

There is a second method called a flume where you measure the liquid flowing through a known geometry. The most common is probably a Parshall Flume (named after Ralph L. Parshall) that uses a combination of horizontal and vertical geometry. There is at least one company, Tracom Fiberglass, that makes fiberglass shapes to help you construct this configuration in your stream. This may require pouring come concrete depending on the volume you have in mind.

If you want something a bit more sophisticated, some ultrasonic flowmeters can be used to measure open streams. Alpha Controls and RS Hydro are companies that offer a selection of such units. Good luck with your efforts.

Peter Welander, pwelander@cfemedia.com