Flumes have a constriction in the flow, and a "hydraulic jump" that causes the head height to be linear and repeatable. There are dozens of kinds of flumes. In modern times, the most common are the Parshall and the Palmer-Bowlus flumes.
Parshall flumes are commonly used for flow measurement at the inlet and outlet to wastewater treatment plants and surface water intakes for treatment plants and power plants. Typically, a Parshall flume is supplied as a fiberglas "liner" that is installed and then cemented in place (Figure 3).
Palmer-Bowlus flumes were originally developed for measurement in sewers and other open conduits with relatively low flow. There is no constriction, as with a Parshall flume, but there is a complex hydraulic jump that forms the point where the height is directly proportional to flow rate, the "H" in our equation (Figure 4).
Making Flumes and Weirs Work
For a flume or weir to be accurate, it must be installed properly. Most error problems with open channel flow measurements are traceable to the accuracy of the flume or weir, not the level measurement or the flow transmitter.
All flumes and weirs must be installed as close to perfectly level inlet-to-outlet as possible, and side-to-side, too. Fiberglass flumes often "bow in" when concrete is poured behind them, and this produces unexpected inaccuracies, which may not be able to be calibrated out, except by in-situ flow testing and the use of a strapping table in the flowmeter.
All flumes and weirs have what is called the "submergence point" where the level goes above the measurement range of the device. Flow may continue to be moving in either direction during submergence, but the measurement is meaningless during that condition.
A Palmer-bowlus flume
In the early 1990s, a special kind of flume was invented, tradenamed Datagator. This primary device is based on a Palmer-Bowlus flume, but is capable of measuring above the submergence point of the standard flume, as well as measuring flow in both forward and reverse directions (Figure 5).
Level Measurement Instrumentation
There are many ways to measure the level behind a flume or weir. The oldest, based on the Nilometer, is the Mark I Calibrated Eyeball with a staff gauge. A staff gauge is a ruler-like device mounted at the measurement point in the flume, weir or channel. The user "eyeballs" the reading on the staff gauge and
records the level for use in calculating volumetric flow.
Other types of level measurement devices that have been used for open channel flow monitoring include ultrasonic (downlooking) and ultrasonic (uplooking); pulse radar, differential pressure transmitters, and capacitance and RF admittance devices.
For many years, floats have been used, with stilling wells, to measure the level in the channel or flume or weir. Each of these types of level measurement devices have their features and benefits, but the accuracy of each as a flow measurement instrument is dependent on the accuracy and correct installation of the flume or weir of which it is measuring the level.
So now you have a quick, back-to-basics look at open channel flow monitoring.
I'm Walt Boyes, and if you are like most automation professionals, you don't do flow measurement design daily or even weekly. So some of the basics may be new to you…especially in the area of open channel flow.