How to calculate water height for a given overflow (weir)

Hotpuppy

In Memoriam
One of the mysteries that plagues reef keepers is figuring out your water height for a given overflow.

Fortunately, this has been solved elsewhere.

An overflow is also known as a weir (pronounced weer). They are very common in irrigation, flood control, and pools.

Here are some links to help you calculate your expected flow rate for a given weir.

Link 1: Easy to use solves for gallons per minute (divide flow rate by 60)
WSU Irrigation

Put in the width and then the desired water height. For our purposes, you want to play with the water height until you see the flow rate you have. This will tell you for a given pump output, how high your water level will be over your overflow.

Link 2: Orifice Flow calculator (aka How much water through a pipe of X inches)
http://www.flowsizer.com/content/orifice-flow-calculator
returns a value in CFS (cubic feet per second)

Link 3: Cubic Feet per second to Gallons per minute:
http://www.sfasoft.com/calc/103/Finding_GPM_with_CFS?

I recommend being conservative with your head pressures.... Remember head is measured in feet, so .25 ft of head is 3 inches... which is alot in our world for an overflow.

You can use orifice flow for estimating flow rate as well. Just measure water height over your intake pipe and plug the numbers in to see what's going on.

If you are using a overflow with teeth, simply add up the space between the teeth... example, an overflow with 10 teeth that are spaced 1/4 inch has 2.5" of flow area.
Enjoy
 
Last edited:
One thing you will notice is that an increase in the water height results in a significant increase in the flow rate.
 
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