Quote:
Originally Posted by Canadian
As a real world example: the overflow in my AIO is 4" long with a flow rate of around 225 GPH. A 3" overflow is adequate to handle that flow rate but even at 4" the surface of the water is around 3/8" above the overflow. The "velocity" of flow is great enough that it "draws" things towards it but the majority of the actual water flowing over the overflow is coming from beneath the water's surface and is therefore woefully inefficient. If I adjust the flow so that I don't get any surface agitation I can see the dust and organics accumulating on the water surface and how little of it is actually drawn over the overflow.
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I find similar results with my tank. The overflow has about 10" of linear distance and at my standard flow rate of about 1200GPH it works great but if I reduce the flow same deal, I get a scum build up. Your theory would seem to suggest you need to increase the overflow size for the smaller flow rate, which to me doesn't make any sense. In fact when I cover part of the overflow at the lower flow rate I can immediately see the surface scum being pulled towards to overflow. Don't get me wrong the theory is sound the larger overflow will pull water from the surface but you need a larger flow rate to create the surface tension so it draws in water from across the tank, not just the area around the overflow.
In the end I guess we have to all do whatever makes the most sense to us. But I would have to disagree and say it's much easier to make an overflow to big than too small. Worst case if the overflow is a little small you'll bring in some more water from below the surface, this doesn't sound like a bad effect as the increased surface tension will still skim the surface efficiently, if the overflow is too big for the flow rate you don't get good surface skim, it's right there in your own example.