Sump physics question

[Cksnffr]

I have a 200-gallon tank with a 40-gallon sump underneath. The setup is pretty much routine: two tank drains empty into two filter socks in the sump; the first third of the sump (with the socks) has a skimmer; the middle third houses heaters for now; the last third houses my return pump, which splits off into two returns.

Given the structure of the sump, the water level is constant in the first two sections. When I add or remove water to/from the whole system, the change is reflected in the water level in the third section with the pump.

The return pump is a DC model with easily variable speed. Here's the question: Why does the water level in sump section 3 drop when I crank up the pump speed and raise when I dial down the pump speed? I expected it to stay at the same level since it's all a closed-loop system. When I turn up the pump and the water level drops in the sump, where is that water going? My drains are not undersized; the water level drops a couple of inches and stabilizes.


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[viboy]

I would think that is just because of the size of the drains. The volume up top creates the pressure in the drain. The drain is draining based on the water volume. When you crank the pump up it increases the water volume in the tank and then takes a second to increase the volume draining. For example. If you fill a water hose and then hold it a foot of the ground it drains slowly; however, if you fill that same hose and attach it to a 20 gallon garbage can above it the water pressure increases and it drains the water faster.

 

[Cksnffr]

That's it! The missing water is _inside_ the drains! When the pump was dialed back, the drains were only partially filled as they did their job.

Thanks!


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