more of a constant level/top off system

[mastertech]

well i have this idea to keep the water level in the sump at a constant level. now this is not really a automated top off system but i think it would be easy for someone to build and maintain in order to keep the water level stable (for proper skimmer tune?) its just a brainstorm so dont be toooo harsh. you either like it or not.
explaination:
there is a rigid sealed container with fresh water in it. there are two 1/8 inch flexable lines from the sealed container to the sump. one will feed water into the sump and one will cause the water to stop flowing.
once the sump water level covers the level of the air bleed line this will cause a vacuume inside the sealed container and stop water flow.
once evaporation causes the water level in the sump to go below the air bleed line this will allow a pressure increase and force water out of the sealed container and back into the sump until the level is constantly maintained.
topping off of the sealed container will be easier than topping off the water in the tank because there is less concern for amount of freshwater entering the tank in a short period of time, hence the reason for 1/8 tubing.
sorry if thats a little hard to follow.
here is a drawing.

 

[scsinet]

I'm usually pretty good at looking at things like this and figuring out whether or not it will work with relative certainty...
But this... I've been staring at it for 5 minutes and I'm not sure. It looks almost too simple to work, but it just might. Have you tried any small scale tests to see how it would work?

 

[mastertech]

I have tried some small scale things and it will work but the sealed container has to be very ridgid otherwise the container will contract and collapse causing all the water to be expelled. there will be a certain amount of "buffer zone" that the water will continue to flow until the pressure differential negates the weight of the water. still..... the i have never tried it with 5 gallons of water weight. on a "re-thought" the best thing to do in theory is to place the bottom of the sealed container at the same height of the water level of the sump reducing excessive pressures.
new pic

 

[beaslbob]

not only will it work but there is actually a comercial product that uses that method.
I would think the output from the water tank should be above the water level of the sump so that salt does not mix with the fw until the fw is added to the system.
also as with all ATOs one thing to consider is overflow failure. When the overflow fails the sump level lowers kicking in the top off. The display then fills with water. That can create a display flood.
my .02