Here is a diagram that shows my system a bit better.
It will work in even the smallest subs,or the big ones.
The tank can be in the wet or not and of any size you need.
For wet aplication a line runs between WTC and tank so that air from the WTC enters the tank with a one way valve preventing the water to enter the WTC.
The gear pump,solinoid valve also can be in the wet and are wired so that they BOTH open at the same time.
Reversing switch for the pump can be electronic or microw switches on the varible radio ch or a 3 position switch center is off,up is serface and down is dive.
The tanks vent can vent at the sail to prevent the sub from going out of ballance or just vent to the rear if there are vents in the upper deck.
To operate turn on the pump to draw water into the tank,air will exit and water will take its place,when full ,you will know because the sub has submerged,turn the pump to the off position.
To serface turn on pump to the suction out or oposite than the above and the one way valves will close not letting any water back into the tank,but the one valve that is turned the oposite direction at the bottom of the WTC with a filter will alow the air in the WTC to take the place in the ballst tank,this creats a vacume in the WTC and if there is water inside it will be evaquated into the ballst tank and the sub serfaces.
Once on the serface at any time open the WTC vent valve to return the WTC to its normal state.Just be sure to do it before you dive again as this would put more vacume on the WTC when you serface and be sure to close the valve before you dive.
Thats it very simple and reliable that works very scale like and a hover can be achieved or any depth desired or water level for ballasting with ease.
Also its best to use one tank in the center if possible,easest way to ballnce and less complicated to use ect.
2 one way valves of 1/8 inch size ,I like the ones from an aquarium,I will post picture.
2 Normaly closed solinoid valves of what ever voltage and size desired.
Aluminum tubbing or large fuel line and 3 fuel filters keep debri out and pump clear and self clean on diving!.
Tank of PVC pipe or what ever and baffles.Note the top vent line has slits down the lenth ,this is so that it will pick upwater/air at any attitude the sub is in pitch wise.
A gear pump like the one pictured works the best or smaller for small subs.
Thats about it .
If you are concerned about the vacume it does not get so bad as to break seals as the usual ratio from ballast tank size to WTC volume is about 3 to one.I use bellow seals and they dont even retract when serfaceing!