Automatic Top Off.. is when you have a method that fills your RODI into a small tank from a low level to a high level (mine fills from almost empty to just over 2.5 gallons) then that tank feeds the sump, usually where your main pump is because that compartment is the one that has the change of level from evaporation, to a specific level adding small amounts as required to account for that evaporation.
The reason for the small tank that feeds the sump is to avoid RODI short cycling. Short cycling is when your RO membrane is turned off and on frequently. Each time it is switched on there is a small amount of less healthy water at the start. So you want to cut that down to less often. So we fill a tank to have less of that lower grade water per volume, the bigger the better but with all the other reactors, sumps, and refuges the space gets tight and that is the exact reason mine is 2.5 gallons.
Then your sump evaporation level can be maintained to a fairly tight tolerance by a simple level device because short cycling is not a factor from the RODI fill tank.
...are you sorry you asked??
-randy-
