DIY auto top off concern

[MGarvo]

I'm slowly completing my first reef aquarium "build." Much of my project has been DIY, but, as many of you are keenly aware, there is a bit of trial and error expected in the process.

My next stage is to either build or buy an auto top off system. I'm quite intrigued with the style in this picture I borrowed from a google search. Simple, clever, and very cheap!

What gives me concern, however, is the thought that there may be a flaw in this design. Here is my thought, and my question: By sealing the air inlet/RO H2O release via negative pressure through contact with the circulating salt H2O in the system, won't the reserve RO H2O become salty as well through osmosis? It seems like this design may be no more than volume replacement, at the cost of dilution of the system as a whole?

If this concern is valid, I've been considering the AutoAqua Smart ATO. Thoughts on this system?

 

[capecodder]

I see how that would occur. Salt isn't going to evaporate into it. The fact that only the fresh water evaporates is the exact reason why you need a fresh water ATO. I think there are some logistics to consider though.
1) It would be good if the tube was height adjustable. Maybe you do a partial water change and your system equalizes your sump at a slightly different level.
2) it should have a shutoff in the tube so that tank water changes and RO refills can occur.
3) I'd be a bit concerned about getting a good reseal each time you refill it. I guess you could put another bulkhead fitting and a ball valve at the top.