Maybe I'm misunderstanding, but why do you push the solution uphill?
And can someone explain the advantage of pushing sanitizer with fermentation gas? I mean, I get that you'll use a helluva lot less bottled CO2 by pushing sanitizer out than you would to do 18 fill and vent cycles, but a mid-gravity five gallon ferment produces more than enough pure CO2 to purge an empty keg (or maybe two?).
Pushing uphill should avoid starting a siphon. Not sure if a siphon is even a possibility with the closed system (sealed keg fermenter, sealed keg with sanitizer, empty keg or bucket with end of hose submerged in pushed sanitizer) but I'd like to avoid it.
The idea of pushing sanitizer is you can be more sure that whatever has displaced the sanitizer is CO2. As long as everything is sealed nicely then there won't be any O2 getting in. In addition, the remaining fermentation CO2 can be used for whatever else you want, continuing purge of emptied keg, spunding, purge another vessel, etc.
If you're just using fermentation CO2 to purge the empty keg you are most likely going to purge that vessel entirely... A mid-gravity 5 gallon ferment will NOT purge two 5 gallon kegs effectively, depending on how you define effectively (5ppb, 50 ppb, 500ppb, etc.)
@doug293cz fantastic post has all the equations you need to calculate this. I think it's also important to note that the equations are worst case scenario and it's possible that the fermentation CO2 purge does a better job than calculated.
For me, depending on how I'm dry hopping and/or spunding, pushing sanitizer out is a must.
I think I've linked this here before but it's a spreadsheet I cobbled together using equations from
@doug293cz post.
Keg Purge Final O2
