If you have a cask breather, you can hook it up to the gas-in side. As the sucking begins it will pull CO2 from your tank.
I have one and used it a few times. It works well.
So i opted for a keg cold crash, after a closed transfer to my corny. This seems like the best way to guarantee zero oxygen pick up during crashing. I use a floating dip tube system that pulls beer from the top, so i won’t really risk much yeast pick up with the beer pulling from there and it will all just sit on the bottom. I also dry hop using a filtering system, so there is no sediment in the beer whatsoever.
In order to transfer I rigged up this system with my big mouth bubbler: Ran the Co2 gas hose directly into the fermenter through the hole where the airlock would typically go and ran the beer into the keg via the liquid out post and a tube attached to the spigot. Prior to this i purged the keg with Co2 and a full 5 gallons of sanitizer. It’s currently crashing in the fridge. going to burst carbonate starting this evening.
If you do not apply internal pressure to the keg while cold crashing I believe it will pull O2 into the keg.
Kegs aren’t designed to hold a vacuum, they’re designed to hold internal pressure, so as the liquid cools air will leak into the keg around the seals just like suck back thru an airlock.
Of course, just like everything I post, this is simply my opinion. Others may disagree which is certainly their prerogative.
Totally. I sealed the keg with a few minutes of Co2 after i transferred everything at around 10PSI. 12 hours later, still pressurized, but I hooked up the co2 to serving pressure anyway just to be sure - before ramping it up to 30/40PSI tonight.