I'm done primary and want to transfer to a keg for aging a few weeks before I dry hop. Do I need to purge o2 after transfer and how do I do this using an airlock on the keg? The batch size is 2.5gal and I'm using a 2.5g keg. Do I even need to purge? Thanks, I'm a fairly new brewer and haven't aged a brew for a month after primary yet so a little unsure.
Absolutely you want to purge. You shouldn't be filling above the gas input tube, so there will be some headspace. After transfer, put on a lid without an airlock and give it three or four "burps", i.e. shoot in the CO2 at 15'ish (or a bit higher) and vent through the PRV. This reduces the oxygen (air) to nil . . . otherwise oxidation is likely. I don't mess around with an airlock when aging in a keg . . . suggest you just let it age. If any CO2 is produced it won't hurt a thing to stay in the keg. When you open to add/pull the dry hops you should re-purgigate. What's the brew?
I've noticed it takes some pressure inside the keg to get the top to seal airtight. Do you pressurize the keg after filling?
It would be fun to watch high PSIs causing the airlock bell to hammer away while purging, but I agree with @PortLargo, skip the airlock for aging. The airlock would slow down O2 ingress, but the normal keg lid would virtually eliminate it.
