Originally Posted by maffewl
I respectfully disagree. Purging the O2 prior to racking creates a "cloud" of Co2 on the bottom of the keg. Opening the keg to rack doesn't allow oxygen in any more than opening the carboy for a gravity reading. You have a cloud of Co2 protecting both. Is is absolutely necessary to always purge the O2 prior to racking, not really, I don't always do it. But it doesn't hurt.
I would think the amount of time the lid is open to fill the keg would allow the co2 cloud to escape and the oxygen to get in just as well, no? As you put the liquid in, the space that the liquid takes up will push any gas, co2 or oxygen out of the top, so in struggling to see the point of putting co2 In and then letting it all escape out as you fill it. I know it's not a lot, but it seems like a waste of co2.
As for opening the carboy for gravity readings, you are still exposing it to oxygen. It's not as if the co2 remains dormant and just sits there, ya know? Hopefully you'll only have it open for a couple seconds and then close it back up. Otherwise, by your point, air locks would be useless.
If I have come off jerky, I don't mean to. Just trying to plea my case