Purging O2

I made a rookie mistake and forgot to purge the oxygen from my keg before force carbonating. I had about 3 inches of clearance to the top when I set the room temperature keg into the fridge and cracked it to 40 psi. I let it go about 4 hours before realizing my mistake.

Its a really strange brew (India Brown dry-hopped with Citra and coffee beans) so I have no comparative flavor to test against.

So the question I guess is, how much oxidation could have occurred during that time?

I have done this before as well, I am interested to see what the experts have to say.

Not much, in my experience. There's really not that much oxygen in 3" of head space.
Just some quick math - 9" diameter by 3" high is 190 cubic inches of volume. "Air" is about 21% oxygen. So that's about 40 cubic inches of actual oxygen in the head space (give or take - at atmospheric perssure)

Lets say you carb to 2.4 volumes. That's about 2772 cubic inches of CO2 dissolved into 1155 cubic inches of beer.

About 1.4% oxygen/CO2 for all of 4 hours IF you actually mixed that air in the head-space with carbonated beer.

EDIT: (My math was off) Since you're not carbonated yet, it's 3.4% O2 to beer - But still...

Next time purge a little, and this time RDWHAHB

By the way, purging the head-space doesn't even displace all of the air in the keg. Pumping a high pressure gas (CO2) into an enclosed space with another gas (air) causes the 2 to mix. Purging could theoretically reduce the air content of the head-space by 1/2 each time you purge 190 (ish) cubic inches of gas from the keg, but the thought of mitigating that makes my head hurt

Well it doesn't really get any more definitive than that. The response is very much appreciated.

Commencing relaxation.

The effects of oxidation take some time. If you drink it fast you are less likely to notice any effects of your blunder. Enjoy!

also, the CO2 will sink creating a blanket between the beer and the O2
so, it was protected anyways.

Thanks for the information! Much appreciated.