Originally Posted by suzanneb
so what you guys are saying is the surface area of the liquid(based on the diameter of the vessel) will affect the rate of oxidation. the height will affect how much oxygen is available. but since the beer is usually filling the headspace with co2 and no new air is getting in. and since fermentation is pretty much done and there is much more oxygen available to invade the beer won't this cause a problem?
During active fermentation, CO2 will be produced and will displace O2, so it's really not an issue. You'll have some protection after fermentation is mostly done because there's still CO2 in solution that will outgas, but eventually you run out of protection. So once fermentation is done, if you're worried about oxidation, you need to take steps to prevent it---the beer can no longer protect itself. The easiest way is to fill the carboy all the way up to minimize both exposed area and reduce the head space volume to a negligible amount. If you can't do that, then adding a neutral blanket (CO2 or N2, or I've heard reports of food grade mineral oil being used, but I really wouldn't go there) is an alternative.
Note that, for all the talk of a CO2 blanket forming, that's not really quite how it works. Unless the beer produces/releases enough CO2 to displace all the air in the headspace, the CO2 it does produce will mix quite thoroughly in there, so you'll get oxygen exposure. When it produces a huge amount of CO2 at the beginning, it mixes as well, but it's continually pushing the mixed air out, so the concentration quickly tends toward pure CO2. If you put a blanket in, you've then got to put your airlock on to keep it in place.
Finally, let me just say that I'm a n00b, so I'm not necessarily saying you will or won't have a problem with oxidation. So you may be fine just letting it ride, my advice is aimed at taking the best action if you decide that you're worried about oxidation in the first place.