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There's a lot of confusion about this issue, which for a while caused me to come to false conclusions about how to deal with CO2/O2 in the brewery, as well. The best thing you can do is pick up a physics book and look at the behavior of mixed gases. (By the way, that gases remain mixed is not caused by rain and wind in our atmosphere.) The reason there’s less oxygen available to you when you climb a mountain is that the air is thinner at those altitudes than at sea level, but the mix of gases remains constant. (Don’t quote me on this, but I think it remains fairly constant up until about 20 km out.)
The "other thread" about this is entitled CO2 Regulator for transfers. Yes, CO2 "flows" out of a fermenter and sort of "settles out" temporarily but will homogenously mix with the air in any enclosed space. The model most people have in their heads is too simple. CO2 is denser than oxygen, nitrogen, etc. but that doesn't mean that a sample from any part of the room (or of a tank), given a little time, will contain less CO2 than another sample. Keep in mind that it only requires a little CO2 to knock someone out. A "blanket" of CO2 can, therefore, refer to a minor increase in the CO2-concentration. However, in fermentation areas, the fans for venting CO2 are close to the floor, because the CO2 does not immediately diffuse throughout the entire room. I don’t have the data in front of me for how quickly this diffusion occurs, but I could probably find it somewhere, if need be. It would be very interesting to find out the amount of CO2 required to “burn” a person’s nose and eyes, if anyone with the proper equipment wants to subject themselves to such a test. When I worked in a brewpub, I judged whether my serving tanks were sufficiently purged by doing the sniff test, and I didn't have any problems. Of course, my beer wasn't packaged. I think the post about the study which stated that after a couple of purges you're wasting your CO2 sounds feasilble. Some O2 will remain in your tank no matter what. If you're moving young beer from your fermentation tank to a maturation/lagering tank and suspect O2 is a problem at this stage, then kräusen. It's good for your beer, anyway. And don't forget to purge your pipes along with your tanks (some brewers forget to do this).
The only sure way I've heard to purge a tank was also posted by someone on this site at some point, but it's not very economical and therefore impractical for most of us (esp. those without water deaeraters): Fill the tank with deoxygenated water and then replace it with CO2 or N2 as the water exits the tank.
The "other thread" about this is entitled CO2 Regulator for transfers. Yes, CO2 "flows" out of a fermenter and sort of "settles out" temporarily but will homogenously mix with the air in any enclosed space. The model most people have in their heads is too simple. CO2 is denser than oxygen, nitrogen, etc. but that doesn't mean that a sample from any part of the room (or of a tank), given a little time, will contain less CO2 than another sample. Keep in mind that it only requires a little CO2 to knock someone out. A "blanket" of CO2 can, therefore, refer to a minor increase in the CO2-concentration. However, in fermentation areas, the fans for venting CO2 are close to the floor, because the CO2 does not immediately diffuse throughout the entire room. I don’t have the data in front of me for how quickly this diffusion occurs, but I could probably find it somewhere, if need be. It would be very interesting to find out the amount of CO2 required to “burn” a person’s nose and eyes, if anyone with the proper equipment wants to subject themselves to such a test. When I worked in a brewpub, I judged whether my serving tanks were sufficiently purged by doing the sniff test, and I didn't have any problems. Of course, my beer wasn't packaged. I think the post about the study which stated that after a couple of purges you're wasting your CO2 sounds feasilble. Some O2 will remain in your tank no matter what. If you're moving young beer from your fermentation tank to a maturation/lagering tank and suspect O2 is a problem at this stage, then kräusen. It's good for your beer, anyway. And don't forget to purge your pipes along with your tanks (some brewers forget to do this).
The only sure way I've heard to purge a tank was also posted by someone on this site at some point, but it's not very economical and therefore impractical for most of us (esp. those without water deaeraters): Fill the tank with deoxygenated water and then replace it with CO2 or N2 as the water exits the tank.
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