BeerBorg

I came across this post on HBT in the midst of a thread where somebody was trying to solve his oxidation issues with his kegs. Along the way, somebody cited the "CO2 is heavier than oxygen, so you should have a blanket of CO2...." statement. Another brewer responded with this:

Time for today's science lesson. Gases spontaneously homogenize due to diffusion, rather than stratifying by molecular weight. Watch the following video. Br2 is 3.6 times heavier than CO2, and it homogenizes with air in about 30 min (for two ~1 ft tall columns.) CO2 is about the same molecular weight at NO2 (44 vs. 46.) As shown later in the video NO2 homogenizes much faster than Br2. CO2 will behave more like NO2.

It definitely makes me re-think the "CO2 blanket" that we've all discussed. Based on this, I'd say that it's highly unlikely that the blanket cannot exist. But just because I saw it on the internet doesn't mean it's true.

Any science guys out there care to comment? If this is true, then how can you ever truly purge a keg?

When I think Co2 blanket I imagine that all of the oxygen has already been purged from the vessel. The video is interesting though, thanks for posting.

I think that's our hope isn't it? But I know we (collectively) always think/say that if you open your fermenter quickly there should be no issue with oxygen exposure because there is a blanket of CO2 covering your beer. I know I've used the expression that way, and always thought that if you didn't fully purge your keg that the oxygen and CO2 wouldn't mix and you'd have CO2 on the bottom with oxygen on the top - like oil and water.

I recall reading elsewhere that to 'fully' purge a keg you'd have to purge it by releasing the pressure valve somewhere around 23 times. And even then, it wouldn't be 100% purged. In any case, I find this interesting.

I always purge again with co2 just before filling my kegs. Just to be sure, ya know

I stopped purging before filling my kegs because I wasn't noticing a difference by doing so. After sanitizing I fill with beer through the open lid, using a racking cane and/or tubing. Then I seal the lid with about 30psi, clamp it down, and purge 3-4 times after it stops pushing CO2 through. This works for me but I've always envisioned the separation of oxygen and CO2 as water and oil inside the keg knowing that I couldn't ever fully 100% purge it.

It's just another piece of data that I will store somewhere in my head replacing something important like a birthday no doubt.

Very interesting. I don't purge before filling the keg, but I do purge after filling.


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I do purge before filling with lid locked down tight, but I fill thru the beer post from the bottom up, have a liquid ball lock on the end of the auto siphon so the beer is pushing any gasses out the vent.

I don't purge my kegs. I do use CO2 to push starsan through, then I fill and pressurize. No oxidation issues. I think that the relatively small amount of air left in a full keg is not a threat. I did not have oxygen issues when bottling either, although there is still an inch of air in the neck.

Bottling is a related discussion, too. If you are bottle priming then the mini fermentation that goes on within the bottle will eat up some of the oxygen in the bottle. How much exactly is for smarter people than me to answer, though.

I've not had oxidation issues either, but find this topic really interesting. I mean, how much oxygen in the keg will cause a problem? How many times do we REALLY have to purge them to avoid issues? I'm sure we've all developed routines that work for us, but I wonder how much is either overkill or not even really addressing the issue.

I've always shot some CO2 into the keg before kegging and will continue to as it's just another weapon to help avoid oxidation. It doesn't take much.