headspace in fermenter question

If you ferment 5.5 gal of 1.040 wort to an FG of 1.010, you will create 83.5 gal of CO2. Your headspace would be 13 - 5.5 = 7.5 gal, so you have enough CO2 to purge the headspace ~11 times. Doing the math of continuous dilution, your post ferment O2 concentration in the headspace would be about 3 ppm (parts per million.) If we assume that there is 0 O2 left in the finished beer, then your TPO (total packaged oxygen) would be 3 ppm * 7.5 / 13 = 1.73 ppm, or 1,730 ppb (parts per billion.) Commercial breweries look for TPO's of 200 ppb or less for good flavor stability (ref: page 21 of http://www.craftbrewersconference.com/wp-content/uploads/2015_presentations/F1540_Darron_Welch.pdf.)

So, that fermenter might not be a good bet for beer you want to age, but if you drink your beer relatively quickly, it probably won't be a problem. Also, you are more likely to get more TPO contribution from your packaging process (racking, bottling/kegging) than from the fermenter, unless you are doing closed system transfers with CO2 pushing.

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For this case you produce 76 gal of CO2 that sweeps thru 1.5 gal of headspace (a 50.6 CO2 to headspace volume ratio.) With such large dilution, the residual O2 is less than 1 part per trillion!

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