Theory with Headspace

[Brewtah]

Not really a theory, a thought. In a fermentation vessel CO2 developes and the headspace is filled. As it builds the gas is pushed out through the airlock. When the production lessens the gas does not have the volume to push through the airlock?
Here is the question. Does CO2 remain at the maximum amount to fill the vessel? Does the CO2 slowly recede, eventually allowing oxidation?

 

[jitteringjr]

CO2 is heavier than air so as the CO2 is produced during fermentation, the O2 is being displaced and pushed out the airlock leaving you O2 only in the ppb amounts. So yes the CO2 fills the void and does not recede. Now when racking to the secondary after too long in the primary might cause some issues as not enough CO2 may be produced...

 

[Brewtah]

I was thinking of fermenting a 3 gal batch in a 5 gal carboy. I have multiple vessel, 5 gal is my smallest. I was going for primary only, no secondary. So then 3-4 week primary should be OK.

 

[jitteringjr]

Shouldn't be a problem. I have done 4 gal batches in a 6.5 gal fermenter without issue.

 

[Yooper]

In primary, there is enough c02 being produced that oxidation isn't an issue. With time, though, this co2 "blanket" that everyone seems to talk about dissipates and equilibrium is reached. The Ideal Gas Law comes into play.

Long term, even airlocks allow small amounts of oxidation and oxygen diffuses through the water into the vessel.

We've discussed this quite a bit in the Brew Science forum. Even with active fermentation, the headspace is never 100% c02, and it does dissipate.

The short answer is that for short term, the c02 does have a protective effect whether in primary or secondary.

 

[jitteringjr]

Sounds more like you are talking about Graham's laws of diffusion and effusion. The ideal gas law is just the PVT relationship of gas.

No doubt you are right that at some point with enough time the volume of gas in a fermenter will have the same make up as the gas outside the fermenter. The question is how long it would take. It would be cool if someone took the time and expense to install a DO meter on a home brew carboy fermenter and graph dissolved oxygen in the solution and charted it over a long period. Have you seen anything like that in the brew science forum?