5 gallons in 15 gallon fermenter?

[mashdar]

Any opinions about fermenting 5 gallons in a 15 gallon keg?

I feel like it should be OK if not spunded from the start, but might impact purging of a second keg a little.

 

[mac_1103]

Just use @doug293cz's equation, right?

Final O2 Conc = 210000 ppm * ((V - 0.0001 L) / V)^4390000

where V is the total volume of the keg(s) being purged and the fermenter headspace in liters.

If I did this right (no guarantee) going from standard fermenter headspace of around 5 liters up to 38 liters increases the worst case scenario O2 ppb for one keg from 0.005 ppb to around 0.012 ppb. With a second keg you'd end up around 0.081 ppb.

 

[doug293cz]

Just use @doug293cz's equation, right?

Final O2 Conc = 210000 ppm * ((V - 0.0001 L) / V)^4390000

where V is the total volume of the keg(s) being purged and the fermenter headspace in liters.

If I did this right (no guarantee) going from standard fermenter headspace of around 5 liters up to 38 liters increases the worst case scenario O2 ppb for one keg from 0.005 ppb to around 0.012 ppb. With a second keg you'd end up around 0.081 ppb.

Your math doesn't seem to work out correctly. The "V" in the equation is the Total Purged Volume, which includes the fermenter headspace plus the volume of all the kegs.

I threw together a simple spreadsheet for the inputs to the equation, and ran 5 L & 38 L headspace volumes each for 1 or 2 20 L kegs. The O2 concentration in the bottom cell is in ppm.

Brew on

 

[mashdar]

I guess my real query is whether the slow purging of O2 from the fermenter might negatively affect the beer. E.g. after lag phase, it has to push out 10gal of air instead of 2-3 gallons.

I'd think yeast would grab any O2 it could, but I've never tried such a big headspace.

The keg purge was a bit if a red herring; apologies.

 

[mac_1103]

Your math doesn't seem to work out correctly.

Well apparently I was off by three orders of magnitude before I even started, so there is that. Guess I should have used a spreadsheet instead of a calculator...