### Poll

#### How do most people carb their kegs?

0 (0%)
2 (13.3%)
force carb for a couple days at 20-30psi
4 (26.7%)
force carb for 4-5 days at ~12psi
9 (60%)

Total Members Voted: 15

Voting closed: April 21, 2012, 08:14:48 PM

### Author Topic: 3/8 cup vs. 3/4 cup for kegging?  (Read 4413 times)

#### ukolowiczd

• Assistant Brewer
• Posts: 144
• Burlington, VT
##### Re: 3/8 cup vs. 3/4 cup for kegging?
« Reply #15 on: April 08, 2012, 03:02:07 PM »
I have not primed a keg yet, generally I go with Sean T's method of set to serving and shake. However I always assumed that the reduced amount of priming sugar had to do with the reduced headspace per ounce. The volume of headspace in a 12 oz bottle divided by 12 is greater than that in a keg divided by... 640? But that's just an assumption I came up with to justify what Charlie P says in the book.

right, that sounds like an actual mathematical answer! The head space per capita is actually greater in the bottle than the keg (if filled all the way). That would mean that you need more sugar to carb in bottles b/c more CO2 is let out of the solution into the head space. hmmmm, can anyone do the actual math to prove this?

#### a10t2

• Official Poobah of No Life.
• Posts: 4351
• Ask me why I don't like Chico!
##### Re: 3/8 cup vs. 3/4 cup for kegging?
« Reply #16 on: April 08, 2012, 06:23:38 PM »
can anyone do the actual math to prove this?

I can do some math that I believe disproves it. Say there's one fluid ounce of headspace per bottle, so roughly 50 fl oz (1500 mL) total, versus maybe 500 mL (one pint) of headspace in a keg. Once fermentation is finished and the dissolved CO2 reaches equilibrium, the head pressure (at room temperature) is going to be roughly 30 psig, or ~3 atm absolute.

The additional CO2 required to prime the bottles' headspace is 1 L * 2 g/L * 3 atm = 6 g.
The additional sugar required to produce that 6 g CO2 is about 13 g (0.45 oz).
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#### Jimmy K

• Official Poobah of No Life.
• Posts: 3646
• Delaware
##### Re: 3/8 cup vs. 3/4 cup for kegging?
« Reply #17 on: April 09, 2012, 12:59:59 PM »
[Say there's one fluid ounce of headspace per bottle, so roughly 50 fl oz (1500 mL) total, versus maybe 500 mL (one pint) of headspace in a keg.

I don't think the difference is even that dramatic. Assuming the headspace is formed by removing the bottling wand, then a 3/8" diameter wand in a 9" tall 12oz bottle would leave 1/2 oz of headspace per bottle.  Also, 1 pint in a keg is 0.44" of vertical space - many homebrewers are leaving more space than that. So headspace is probably close to equal or even greater in a keg.

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