Can anyone explain why the rule of thumb for adding corn sugar in kegging is 1/2 of the norm (i.e. 3/8 cup vs. 3/4 cup)? I’ve just been adding the 3/4 cup in my kegs and am carbing fine. I’m worried that the 3/8 cup would be too little and I’d end up having to force carb a bit.
The fermented beer will have some dissolved co2 in it, and maybe some of it is forced out of solution during bottling. A bottling wand is a pretty right constriction. Perhaps combined with a preference for kegged beer to have lower carbonation?
As far as I know there’s no reason to use less. I suspect that people who are making that recommendation are finishing the beer off by force carbonating.
I added a poll about how people mainly carb their kegs.
Serving pressure and shaking. No need to wait.
Set at serving pressure and forget. Unless I’m in a hurry then I’ll set it at serving pressure and shake.
You mean you set it around 12psi and then shake it? For 20 minutes or so?
Do you mean set at serving psi? Serving temp? I don’t understand.
Yeah, that’s a typo. Fixed.
You shouldn’t measure but weigh your priming sugar. If you use 3-4 oz per 5 gallons it should prime up perfectly for normal volumes such as 2.2-2.5 depending.
Sometimes I prime my kegs with sugar and push with the tank.
I do weigh my priming sugar. I was just using the 3/8 and 3/4 as points of reference so as to not confuse and get specific with oz.'s of sugar.
More like shake it for a few seconds at a time, every few minutes, for a few hours. The exact pressure depends on the temperature and carbonation level.
And ended up being more confusing. 
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.
I set the CO2 regulator to 60psi and force carbonate at 33F for 2 days.
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?
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).
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.