Naturally carbonate 3gal in 5gal keg

[chillaxis]

Hey all,

I typically make 2.5-3.5 gallon batches (I go through them relatively slowly, so if I brewed 5gal I wouldn't be able to brew as often as I'd like!) and want to start kegging sometime soon.

I understand that normally, with a 5gal batch in a 5gal keg, you need less priming sugar than you would if you were bottling, because there's less headspace overall. But it seems like it would be the opposite situation if I was to try to naturally carbonate 3gal in a 5gal keg, because there would be way more headspace, relatively. Is there any kind of formula or rule of thumb that I could use to calculate the right amount of priming sugar in this kind of situation? Or should I just skip the natural carbonation plan and restrict myself to force carbing?

 

[chillaxis]

Since no one has a reply for this, how about this idea: I put a lot of priming sugar in, definitely more than I need, and use something like this: http://morebeer.com/products/ball-lock-qd-adjustable-pressure-valve-wgauge.html to relieve any excess pressure (beyond what a force-carbing chart would specify). Any reason that wouldn't work?

 

[xumbi]

Just purge with extra co2 when you rack your beer. 3 gallons in a 5 gallon carboy is fine.