# Temp swings and forced carbonation

Like some, my chest freezer is my fermentation chamber, lagering chamber, and in cases where I keg, my serving chamber. I have a keg of Oktoberfest in it now lagering and carbing up at 12psi and 35 degrees.

My question is, if I want to use the chest for a fermentation and remove the carbed kegs, will I need to dial up the psi on the regulator? What I would probably do is pull a growler's worth of beer at a time and put it in the fridge for serving the Okfest (and likely an IPA I will be kegging).

My two questions are 1. do I need to dial up the psi when the beer is warmer (~65 degrees) and go by the carbonation chart? 2.) will this cause any detriment to the beer?

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## 1 Answer

In a nutshell, if you want to maintain the same level of carbonation when raising temperature, then you'll need to increase the pressure applied. Similarly, when cooling, you'll need to reduce the pressure by venting some of the CO2 in the headspace.

Although the CO2 pressure inside the keg will naturally increase with the higher temperature, the rate of solubility decreases faster, so you need to apply more pressure to maintain the same volumes dissolved CO2. If you don't, about 40% of the dissolved CO2 will come out of solution and build up in the headspace as you increase from 35F to 65F. See here for a detailed analysis of CO2 pressure and dissolved volumes, in particular Figure 3.1 which shows a graph the ratio of volumes to pressure over a range of temperatures - at 35F is about 0.10, while at 65F it's 0.06. So, if you didn't add any CO2 you'd get only about 60% of the carbonation level you want.

The converse is true when you chill the beer - since it was carbed at a higher pressure, and then chilled, the volume to pressure ratio is now higher at the lower temperature, and so you get more volumes of CO2 dissolved. This is easily fixed by bleeding the excess pressure from the keg over a course of a few days.

There's no long-term negative affects on your beer, although according to Charles Bamforth, the rate of oxidization increases by 3-fold for every 10C/18F increase in temperature, so the beer will oxidize faster at the increased temperature, but at these temperatures oxidization times are measured in months, so in practice I doubt you'd notice if only keeping it at room temp for a couple of weeks.

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so if i'm going to carb it while at 38 degrees, warm it up (due to no space in the fridge), then cool it back down to serve it, should I mess with the carb levels, or just leave it?x – Pietro Oct 18 '12 at 14:32