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Old 11-14-2011, 01:26 AM   #1
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Default Noob kegging PSI question

Hey all,

OK, so I've read, and experienced, that 10-12 PSI is a good level to set your kegs at for carbonation.
However, I've also read that 5ish PSI is ideal for dispensing...

Wouldn't the keg, once carbed at 12 PSI and left so sit afterwards at 5 PSI for serving lose carbonation after a while? Wouldn't it balance out to somehwere between 5 and 12 given that the co2 pressure being put ON the beer is lower than the co2 pressure IN the beer.

Or am I a total noob and this is not how it works? lol

Thanks,

Nick


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Old 11-14-2011, 01:48 AM   #2
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The beer is absorbing CO2 with it set at 12 psi (the carbing). It is still absorbing some CO2 when set to 5 psi, but that is mainly just used to push the beer out. I'm not sure the beer is going to lose CO2 unless it has somewhere to go (take the top off the keg). I know what you are getting at with thinking equilibrium, but even keeping it at 5 psi you are forcing CO2 into the beer.


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Old 11-14-2011, 02:24 AM   #3
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Your instincts are correct, and the pressure at which the beer is carbonated needs to be the same as the serving pressure to maintain the same carbonation level. It's also true that a lower psi results in a better pour in many kegerators, but that's because they aren't properly balanced (usually the lines are too short). I suggest using a chart like this one to determine what pressure will give you your desired carb level, and then choosing a beer line length that provides enough resistance to balance that pressure. If you search for "draft system balancing" you'll find a lot of info about how to do this.

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Originally Posted by barleyhole View Post
The beer is absorbing CO2 with it set at 12 psi (the carbing). It is still absorbing some CO2 when set to 5 psi, but that is mainly just used to push the beer out. I'm not sure the beer is going to lose CO2 unless it has somewhere to go (take the top off the keg). I know what you are getting at with thinking equilibrium, but even keeping it at 5 psi you are forcing CO2 into the beer.
This is incorrect. If you set the serving pressure lower than the carbonation level, every time you pour a beer CO2 will come out of solution into the headspace, until the carbonation level reaches equilibrium with the new lower pressure.
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Old 11-14-2011, 02:30 AM   #4
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It will slowly lose carbonation. I don't know the math, but I think this is how it should work:

You carb to 12 psi, meaning there's 12 psi sitting on top of the beer, and a bunch of gas in the beer, sitting at an equilibrium. When you lower the pressure to 5psi, that equilibrium is thrown off, what with there being more CO2 dissolved in the beer then in the headspace above the beer. Over time, this imbalance will equalize, by the CO2 coming out of solution from the beer, and hanging out in the headspace. As you drink more beer, the headspace grows, further reducing pressure. This will result in the equilibrium being thrown off again, and the process repeating. I think (this is where my knowledge runs out, someone correct me if I'm wrong) that eventually, your beer will settle out to however many volumes of CO2 you get from 5psi at the temperature of your kegerator.

The way to get around this, other than getting longer beer lines, is to store it at 12 psi, then reduce it to 5 when you want to drink some.

Again, my memory of gas and pressure is pretty sketchy, seeing as I haven't learned anything about it since high school, but I think this is what should happen.

**EDIT** Damn, I see I got beat!
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Old 11-14-2011, 02:30 AM   #5
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Quote:
Originally Posted by JuanMoore View Post
Your instincts are correct, and the pressure at which the beer is carbonated needs to be the same as the serving pressure to maintain the same carbonation level. It's also true that a lower psi results in a better pour in many kegerators, but that's because they aren't properly balanced (usually the lines are too short). I suggest using a chart like this one to determine what pressure will give you your desired carb level, and then choosing a beer line length that provides enough resistance to balance that pressure.

This is incorrect. If you set the serving pressure lower than the carbonation level, every time you pour a beer CO2 will come out of solution into the headspace, until the carbonation level reaches equilibrium with the new lower pressure.
Right. The only reason, ever, to have a "serving pressure" that is different is when you have an unbalanced system. That might work for one keg, but not in something like I have. I have 5 kegs. My psi is at 12, with the temp at 40 degrees. If I had a different 'serving pressure', that would be terrible!

If I wanted a glass of beer A, I'd have to close the gas to that keg, reach in and purge it and reset the regulator to 5 psi. Then turn off the gas to other kegs? So it's at 5 psi to push that one beer. Then, for the second glass, I want beer B. I'd have to turn off that gas, and purge, and set it to 5 psi? And now I'm done- I have to reset them all at 12 psi, and reopen the gas to all 5 kegs.

What a huge pain that would be! I have my regulator set at 12 psi and I have perfect pours. That's because you should never have to change it!

You don't go to a bar and have them run in the back room and "turn down" the gas to pour! The key is balancing the system.
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Old 11-14-2011, 02:48 AM   #6
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OK, cool, thanks guys.

Now I just need to figure out what the heck is wrong with my regulator.
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Old 11-14-2011, 05:47 PM   #7
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Quote:
Wouldn't the keg, once carbed at 12 PSI and left so sit afterwards at 5 PSI for serving lose carbonation after a while?
assuming the temperature is constant, yes. after a while CO2 will come out of the beer and it will be as if you carbonated it at 5psi.

there are two variables to carbonating beer (temperature and pressure)
there are three variables to balancing your system (temperature, pressure, and line legnth)

if your line legnth is not adjustable, then you can adjust your temperature and pressure to match it (but obviously you can only adjust temperature so far before you are drinking warm beer or sucking on a beer ice cube). having a certain line legnth means you will have a certain pressure that will give you the best pour. you can then look on a carbonation chart to determine what temperature your kegs should be, at that pressure, to get the desired carbonation level.

if your lines are too short, you wont be able to set the pressure high enough to maintain the proper carbonation level because the beer would be exploding out of the tap (so you would have to decrease temperature to make up for that, but you can only go downto 32 degrees). also if your lines are too long, you will need to use a lot of pressure to push the beer thru them, and have to increase the temperature of the kegs to match that pressure, to prevent over-carbonation. if your lines are wayyyy too long, the temperature required will be too high to enjoy the beer at.
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Old 11-14-2011, 05:59 PM   #8
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Thanks for that. My lines are 4.5 feet long...that's pretty standard, isn't it?
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Old 11-14-2011, 07:20 PM   #9
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Originally Posted by audger View Post
also if your lines are too long, you will need to use a lot of pressure to push the beer thru them, and have to increase the temperature of the kegs to match that pressure, to prevent over-carbonation.
You'd need some seriously long lines to require increasing the pressure. The only downside to overlength lines in practical use is that it slows the pour down. For us the speed of the pour usually isn't that important like it would be for a bar. I cut my lines overlength intentionally for the times I want to serve a hefe or belgian at a high carb level. If I have time to drink a beer, I can wait an extra few seconds for it to pour.

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Thanks for that. My lines are 4.5 feet long...that's pretty standard, isn't it?
A lot of people have foaming issues with most beers using lines that short, but unfortunately that's the length a lot of kegerators and kegging conversion kits come with. Some of it will of course depend on the beer temp, particular resistance of your line, rise to faucet, etc. 8-10' is a`more common length.


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