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Keg carbonation: CO2 - temp relationship.

Discussion in 'Homebrewing' started by yinzer, Sep 17, 2012.

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  1. yinzer

    yinzer Initiate (0) Nov 24, 2006 Pennsylvania

    Instead of going into detail I'll ask a simple question.

    At 72*F for 2.5 volumes you need 30 PSI. At 34*F you need 9 PSI.

    So I put a corny that was at 72*F and 30 PSI and crash it to 34*F. Two days go by. And it sits at 20 PSI. Why?

    Yes, the correct answer is to get a kegerator. But until then....
     
    #1 yinzer, Sep 17, 2012
  2. IceAce

    IceAce Pooh-Bah (2,232) Jan 8, 2004 California
    Pooh-Bah


    Most likely because you believe your pressure guage is telling you the truth on both ends of this spectrum.

    PV=nRT
     
    #2 IceAce, Sep 17, 2012
  3. yinzer

    yinzer Initiate (0) Nov 24, 2006 Pennsylvania

    Since there is so little to believe in these days, I feel a need to start somewhere.

    Oh, I'm not using the pressure guage on my tank. It's a bleeder type set-up.
     
    #3 yinzer, Sep 17, 2012
  4. mikehartigan

    mikehartigan Maven (1,409) Apr 9, 2007 Illinois

    Needs more time for the CO2 to dissolve. It typically takes a week or so to carbonate a keg of beer. You also need to keep the gas connected or the pressure will eventually drop below 9 psi - the pressure you saw at 72F was only for the volume of gas in the headspace, which is not enough to carbonate the beer to 2.5 volumes.
     
    #4 mikehartigan, Sep 17, 2012
  5. yinzer

    yinzer Initiate (0) Nov 24, 2006 Pennsylvania


    Thanks, but the beer was already carbonated. It had been at room temp for about three weeks.

    I'm not sure that with my scenario why it would matter. My question is that the pressure is too high after being cooled. As the temp of the keg drops, shouldn't the pressure change in step? If the beer wasn't at the assumed co2 volume and under carbonated, given enough time and that colder beer accepts the co2 easier, then I could see a lower calculated pressure.
     
    #5 yinzer, Sep 17, 2012
  6. mikehartigan

    mikehartigan Maven (1,409) Apr 9, 2007 Illinois

    Was the keg at equilibrium? IOW, how long had it been sitting at 70F/30psi? Sounds like it might be over carbed. How does it pour? Give it a few more days and see where the pressure is.
     
    #6 mikehartigan, Sep 17, 2012
  7. telejunkie

    telejunkie Savant (1,107) Sep 14, 2007 Vermont

    patience may be the answer...i'll agree with mike that you should test the cold beer to see how it pours and if it's overcarbed.
     
    #7 telejunkie, Sep 17, 2012
  8. VikeMan

    VikeMan Grand Pooh-Bah (3,055) Jul 12, 2009 Pennsylvania
    Pooh-Bah

    30 PSI at 72F will stop at 2.5 volumes. Since that's what OP wanted, and assuming the temp and pressure were true, how could it overcarbonate, regardless of how long it sat?
     
    #8 VikeMan, Sep 17, 2012
  9. telejunkie

    telejunkie Savant (1,107) Sep 14, 2007 Vermont

    it shouldn't...but that's not to say that a faulty guage or some other factor is coming into play...
     
    #9 telejunkie, Sep 17, 2012
  10. kjyost

    kjyost Initiate (0) May 4, 2008 Canada (MB)

    Also a thought... The pressure in the headspace behaves according to the ideal gas law (PV = nRT) but the gas' solubility is according to a different relationship.

    A drop of 40 F from 70 to 30 should not decrease the pressure in the headspace to 9 PSI, and not even 20 PSI as the temp needs to be converted to Kelvin. You may get a few hard pours to start but it should reach equilibrium at 9 PSI once the headspace expands.
     
    #10 kjyost, Sep 17, 2012
  11. yinzer

    yinzer Initiate (0) Nov 24, 2006 Pennsylvania

    other factor?? yeah .. ME!!!!!


    My logic was using beersmith and looking at different scenarios. Maybe this is where I'm out in the weeds. See I thought since it says 9spi @ 32F = 2.5 vol. means that this is what the pressure would drop too. It seems like taking the keg and crashing it will make it over carbonate to ~3.5 volumes. .

    I don't have the proper way to dispense beer. I'm often crashing to 32 to fill bottles, so when I do that I drop the pressure to 5 psi for the beer gun. Also without venting aftre crashing and I just want to fill a glass, the keg is at too mush pressure so I get all foam. Maybe for glass filling I should just use a longer hose to balance the proper psi at ~33*F

    Maybe this question will help. I just filled some bottles, the keg is at 5psi and at 32*F. What psi should I fill it at before placing it back at room temp?
     
    #11 yinzer, Sep 17, 2012
  12. mikehartigan

    mikehartigan Maven (1,409) Apr 9, 2007 Illinois

    Not enough information. If, for example, the beer was overcarbed to begin with, say 50psi at 72F, and the OP tried to 'fix' it by simply venting it until the pressure in the headspace was 30, then the beer would still be over carbed. If, on the other hand, it was at 30 at 72F after having sat untouched for a week or two, then the carbonation would be correct and the pressure should have dropped to 9-ish when the temp was lowered.
     
    #12 mikehartigan, Sep 17, 2012
  13. VikeMan

    VikeMan Grand Pooh-Bah (3,055) Jul 12, 2009 Pennsylvania
    Pooh-Bah

    Okay, I guess if he did some other things first that he didn't mention, then it could be overcarbonated.
     
    #13 VikeMan, Sep 17, 2012
  14. mikehartigan

    mikehartigan Maven (1,409) Apr 9, 2007 Illinois

    Not saying that's necessarily the case, but the OP indicated that he doesn't have a proper kegging setup, so it's possible he simply doesn't have the experience to draw on to predict how these things will play out.
     
    #14 mikehartigan, Sep 17, 2012
  15. yinzer

    yinzer Initiate (0) Nov 24, 2006 Pennsylvania

    Right, but I kept it simple because I'm looking for a yes/no answer to the question of "is 20 psi the proper pressure?". Looking back I shouldn't of asked "why", but just asked if that would be the correct reading.

    I think that my question has been answered. I believe that my mistake was thinking that this graph would be linear.

    [​IMG]

    Anyway I'm off to the Euro zone tomorrow and I doubt if I'll be looking in. Thanks.
     
    #15 yinzer, Sep 18, 2012
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