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Home Brew Forums > Home Brewing Beer > Bottling/Kegging > CO2 pressure drop
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Old 06-09-2011, 05:32 AM   #1
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Default CO2 pressure drop

Hey All,

Thanks for all of the help so far. Finally got my co2 tank filled. Hooked everything up, stayed about the same level. Came back after about 4 hours of it being in the fridge, the gauge had dropped to half of the level. Does the cold of the fridge effect the meter in anyway or do I have a lose connection somewhere.

It's a hard thing to get the tank filled with my schedule, so I'm really hoping that it is just the fridge and note a loose connection somewhere. I checked them all and it looked like they were solid.

Thanks,
MDF

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Old 06-09-2011, 05:44 AM   #2
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the fridge will drop the pressure on of the co2 bottle thus it will show on the gauge. No worries though because mine has been showing right there at the red for about 3 batches now.

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Old 06-09-2011, 01:50 PM   #3
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Ideal gas law. PV=nrT with everything else being constant, if you lower the temp by 50% (say 80 to 40) The pressure will drop by 50% (say 1000psi to 500psi)

EDIT: I should add that since some of the CO2 is in liquid form this isn't 100% true, it will be something less than an exact ratio

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Old 06-09-2011, 04:55 PM   #4
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Great, thanks everyone. So if it isn't empty after a full night I'm probably alright and the shift was just from temp change.

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Old 06-09-2011, 05:14 PM   #5
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Pressure has nothing to do with volume. CO2 is a liquid under pressure. You still have the same volume of liquid in your tank.

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Old 06-09-2011, 07:05 PM   #6
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Quote:
Originally Posted by AnOldUR View Post
Pressure has nothing to do with volume. CO2 is a liquid under pressure. You still have the same volume of liquid in your tank.
Even when considered full, the CO2 in liquid form in your tank is only about 40%. Most won't go above that due to liability from rupture.
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Old 06-09-2011, 07:26 PM   #7
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Quote:
Originally Posted by Gunfighter04 View Post
Even when considered full, the CO2 in liquid form in your tank is only about 40%. Most won't go above that due to liability from rupture.
not exactly, but it the point is moot because they go by weight of CO2 and not volume. 5lb in a big tank is the same as 5lb in a small tank; only the small tank would have a greater % of liquid in it. doesnt have anything to do with rupturing, as its all the same pressure. you cant overpressurize a CO2 tank unless you either; forcibly pump more liquid CO2 in then it can hold, or heat the tank so that the gas-to-liquid transition pressure gets to a point that will rupture the tank.

CO2 will convert to liquid, at room temperature, when its around 1000 PSI, +/-. when the pressure of the gas drops, the liquid will gassify to maintain the same pressure. when the pressure goes up, some gas will liquify to maintain that pressure.

when you refrigerate the whole tank, that gas-to-liquid transition point will get lower according to gas laws mentioned above.

this is the reason that your high pressure gauge will read almost the same for a very long time, and then suddenly start to fall to "E". that is because the liquid CO2 maintains a constant pressure until it runs out of liquid, then the pressure rapidly falls when all that remains is gas.
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Old 06-09-2011, 10:20 PM   #8
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Makes total sense now. The tank has been holding at 500 all day so my leak fears have dissolved. Thanks for the science behind it, I love this forum and these little bits of information.

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