CO2 pressure drop question....

[Spidermonkey]

Hello everybody,
I'm new here so I hope I'm not making any violations.
I kind of need some advise.
So I finally set up my system last night.
It's a kegerator with double taps for some home brew.
My question is last night after putting everything together and checking for leaks( there were none) my system was at 750 psi.
I have a 20 lb tank. The tank and fridge were warm when I set it up.
This morning when I woke up everything was nice and cold inside
HOWEVER... I noticed that my tank pressure dropped from 750 psi to 525psi...?????
I was like WTH....
I checked for leaks and re tightened all the clamps , still no leak....

Is this normal or something I should be concerned with?

Thank you again for your help.
Eric

 

[govner1]

First, I hope you meant 7.5 PSI & 5.25 PSI! The max pressure on kegs is 130 PSI.
As for the change with temp, that is normal. You should have also observed that the indicated volume on your cylinder dropped. This is also because the temp of your CO2 dropped.
Check your pressure settings and connect to a keg. Vent the keg and adjust from there.

 

[mirthfuldragon]

This morning when I woke up everything was nice and cold inside
HOWEVER... I noticed that my tank pressure dropped from 750 psi to 525psi...?????
[...] Is this normal or something I should be concerned with?
Eric

Completely normal. At or below a certain pressure CO2 becomes a liquid and cannot be pressurized further. At a warmer temperature, the pressure is higher than at a colder temperature. The total amount of CO2 you have is consistent. The CO2 gauge does not measure volume of CO2 and is basically "you have CO2 / you do not have CO2".

The same principle applies to your regulators. If you set a regulator temperature to x PSI warm, it will not be x PSI when cold.

 

[Spidermonkey]

Ok.
Thank you . It's fluctuating up and down. Gets higher if I leave the door open and drops back to 525 when it's cold again. Never goes below 525.

 

[cegan09]

Ideal gas law
P*V=n*R*T
P=pressure
V=volume
n=number of moles
R=constant
T=Temp

All things equal, when you change one variable, something else must change. So when you drop the temp, since the volume stays the same, the pressure will change. As you heat the bottle back up, the pressure will rise again. Still the same amount of CO2 in there, just at a lower pressure. Your regulator will keep the feed pressure where you want it.

This also is why when you release a lot of gas quickly the bottle cools off. Changing volume and but hte pressure doesn't change quickly, so temperature does. Similarly gasses heat up as you compress them.

 

[yowzers]

You must be looking at the cylinder side gauge on your regulator. It will read roughly 725 psi at room temperature and will drop to 500 psi or so if you keep it in your kegerator and chill it. That is normal. You should set your outlet pressure to roughly 10 to 12 psi for normal serving pressures.

 

[doug293cz]

Ideal gas law
P*V=n*R*T
P=pressure
V=volume
n=number of moles
R=constant
T=Temp

All things equal, when you change one variable, something else must change. So when you drop the temp, since the volume stays the same, the pressure will change. As you heat the bottle back up, the pressure will rise again. Still the same amount of CO2 in there, just at a lower pressure. Your regulator will keep the feed pressure where you want it.

This also is why when you release a lot of gas quickly the bottle cools off. Changing volume and but hte pressure doesn't change quickly, so temperature does. Similarly gasses heat up as you compress them.

The ideal gas law only applies when you only have the gas phase present. Under normal circumstances you have both gas and liquid CO2 present in your cylinder. So, you have gas and liquid at equilibrium with each other, and in this case the pressure is determined only by the equilibrium vapor pressure of CO2 at the temperature of the cylinder. The equilibrium vapor pressure is affected by temperature, but it does not follow PV=nRT. Pressure actually changes much faster with temperature than the ideal gas law rate.

Under the ideal gas law going from 20°C (293.15°K or 68°F) to 0°C (273.15°K or 32°F) would drop the pressure by a factor of 273.15/293.15 = 0.932. Since pressure is 831 psi @ 20°C, then PV=nRT would make the pressure 831*0.932 = 774 @ 0°C. But in fact, when gas and liquid are present, the pressure at 0°C is 505 psi. Quite a difference.

Brew on

 

[cegan09]

The ideal gas law only applies when you only have the gas phase present. Under normal circumstances you have both gas and liquid CO2 present in your cylinder. So, you have gas and liquid at equilibrium with each other, and in this case the pressure is determined only by the equilibrium vapor pressure of CO2 at the temperature of the cylinder. The equilibrium vapor pressure is affected by temperature, but it does not follow PV=nRT. Pressure actually changes much faster with temperature than the ideal gas law rate.

Under the ideal gas law going from 20°C (293.15°K or 68°F) to 0°C (273.15°K or 32°F) would drop the pressure by a factor of 273.15/293.15 = 0.932. Since pressure is 831 psi @ 20°C, then PV=nRT would make the pressure 831*0.932 = 774 @ 0°C. But in fact, when gas and liquid are present, the pressure at 0°C is 505 psi. Quite a difference.
Brew on

Yes yes, it's a touch more complicated with the dual phase situation. However, the principle is the same. A change in temp will require a change in some other variable. And since volume and amount of CO2 stays the same, that change is pressure.

 

[SleepyCreekBrews]

As reported above, totally expected when you chill the tank.
Personally, I don’t think the cold and moisture is good for the regulator, so I leave mine outside the kegerator at room temp.
YMMV