Can temperature change cause CO2 pressure to drop...

[tidehouse]

or did I just lose half my co2 tank to a leak? I took my keg out of the fridge to force carbonate by shaking it. The co2 tank is in the fridge and it was out in the warm air maybe 5 minutes. I looked at my gauge and it was in the red and I just had it filled. Could it be that the tank warmed up or did I just lose most of my co2?

 

[rsmith179]

No need to be concerned. The pressure will drop with lower temperatures but the amount of CO2 in the tank should be the same.

 

[Arkador]

https://www.homebrewtalk.com/f35/psi-problem-normal-118765/
https://www.homebrewtalk.com/f35/gas-pressure-drop-104913/
https://www.homebrewtalk.com/f35/first-kegging-120920/

at 90ish degrees, 1000-1100psi
at 38degrees , about 550psi

 

[tidehouse]

Thanks guys, I realize the title of the thread is a stupid question, of course it will make the pressure drop, I tend to forget science when I'm panicking. I guess I didn't check the gauge before and was worried that I lost CO2 while shaking the keg. Thanks for letting me breathe easy.

 

[IrregularPulse]

Although pressure should go up and feel lighter when the tank warms up. Pressure should drop from from warm to cold.

*reread post* n/m

 

[Brewin_CRAZY]

PV=nRT as T goes up, P goes up, as T goes down, P goes down.... woot science

 

[rsmith179]

Correct me if I'm wrong, but the tank should not feel any lighter regardless of whether it is stored cold or warm. According to the equation above, only the pressure is affected by the temp, not the actual amount of CO2 in the tank.

For example, say I just got my 5# tank filled. The pressure will read quite high until it is brought down in temperature. However, if you were to weigh that tank, it would read the exact same weight, regardless of temperature.

 

[ifishsum]

Remember that the gauge on the high side is pretty much useless as an indicator of how much CO2 you have left in the tank. By the time it starts really dropping (ignoring the drop for temperature change), you're basically out. Weighing the tank is the only way to know how much is left (I wrote down the weight of my tank when it's full).

 

[Brewin_CRAZY]

However, if you were to weigh that tank, it would read the exact same weight, regardless of temperature.

Correct, its what whole conservation of mass thing. If you find a way to reduce mass by cooling it, patent that **** cuz you just found yourself a scientific first.
If you are loosing mass when you put it in the fridge its cuz you have a leak somewhere. You can gain a little mass due to condensation on the bottle (outside of the bottle) but nothing more than a few ounces.
Personally I leave my CO2 outside the fridge, lasts longer that way.

 

[Brewin_CRAZY]

Remember that the gauge on the high side is pretty much useless as an indicator of how much CO2 you have left in the tank.

ifishsum, if you could elaborate on this one. I deal with gas bottles everyday (CO2, Ar, C2H2, He, H, O2, everything) and my indicator for how much I have left in the bottle is the high pressure gauge, i.e. the pressure left in the cylinder. How is this useless?

 

[ifishsum]

Assuming a constant temperature, as long as there is liquid CO2 in the tank it will produce the same vapor pressure - and that pressure won't fall significantly until the liquid is gone. You're basically "running on fumes" at that point. Also true of LP gas and other self-pressurizing liquids. Some LP tanks are equipped with a float that indicates the liquid level in the tank, and that will give an accurate idea of how much is actually left.

I'm not entirely sure about the other gases you mentioned, I only have experience with LP and CO2. I think hydrogen, helium and oxygen are not normally liquefied in the tank, so the pressure would start high and drop as it gets used up, making the high pressure reading an indication of how much is left.

 

[Brewin_CRAZY]

Hmmm, never experienced this with a comp gas before, guess CO2 is pressurized a little different that what im used to. I always assumed that CO2 was in gas form not liquid (as all my other gasses are, well gasses lol). CO2 usually goes through sublimation, however (never looked at a CO2 phase diagram until today) at high pressures apparently CO2 is in the liquid state. It would totally make sense to have a liquid gauge then, just like with LN2 (liquid nitrogen) the only way to assess how much gas is left in our tanks is the liquid level, pressure makes no difference until the liquid is used up. Makes sense, gotta look into this one a little.

 

[ajwillys]

Yes, CO2 is stored in tanks as a liquid except for the headspace that is required to equalize the pressure at the CO2 'boiling point'. The best way to tell how much is left in your tank is to weigh it (without any gauges attached) and subtract the tare weight (stamped on the tank) from the weight you measured. That will tell you how many pounds of liquid CO2 are left.