My CO2 tank reads low gas when cold

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meylo

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I notice that both times I bring my CO2 tank into a refrigerated area, it reads low gas (near the red). Once I get back to room temp, it goes back into the green (still full). Has anyone else seen anything like this?
 
co2pv.gif


Cheers!
 
Gas laws. Pressure is directly related to temperature...
 
This makes sense. So please correct me here. The reading shows lower because of the temp change right? So do I need to make any adjustments to the PSI meter to compensate in any way or should I trust the PSI reading level (20 psi)? FYI my regulators max setting is 30 PSI.
 
You may experience some drift, typically downwards, if you stick a regulator in a fridge/keezer and it chills down.
Otherwise, note even at 32°F temperature and a nearly empty cylinder, the vapor pressure is at least a few hundred PSI above your regulator setting...

Cheers!
 
Gas laws. Pressure is directly related to temperature...
That would be true if you were dealing with a fixed volume of gas, but under normal storage conditions CO2 liquefies. For a given temperature the gas pressure is limited by the pressure at which the gas liquefies. At 0°C the liquefaction pressure is ~12.5bar (181psi) . At 20°C the liquefaction is pressure is 60bar (870psi). Once the tank is sufficiently empty that it can't reach liquefaction pressure then the pressure will be approximately proportional to temperature.

636px-Carbon_dioxide_pressure-temperature_phase_diagram.svg.png
 
At 0°C the liquefaction pressure is ~12.5bar (181psi) . At 20°C the liquefaction is pressure is 60bar (870psi).

Soooo....Like I said, and like you reiterated here^^. Pressure is directly related to temperature. Considering our CO2 is evaporating from liquid to gas we aren't concerned with liquification unless we are filling our tanks from a gaseous CO2 into liquid CO2. Even so, the relationship still exists.
 
As a senior member of the Dead Horse Committee I will keep this going and likely demonstrate my ignorance on the subject at the same time.

Regarding the original question; the pressure in the CO2 bottle is controlled by the vapor pressure unless there is no liquid CO2 left.

What I experience as well is that the line pressure after the regulator drops as well (when the entire bottle and regulator is the cold box along with the beer keg). I suspect this is due to the regulator design and its temperature coefficient rather than the temperature of the gas. Also, the pressure gauges likely have their own temperature coefficients as well further complicating the issue.
 
Yes, pressure is related to temperature, but in this case not by any of the gas laws. Which gas law were you thinking of?

Well, Guy-Lussac's law explains this direct relationship between temperature and pressure of a gas at a constant volume. Go stick your CO2 tank in the fridge and compare the pressure readings to when it is at room temp. A cold CO2 tank will show lower pressures than that of one at room temp. I think you're getting tripped up by the fact that CO2 exists as a liquid and gas in our tanks. Even so, liquids will also expand as temperature increases and thus pressure will increase at constant volume. I will say it again, a direct relationship exist between temperature and pressure.

What point were you trying to make with your initial response?
 
Well, Guy-Lussac's law explains this direct relationship between temperature and pressure of a gas at a constant volume. Go stick your CO2 tank in the fridge and compare the pressure readings to when it is at room temp. A cold CO2 tank will show lower pressures than that of one at room temp. I think you're getting tripped up by the fact that CO2 exists as a liquid and gas in our tanks. Even so, liquids will also expand as temperature increases and thus pressure will increase at constant volume. I will say it again, a direct relationship exist between temperature and pressure.

What point were you trying to make with your initial response?
Guy Lussac's Law states that pressure is proportional to absolute temperature. In this case it is not. The pressure is due to the phase change. You can see how the pressure is related from the phase diagram. We're in agreement that pressure is related to temperature it's just nothing to do with gas laws.
 
Regarding the original question; the pressure in the CO2 bottle is controlled by the vapor pressure unless there is no liquid CO2 left.

That (and what I said previously) is almost true. The chart that day_trippr posted shows that above the critical temperature (31.1 °C) all the CO2 in the tank is a supercritical fluid and starts to behave like a gas. This means that above 31.1 °C you can tell how full the tank is from the pressure. Also from the gradient of the contours on the far left and far right of that chart you can see that liquid CO2 is almost as compressible as CO2 gas.
https://www.homebrewtalk.com/forum/members/day_trippr.81618/
I've flogged more dead horses than Findus.
 
My super basic and not total accurate way of remembering this stuff goes back to my 10th grade chemistry teacher who used to say "pv=nrt it's not just a good idea, it's the law". Since it's the ideal gas law it doesn't really apply but always reminds me to consider pressure and volume in relation to temperature.
 
Henry's law can also help in understanding this scenario. It pretty much quantifies what was mentioned above with partial pressures with simple equations. The CO2(gas) inside of your canister will dissolve more readily into the CO2(liquid) inside of the container as temperature drops, leaving a lower pressure in the headspace.
 
While we're on the subject, I recall reading threads where people said that storing the co2 tank in the refrigerator will make the tank last longer. I never understood this, and don't know if it's even true. Does anyone care to explain? I have always kept my tank outside the fridge because of space issues.
 
While we're on the subject, I recall reading threads where people said that storing the co2 tank in the refrigerator will make the tank last longer. I never understood this, and don't know if it's even true. Does anyone care to explain? I have always kept my tank outside the fridge because of space issues.
There's no truth to that. The mass of CO2 used has no relation to the temperature the CO2 is stored at. It will last the same stored in or out of the fridge.
 

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