Co2 pressure change at temperature

Bolted together my Keezer this weekend. Came down stairs to check it this AM. Opened the lid, and was shocked to find the brand new co2 bottle had dropped from 800 PSI to 600 PSI!! Oh crap, at this rate the 10# bottle will only last 4 days NO WAY!!! After stressing for about an hour I rememberated back to high school chemistry and the gas law. Given a volume of co2, pressure will fall as temperature is reduced. I turned on the freezer and dropped the temp from 68F to 36F overnight.

Does anyone know the volume of co2 in a 10# tank at 800 PSI at 68F? I'd like to poke my temp drop in and verify the current reading does not suggest a leak. I found this calculator: http://www.1728.org/combined.htm I assume it needs the gas volume at atmospheric pressure. I have no idea how to calculate that. I have a 10# cylinder that at 68F holds 800#

Ideal gas law isn't what you need. You need Henry's law. CO2 in a cylinder is in liquid phase, not gas. So the pressure you read is the equilibrium vapor pressure at your storage temp.

Your tank will sit at the same pressure for weeks and weeks and weeks (hopefully if you don't have any leaks). Then one random day, the liquid will be gone and only vapor will be left. Your pressure will fall like a rock and be gone in a day or two.

ok, henry's law it is. Is a 200# pressure drop reasonable given the temperature difference I have stated?

Easily...



Cheers!

Those are precisely the numbers I'm seeing. Thank You

Dear lord, I prayed I would never have to see one of those 3-phase charts again....

Just to nitpick further, it's actually not Henry's law. Henry's law describes the solubility of a gas (substance A) into a different liquid (substance B); what we have here is CO2 vapor and liquid (both substance A) in equilibrium with each other.

Day_trippr's chart is what I always use, you can also look at the vapor pressure of CO2 as a function of temperature:

https://en.wikipedia.org/wiki/Carbo...Liquid.2Fvapor_equilibrium_thermodynamic_data

If you graph that data it looks like the chart at the bottom. Since the density of the liquid also changes with temperature and there's a fixed volume inside the tank, you end up getting some slightly different behavior, which is why day_trippr's chart above is the way to go.

So, to further digress into the land of brew science, when I purchase a cylinder of co2, am I actually buying co2 gas that has been compressed into a liquid, or a gas that has been dissolved in a liquid? I assume the substance in the cylinder is a liquid that turns to co2 gas when pressure is reduced.

william_shakes_beer said:

So, to further digress into the land of brew science, when I purchase a cylinder of co2, am I actually buying co2 gas that has been compressed into a liquid, or a gas that has been dissolved in a liquid? I assume the substance in the cylinder is a liquid that turns to co2 gas when pressure is reduced.

Click to expand...

The former, you've got it right! You have liquid CO2 with nothing else dissolved in it (ideally...) and a small headspace of CO2 gas. As you pull off the CO2 gas, the liquid boils to maintain the headspace at a certain pressure. That pressure is basically the vapor pressure of liquid CO2 at a given temperature.

The cylinders are filled by weight, not pressure, since CO2 is a liquid in the cylinder. If you want to know how much you have left in the tank, you have to weigh the tank and subtract the tare weight. This is stamped on your tank (it may say T.W.) but remember it doesn't include the regulator.

william_shakes_beer said:

So, to further digress into the land of brew science, when I purchase a cylinder of co2, am I actually buying co2 gas that has been compressed into a liquid, or a gas that has been dissolved in a liquid? I assume the substance in the cylinder is a liquid that turns to co2 gas when pressure is reduced.

Click to expand...

CO2 in your tank is liquified CO2. Or CO2 gas that is at sufficient pressure to have turned it into a liquid. It is not dissolved, it is (mostly) pure.