# What is the CO2 tank pressure?



## SpaceBug (Jan 10, 2007)

It is my understanding that the pressure insode the CO2 tank remains relatively constant as long as there is still liquid CO2 in it. Is that correct?
My new regulator reads a pressure of 2000 PSI although the instructions that came with the regulator says it should be in the 700 to 1100 PSI range. No matter how little I open the tank valve, the pressure still rises to 2000 PSI.
So I'm guessing that either my gauge doesn't read accurate, or I have a potential problem. Any thoughts? Will an over-filled tank exhibit higher pressure?


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## hoppycalif (Apr 7, 2005)

The pressure in a CO2 bottle should be around 700-800 psi, as long as there is any liquid in the bottle. If you truly do have 2000 psi, you have a major problem. I suggest taking the tank back to whomever filled it and asking them to verify that it isn't overfilled. You do have to be careful in reading the pressure gage too, since there are almost always two scales on the gage, only one of which is a psi scale.


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## Rex Grigg (Jan 22, 2004)

You don't have to take it back. Look at the cylinder. Find the markings TW or something similar. This is the Tare Weight. What the cylinder weighs empty. Once you have found this then remove the regulator and weigh the cylinder. If the TW is say 10 lbs and it's a 5 lb cylinder it should weigh about 15 lbs. More than that then you have a problem.


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## SpaceBug (Jan 10, 2007)

Okay, it looks like it was overfilled. The TW is 14.8 lbs and it is supposed to be a 10 lb tank. So it should weigh 24.8 lbs when full. But it actually weighs 30 lbs. I did not remove the regultor for this but I don't think that weighs 6.2 lbs!


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## slickwillislim (Oct 11, 2005)

Thanks for the tip Rex. To be sure I would remove the regulator. Just to see how over filled it is. I just removed mine because I thought it was empty. I just forgot to turn it on after a big extended water change. oops. No wonder the bga started to take off.


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## random_alias (Nov 7, 2005)

So you should completely close the 320 valve on the Co2 cylinder and then remove the regulator and washer. 

Take the cylinder outside and slowly open the 320 valve until it begins to bleed out Co2. It will eventually spit ice and snow, this is normal and nothing to be concerned about. Every 3-5 minutes (this varies because different people open the valve to different levels), place the cylinder on a scale (or better yet, have it sitting on a scale from the get-go) and measure its weight. It'll probably take about 6-10 minutes to bleed off 6lbs or so. When the weight is down to whatever it should be, close the 320 valve, attach the washer and regulator, slowly open the 320 completely and check your new pressure. Then check the regulator to 320 valve connection to be sure you have no gas leaks. 

Now you're good to go. I had a 10lb cylinder overfilled once, my first time actually. My pressure was over 2k. I called the welding supply place and this is the procedure I was told to follow. It worked. I now weight all my refills before attaching the regulator.


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## SpaceBug (Jan 10, 2007)

Bleeding off the excess CO2 is no problem. What surprises me about this experience is with the tank pressure being affected by the amount of liquid CO2 in the tank. I would think that regardless of the amount of liquid CO2, as long as there is space for some of the liquid to change to gas, that it will do so until the pressure reaches an equilibrium point. The pressure at that equilibrium point ought to be a constant (for a given temperature).


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## hoppycalif (Apr 7, 2005)

If the tank is actually full of liquid CO2 the pressure will be that at which a fixed volume of CO2 remains liquid at the temperature of the tank. I can't find my Handbook of Chemistry and Physics, so I can't look that up. As I recall, that pressure can get pretty high, hence the need to always have a space for gas in the tank - so the volume of liquid CO2 can increase as the temperature goes up.


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## Rex Grigg (Jan 22, 2004)

And if the tank was filled with liquid CO2 the pressure will be quite high as the tanks get pretty cold when filled. Then it warms up and there is no room for the liquid CO2 to expand. 

I'm really surprised that the burst disc on the CGA 320 valve did not function at that pressure.


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## dennis (Mar 1, 2004)

Well, yes and no. CO2 turns to liquid at ~830psi at standard temperature (25 C or 77 F) Higher or lower temps will create higher and lower pressures respectivly. In my house with a overnight temp of ~60F and a daytime temp of ~68, my guage only reads above 800psi for a short while after I get it. In the summer though, with temps in the upper 80's, it will read over 1000psi. This is only a direct effect of temp, not volume. The fil temp can factor intot this though and some places will bleed off the excess CO2 in order to get more gas into the tank, paintballers sometimes do this. If the tank and gas are both cold for the fill, the liquid is smaller and more will fit into the same space, as it warms up the liquid can't go anywhere and the pressure is higher as a result.

The ideal gas law will also explain this, though I don't remember enough off the top of my head to actually do the math. Basically though, PV=nRT or Pressure*Volume=moles*constantR*Temp If you compare an overfilled tank and a properly filled tank, R is the same for both so:
P1V1/n1T1=P2V2/n2T2, since the temp remains the same for both situations in our though experiment, and since V remains the same since the tanks are of a specific size, then n (the amount of gas in moles (atoms for the sake of argument) is proportional to P and V. Therefor, you get:

P1/n1=P2n2 So, if we pretend, for sake of easy math that a correctly filled tank at room temp and 830psi contains 100moles and the incorrectly filled tank contains 150 moles(50% more) then, since the 2 sides are set equal:

830/100=P2/150
8.3=n/150
n=8.3*150=1245psi

Another way, which much simpler math though I did not feel it showed all the connections, is to multiply the correct pressure by the %amount of extra in the overfilled tank and add the difference to what should be...

830+(830*50%)=1245psi

These numbers are not made to correspond to your particular readings, meerly as an example of the proportional relationship between volume and pressure. The exact relationship would have to take into account the individual pressures at the correct number of moles, the constant and the expansion properties of CO2 liquid. Also, the pressure needs to be in atmospheres and the temp expressed in kelvin, IIRC. Regardless, you get the idea

In short, what I am trying to say is that the "volume" or more technically the physical amount of molecules of the gas present in the tank will determine the final pressure. Also, this as when considering beyond the gas/liquid minimum pressure. For example, if you put a full tank in the fridge at 35F the pressure will read very low but the CO2 in the tank will still be a liquid because the proportionality between pressure and temp. dictates when the CO2 is gas or liquid. Considering this idea with our ideal gas law with a tank at 77F and at 35F... since the volume, moles and R are the same and only the temp and pressure change;

P1/T1=P2/T2 

Sorry to ramble!


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## SpaceBug (Jan 10, 2007)

But doesn't your discussion of the "ideal gas law" only hold for cases where the gas remains in the gas state? If I have a tank of CO2 with some liquid CO2 and some in the gas state and the temperature rises such that the pressure goes above the 830 psi point, then won't some of the gas turn to liquid and thus reduce the pressure back down? I realize this isn't really correct since as the temperature rises so does the pressure at which CO2 turns to liquid.

And then there is, of course, my over filled tank at 2000 psi which is proof that I know nothing!


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## dennis (Mar 1, 2004)

To be honest, I am not sure. I was going on the assumption that I believe the liquid is held at the liquid by the pressure of the tank, a pressure which is a result of the gas itself. I had not thought that through much so I do apologize if I am wrong.

I realize that the liquid/gas under pressure was not something I though of, nor do I know much beyond what I wrote. However, the principles behind pressure and volume/quantity remain the same. CO2 will always be a gas when pressure is over ~830psi at 25'C. Adding more pressure will not change that. CO2 is one of those funny compounds that sublimate. Dry ice, solid CO2 is not frozen liquid CO2 but rather frozen gas. Pressure has nothing to do with that. The ice does not melt, ie go from solid to liquid to gas, but rather sublimates directly from solid to gas at a specific temp.

Although your right, I had not thought about the liquid part. Sorry.


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