# 721 psi in 20 lb cylinder.....Newbie Question



## putty (Nov 19, 2003)

From what I recall, the standard "full" pressure is around 1000PSI, so I think you are pretty full.


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## bharada (Mar 5, 2004)

Liquid CO2 maintains a pretty steady pressure reading at any given temp. So unless the room your tank is in is below 60° you're out of liquid CO2 and now running only on gaseous CO2. How long before you run dry depends on your injection rate, but typically you get a week or two.

My rule of thumb is that once the tank pressure drops below 750 psi to get it refilled at my earliest convenience.


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## froghair (Jan 5, 2008)

Putty and Bill, I appreciate the replies.


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## EdTheEdge (Jan 25, 2007)

I think I'm confused about just what went on here.....

One said it was near full and the other said it is near empty.

Can someone please explain before my head explodes?

Thanks.


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## bharada (Mar 5, 2004)

EdTheEdge said:


> I think I'm confused about just what went on here.....
> 
> One said it was near full and the other said it is near empty.
> 
> ...


What it boils down to is that the liquid CO2 you get in a filled tank will maintain a constant pressure for a given temperature. For example, a tank containing liquid CO2 kept at 70° will have a pressure of ~850 psi. At 80° the tank pressure is ~970 psi. This is why a CO2 tank will seemingly maintain the same pressure even after months and months of continuous use. It's not because you've use so little CO2 during that time period, but because you still have liquid CO2 in the tank.

At some point the liquid CO2 will have all converted to gaseous form. At this point the pressure gauge will drop in a linear fashion until all the CO2 gas has been exhausted. I use 750 psi as my guide simply because that's the approximate pressure of liquid CO2 at 60°. Since the room my tanks are in are never lower than 65° it's a safe bet for me that they've converted all the liquid CO2 to gas. For me it simpler than taking the tank out and weighing it.


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## putty (Nov 19, 2003)

I think bharada knows way more about this than me. 

I was basing my answer on the fact that my gauge shows about 1100 psi on a full tank.. It does seem stay there for quite a while and drops quite quickly once the tank is almost empty.


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## Rex Grigg (Dec 10, 2002)

The pressure of a CO2 cylinder has a direct relationship with the temperature of the cylinder.

See here and here.

The same cylinder at 40°F will read around 550 psi and 1100 psi at ~85°F.

If the temperature remains constant then the pressure will remain constant till the point where there is no liquid CO2 left in the cylinder. I find that when this happens with my 5 lb cylinders that there is about 1.5 lbs of CO2 left in the cylinder.

putty, the only way your cylinder can read 1100 psi when filled is due to one or more of the following.

The temperature of the cylinder is 85°F.
The cylinder is over filled.
The gauge is wrong.

Note that CO2 cylinders are based on weight. A fully filled cylinder is only filled up to about 2/3 volume. The rest of the area is for gas expansion.

A cylinder cools when it is filled. If you get your cylinder filled while you wait you know this.

If a cylinder is filled up to the top volume wise then the CGA 320 valve burst disc will soon be bursting to save you the problem of the cylinder failing. This disc bursting is going to be noisy and scare the crap out of you. You will also notice a very large white cloud when this happens.


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