# Figuring out the amount of CO2 in a cylinder



## saugata (Nov 30, 2005)

Hi! All,

I use a double gauge CO2 regulator. This regulator has two gauges. The first one indicates the pressure inside the cylinder with a scale of 0, 50, 100, 150, 200, 250 kg/cm2. In this the area from 150 to 250 is marked in red indicating danger zone. The second gauge indicates the pressure at which the gas is getting into the bubble counter and has a scale of 0, 2, 4, 6, 8, 10, 12, 14 & 16 and again the units are kg/cm2.

Now my question is what reading should the first gauge show when the cylinder is completely full with CO2 (my cylinder capacity is 2.5 kgs) and how do we know how much gas is there in the cylinder at any given point of time?

Regards,


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## cwlodarczyk (Sep 13, 2005)

You can't rely on the pressure of the tank to tell you how much gas it contains. The only reliable way to determine the tank contents is to put it on a scale. Somewhere on the tank there is usually a marking that represents the tare (empty) weight of the tank - subtract this from the total weight to know how much liquid is inside.


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## KevinC (May 24, 2004)

CO2 inside the tank is under enough pressure to liquefy it. So the head pressure (measured by your gauge) will *stay at the same level as long as there is still liquid CO2* in the tank. As you remove CO2 (gas) from the tank, more of the liquid CO2 will evaporate to take its place. So the only thing the gauge indicates is if you are out of liquid CO2 in the tank (at which point it will start to drop). At room temp with any liquid CO2 remaining in the tank it should read between 63 kg/cm^2 and 77 kg/cm^2 (based on my experience of between 900 and 1100 psig - converted to your units).


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## Left C (Nov 15, 2003)

Here's a little more about CO2. I got it from a reference book at the library. 

Air is 0.033% CO2 by volume. 

CO2 is liquified at 70 degrees F (21.1 degrees C) by being compressed to 838 psi (58.92 kg/cm2). 

At about 87.9 degrees F (31.05 degrees C) or higher CO2 exists only as a gas. 

CO2 is normally stored at 225 to 325 psi (15.82 to 22.85 kg/cm2) by refrigeration. 

A cylinder 100% full of liquid CO2 at 87.8 degrees F (31.0 degrees C) is 1071 psi (75.3 kg/cm2). 

saugata, you read the guage marked with a scale of 0, 50, 100, 150, 200, 250 kg/cm2 to know the volume of gas in the cylinder. You can also keep up with the weight like cwlodarczyk mentioned. 

The area from 150 to 250 kg/cm2 marked in red is indicating the danger zone. This exceeds the limit for a tank 100% full of CO2 which is 75.3 kg/cm2. I don't know what the upper limit is for the other gases that your gauge can be used to measure. 

Here's a conversion: You multiply kg/cm2 by 14.22 to get psi.


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## Hoppy (Dec 24, 2005)

Left C said:


> Here's a little more about CO2. I got it from a reference book at the library.
> 
> Air is 0.033% CO2 by volume.
> 
> ...


The part in bold isn't correct. The pressure in the cylinder indicates nothing except the temperature of the contents, until all of the liquid CO2 is changed to gas, which is when the tank is essentially empty. Then the pressure drops fairly quickly as the gas is used up.


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## Left C (Nov 15, 2003)

Hoppy said:


> The part in bold isn't correct. The pressure in the cylinder indicates nothing except the temperature of the contents, until all of the liquid CO2 is changed to gas, which is when the tank is essentially empty. Then the pressure drops fairly quickly as the gas is used up.


I've noticed that as the weeks go by, the high pressure gauge shows a lower reading and then the next week, it's a little lower still and so on. 

Why is this? 

I know that it drops quickly as the CO2 is used up.


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## Hoppy (Dec 24, 2005)

Maybe its because summer is over and the cooler days of autumn are upon us?? The only reason I can see for a drop in pressure is a drop in temperature.


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## Left C (Nov 15, 2003)

Thanks Hoppy for responding. I'm still a little confused. Let me see if I have your response right. I know that you have been working with pressurized CO2 longer than I have.

You're saying that if a CO2 cylinder stays at the same temperature that the high pressure guage will stay the same and not show a pressure drop as the CO2 is used up. Then, when the CO2 is practically used up, the high pressure guage will drop quickly. Is this what you mean?

This is what both Carl and Kevin are saying also.

Thanks for your help. I wasn't aware that it worked that way.


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## Rex Grigg (Dec 10, 2002)

That is what Hoppy means.


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## Hoppy (Dec 24, 2005)

Yes. When my 5 pound cylinder ran out the first sign was that the bubble rate went up a little - just enough to be noticeable. When I checked the pressure it was a hair under 800. The next day I checked again and it was a bit lower again. The third day, the same, so I realized I needed a refill. But, it was Saturday, so I ran it through Sunday, and the pressure kept dropping, down to around 300 or so psi when I took it in for the refill on Monday. So, it does drop rapidly compared to its total life, but it still took several days to drop all the way. This was with a bubble rate of about two big bubbles per second, or about 4 or so small bubbles per second (the second being my bubble counter, and the first my Barr internal venturi reactor CO2 tube)


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## LS6 Tommy (May 13, 2006)

Hoppy said:


> The part in bold isn't correct. The pressure in the cylinder indicates nothing except the temperature of the contents, until all of the liquid CO2 is changed to gas, which is when the tank is essentially empty. Then the pressure drops fairly quickly as the gas is used up.


There are a few other incorrect statements in that post.
1) You _*never*_ fill a pressurized cylinder 100% with liquid. 
2) The statement that CO2 can only exist as a gas at 87.9* F is only true at atmospheric pressure (14.7 psig or 0 psia)
3) All the pressure readings in the statement refer to cylinder pressure, which is generally expressed as psig, not psi.

I think I'd get a new reference book.:wink: 

Tommy


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## Left C (Nov 15, 2003)

I remember seeing psig and psia in the reference book. I just wrote everything as psi. I really wasn't familiar with those terms.

Thanks guys for correcting my mistakes.


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## Left C (Nov 15, 2003)

I just found this in one of the stickies. Momotaro posted it this past July. It's about pressure of CO2 in tanks at different temperatures, PSIA and PSIG.

I thank all you guys for correcting me.

http://www.plantedtank.net/forums/p...re-relation-room-temperatures.html#post287886


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