Cylinder Volumes in North America

[Mark Vlahos]

Lately I have seen lots of posts that seem to have at their root the issue of the volume of gas in one cylinder or another. I have written an Excel spreadsheet that indicates the quantity of gas (described in cubic feet) for each 100 pound increment in cylinder pressure (described in Pounds Per Square Inch). The green numbers reflect the quantity of gas in a cylinder that is either filled to its rated pressure or lower. The red numbers reflect the quantity of gas in a cylinder that has been overfilled. I make no endorsement of overfilling any cylinder, I have simply included the numbers for comparison purposes.

The chart is actually two charts, the top area is for single cylinders. The bottom is for double cylinders. Pay attention not to confuse the two charts, an error on this could be extremely dangerous.

I am including two versions of the chart. The .xls version is the actual Excel spreadsheet in an unlocked state, you can edit this chart to suit your own needs.
I am also including a .pdf version for those who do not have Excel or a program that will allow you to view the Excel file.

Please take note that there may be errors or omissions in the original file, and that you the reader need to verify the information and make any corrections yourself. It should also be noted that since the file is available as an unlocked document others may make changes to the file having unknown consequences. The author assumes absolutely no responsibility for the information provided.

Have fun,

Mark Vlahos

 

[WarmWaterDiver]

Did you use ideal gas behavior in your calculations or observed gas behavior?

 

[Hoosier]

Nice Job....

Absolutely, it is an ideal gas number....

 

[H2Andy]

great chart

 

[Mark Vlahos]

Did you use ideal gas behavior in your calculations or observed gas behavior?

If you downloaded the Excel version, and you also understand Excel, the calculations are right there in the spreadsheet. Just click on the cell and look up at the formula bar to see what is being done to give you the result. The math is right there for you. This is also why I put in the warning about what others might do to change the result since the formulas are not hidden or locked.

By the way, if you have a cylinder that is not on the chart just insert a blank column where you think it will fit in with the rest of the chart, then copy the contents of the neighboring column over. Then manually change the "Common Name", "Actual Volume", and "Rated Pressure" and the chart should calculate the base line for you and fill in all the values. You would need to manually change the color from green or red to the appropriate colors since the color choice is not automated.

Mark Vlahos

 

[WarmWaterDiver]

Just a simple question Mark - no worries, I won't download your chart. It seemed simple to ask rather than dissect.

 

[Mark Vlahos]

Just a simple question Mark - no worries, I won't download your chart. It seemed simple to ask rather than dissect.

If I gave you the wrong impression, I apologize. I posted the chart for anyone to download, and I left it unlocked so it could be customized for any cylinder I may have left out.

Download it, modify it, use it. I just wanted to make it clear that I can't be responsible for someone making changes and then saying that my chart did not leave them enough decompression gas or back gas for a dive they planned poorly.

I posted to try to illustrate that cylinder pressure by itself has nothing to do with available gas. It is the size of the cylinder and the pressure for that cylinder that will tell a diver how much gas they have available. WWD, it think you get it, based on your question, but lots of other divers do not.

A good example would be that a PST E7-80 @ 2800 PSI will contain approximately 65 cubic feet of gas, but a PST Low Pressure 104 @ 1900 PSI will contain approximatley 75 cubic feet of gas. The 104 at a significantly lower pressure will contain more gas. This simple example is lost on lots of divers, so I made this chart so they could compare lots of different cylinders at various pressures in one place.

Mark Vlahos

 

[WarmWaterDiver]

No problem - some of the European board members have posted where this comes into play using high pressure tanks in their area. The incremental volume gain isn't always linear with pressure increase.

It could also vary by mix composition for observed gas behavior (not all non-ideal gases and / or mixtures are equally non-ideal under the same set of actual conditions).

These are just some things to keep in mind when gas planning using tanks filled to high pressures, and with different gas mixtures. I'm not sure any agency or vendor has this in their dive planning software.

If you're sticking to US tank rated pressures and air, it's negligible.

I have tried to get this same concept you've illustrated clearly in your tables (nice job!) across to dive operators who have aluminum 100's available, but fill them to 3000 PSIG alongside the aluminum 80's on their whip lines. Now you only have an aluminum 90 since it's rated at 3300 PSIG for 100 ft3.

A few have gone ahead and pumped it up to 3300 for me after taking the 80's off. Many others have not.

 

[Mark Vlahos]

No problem - some of the European board members have posted where this comes into play using high pressure tanks in their area. The incremental volume gain isn't always linear with pressure increase.

It could also vary by mix composition for observed gas behavior (not all non-ideal gases and / or mixtures are equally non-ideal under the same set of actual conditions).

These are just some things to keep in mind when gas planning using tanks filled to high pressures, and with different gas mixtures. I'm not sure any agency or vendor has this in their dive planning software.

I am aware of the nonlinear compressability of gasses at high pressures, and the differences of helium, for example, when compared to other gasses commonly found in breathing mixtures used in diving. My chart does not take this into account. Frankly, I would assume that any diver who is planning a dive using helium at extremely high pressures would not be using this chart in the first place.

The chart is close enough for general estimation purposes when using air in cylinders at their rated pressures.

Mark Vlahos

 

[WarmWaterDiver]

Agreed - sorry if I gave the wrong impression as well.