Can I consider ft^3 to be linear with PSI?

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android

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I know it's not the precise formula, but can I assume that 1720 PSI in a 3442PSI 100CF tank means I have 50 CF of air remaining?

My math has an AL80 holding 13CF at 500 PSI and that is about equal to about 450 PSI in the 100.

Is this close enough for working numbers? Any handy calculators or formulas anyone can point me to?

Thanks,
Andy
 
yes... pressure and volume have a linear relationship.

For handy rules of thumb you can divide the rated volume in cf by the rated pressure in psi multiply that by 100 and get a cubit feet per 100 psi for the tank you are using.

For example a low pressure steel 104: 104/2640*100=~4

So I know that each 100 psi mark on my SPG equals 4 cf in a single 104 and 8 cf in double 104s

Very handy not only for gas planning but also for keeping track of gas during the dive.
 
partridge:
Yes, but I dont think an AL80 is actually 80 cubic foot. More like 77. Not sure though. Hope someone will chime in here.

Someone (Jonnythan) told me in another thread that the normal AL80 was 77.4cf.
 
Uncle Pug:
yes... pressure and volume have a linear relationship.

For example a low pressure steel 104: 104/2640*100=~4

Ah, so if we use the formula:

Volume in CF / Working pressure * PSI between Tick Marks = CF per Tick Mark

My marks are in 250 PSI increments so I would use:

100/3442*250 = ~7 CF per mark.
 
Yes... that would work I suppose during a dive...though it is more coarse than I prefer.

Having an SPG that has 100 psi marks allows you to easily resolve a 50psi change which for me is 2 cubic feet.

However for planning purposes it is helpful to have a rule of thumb that easily allows you to convert PSI to cubic feet and vise versa. cf per 100 psi is a very easy number to work with.
 
This is another way the "tank factor" number is handy. The number is around 2.5 per 100 psi for an Al 80... so every 1000 psi is about 25 cf, every 250 psi is just over 6 cf, etc.

So when you're diving an Al 80 and see that you have 1500 psi left.... halfway between 25 and 50 is about 37 cf left, or just half of 77.4 is about 38-39 cf....
 
Uncle Pug:
Yes... that would work I suppose during a dive...though it is more coarse than I prefer.

Having an SPG that has 100 psi marks allows you to easily resolve a 50psi change which for me is 2 cubic feet.

Good point. I may look into getting a different gauge.
 

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