Should PSI:volume ratio deviate from linear??

[dlwalke]

Shouldn't the PSI to volume ratio be linear? I just noticed that DiveRite Express lists, for the E7100 tank, the following:

3442 PSI = 100 ft3
3000 PSI = 89 ft3
2640 PSI = 80 ft3

-which means

(34.4 PSI = 1 ft3)
(33.7 PSI = 1 ft3)
(33 PSI = 1 ft3), respectively.

Just curious,
Dave

 

[ClevelandDiver]

This link was posted by another memeber recently. I saved it to my favorites and do not recall who the poster was (sorry). http://www.babilim.co.uk/pages/gas_laws.html

The information is in bar instead of psi.

 

[Jason B]

If you take your tanks volume (in this case 100 ft3) and divide it by the fill pressure (in this case 3442) then multiply by 100 you will get the base number for your tank.
In this example we get a 2.9. That means every 100 psi of gas in your tank is equal to 2.9 ft3 of gas. So, at 3000psi you have 30 x 2.9 = 87 ft3, 2640psi you have 26.4 x 2.9 = 76.5 ft3, at 500 psi you have 5 x 2.9 = 14.5 ft3. Get the idea.

So let's examine the other figures you threw out there....
34.4 psi would be .344 x 2.9 = .99 ft3, 33.7 psi would be .337 x 2.9 = .97 ft3, and 33 psi would be .33 x 2.9 = .95 ft3 of gas.

so that means...
(34.4 PSI = .99 ft3)
(33.7 PSI = .97 ft3)
(33 PSI = .95 ft3)

Hopefully you'll be out of the water long before you hit 34 psi.

Jason

 

[dlwalke]

This link was posted by another memeber recently. I saved it to my favorites and do not recall who the poster was (sorry). http://www.babilim.co.uk/pages/gas_laws.html

The information is in bar instead of psi.

Very interesting.

Dave

 

[Rick Murchison]

You can flatten your curve just a bit by using the right (actual) pressures rather than gauge pressure... add 15 psi to each gauge pressure. As you can see in the cited article (a good one, by the way), real gas behavior isn't quite in line with the ideal gas law, but the bottom line is that the variance for breathing mixes at Scuba diving pressures from the ideal gas law is less than physiological variances from dive to dive, so for gas planning purposes you can reliably use the ideal gas law.
---
You cannot use the ideal gas law for accurate mixing of breathing gasses, however, so if you're going to mix your own get the charts & use 'em.
Rick