Can you help me learn about: gas management and basic deco therory?

[lamont]

OK I think I get it, you have to factor in the physical size of the tank, not just the max psi. The 26cf of "extra" gas is due to the max psi, not a bigger tank.

Actually its probably due to some combination of the incompressibility of the gas at higher pressures, differences in the physical water volume of the tank, and manufacturer's rounding off choices. Someone would need to do the work to accurately measure the water volume of a sample of both of those tanks to determine what the real truth is. In practice, its not a significant enough difference to care about. (If you are conservative, you should probably pick the HP tank value which is more conservative even if you're diving LP tanks, YMMV).

 

[JasonH20]

Here is a spreadsheet I put together a couple years ago. The results are similiar to Lamonts page, but let's you tweak the values around a bit.
Rock Bottom Excel Spreadsheet

 

[JasonH20]

If you just downloaded the spreadsheet in the last 20 minutes, you might want to redownload. I just tweaked some of my numbers (ascent rate mainly) and now it jives exactly (well, within 1psi) with Lamont's work.

Enjoy!
Jason