Question about gas calculation in Subsurface Dive Planner

[Michael Ott]

In the Planner, I entered a twin set D7 at 300 bar. Subsurface shows the cylinder size as 14.0 L and the starting pressure as 300 bar.

Mathematically, the available gas volume should be:

7 L × 2 × 300 bar = 4,200 L

However, Subsurface shows only around 3,730 L, and the tooltip displays:

807 L (3,730 L → 2,924 L)

Why does Subsurface not calculate the full 4,200 L?
Is there an automatic reserve or deduction being applied here?

Also, why is it not possible to set this reserve manually? For dive planning, it would be useful to define the reserve or minimum pressure directly.

[Dirk Hohndel]

This has been discussed dozens of times on the forum.

Real gases don't behave like the ideal gases in Boyl's formula that you use below.

Lots of details at https://thetheoreticaldiver.org/wordpress/index.php/2017/11/02/when-real-gas-corrections-matter/

Essentially, your cylinder only contains 3730L because real gases aren't ideally compressible.

/D

[Dirk Lehmann]

Real Gas calculation.

For 300bar you need to correct via appr. factor 0.9

Your calculation is for "Ideal gases" which don't exist in reality.

BR

Lemmi

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[Michael Ott]

I understand that at 300 bar, real gases are not perfectly compressible, so Subsurface applies a real-gas correction and calculates about 3,730 L instead.

My remaining question is: where can I see or adjust the actual reserve/minimum gas setting in Subsurface? The tooltip shows 807 L (3,730 L → 2,924 L), and I would like to understand exactly what determines this remaining/reserved amount.

[Robert C. Helling]

That is the gas usage of the dive you planned. You start with 2x7 L cylinders filled to 300 bar. If you would expand that gas to 1 bar you would obtain 3730 L. That is the real gas calculation. Then the dive that you planned, breathing absent pressure with the SAC you entered uses 807 L of that gas. And the 2924 L of gas is what is remaining in you cylinders after the dive (which is the converted again into a pressure). There is no reserve in this calculation. 

The only reserve is on the bottom of the dive plan details: There it calculates at which pressure you have just enough gas for gas sharing with a buddy and do the specified time of "problem solving" and then ascending to the next gas change or surface breathing at an increased panic rate specified by the SAC factor.. 

Hope that helps.
Robert