Rock bottom gas calculation in planner

Stefan Fuchs sfuchs at
Tue Jan 3 10:38:56 PST 2017

Hi All,

don't know if this was ever discussed here:
Today I installed and tested the 4.6 beta 2 a little bit and planned
some dives to see how long I can stay at 60m with and w/o bottom stage :-)

At this moment it became clear to me in which way subsurface already
today supports you to do rock bottom gas calculation and how this could
be improved.

What we already have today:
We have "Bottom SAC" GUI user input, cylinder size of last cylinder used
in "dive planner points" and we already have tank pressure consumption
from this cylinder in planned ascent including all calculated deco stops
until we reach first deco gas switch. That's a good starting point.

What we would need for the rock bottom calculation:
- Problem_solving_time (including some margin) as GUI input, integer
number of minutes, default e.g. 4min
  This is the worst case time the buddy team will stay at max. depth
after OOG situation happened.
- SAC_factor
  This is the increase factor for bottom SAC rate in OOG situation. I
would multiply bottom SAC by 2 already hard coded and then again
multiply by this factor.

Rock bottom cylinder pressure would then calculate as:
(Problem_solving_time * Bottom_SAC * 2 * SAC_factor *
ambient_pressure_end_of_bottom_time / cylinder_size) +
(tank_pressure_consumption_ in_planned_ascent * 2 * SAC_factor)

Example1 (60m, D12, ~25bar for ascent (deco below 21m!), 18barl/min SAC):
(4min * 18barl/min * 2 * 2 * 7 / 24) + (25bar * 2 * 2) = 184bar

Example2 (40m, D12, ~6bar for ascent (direct ascent to 21m), 18barl/min
(4min * 18barl/min * 2 * 2 * 5 / 24) + (6bar * 2 * 2) = 84bar

The result could be simply printed to the dive plan details together
with the info about the corresponding cylinder (for cross check only!).
One could also compare cylinder pressure at end of bottom time with
calculated value and give a warning but this IMHO is optional.

Maybe too late for V4.6 but for a later version? :-)

Best regards


Stefan Fuchs
Banzhaldenstr. 66
70469 Stuttgart
Tel.: 0711/4116289
Mobil: 0172/8922025
E-Mail: sfuchs at <mailto:sfuchs at>

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