Rock bottom gas calculation in planner

Rick Walsh rickmwalsh at
Thu Jan 5 18:24:13 PST 2017

Hi Stefan,

On 4 January 2017 at 05:38, Stefan Fuchs <sfuchs at> wrote:

> 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
> SAC):
> (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? :-)
> Do you feel like implementing the rock bottom gas feature and sending a

While it's not hard to do the calculation, or a simpler approximation of
it, in your head, I think this is worth looking at after the 4.6 release.
Of course the argument against doing so is that it introduces yet more
planner preferences, or someone else will propose a different method (e.g.
I don't think anybody feels like implementing automated contingency
planning).  But what you've proposed should be simple and flexible enough
for basic use.

FWIW, for an ocean dive I'll typically consider 3x gas for ascent as
minimum (equivalent to SAC_factor =1.5 in your equation, but ignoring
problem solving time), but will also check a separate contingency for lost
deco gas (which almost always governs).

Currently, a warning is printed if end pressure is less than ascent gas
(i.e. not enough gas to share on ascent, assuming sac factor of 1), or if
end pressure is less than 10bar.  Your proposal should be in place of these


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