Rock bottom gas calculation in planner

[Stefan Fuchs]

Hi Rick,

Am 06.01.2017 um 03:24 schrieb Rick Walsh:
> Hi Stefan,
>
> On 4 January 2017 at 05:38, Stefan Fuchs <sfuchs at gmx.de
> <mailto:sfuchs at gmx.de>> 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 patch?
I have to start with the unhappy part of my answer: I would like to send
a patch but unfortunately I won't be able to. I already failed multiple
times during my life to teach myself even basic application programming
skills. To explain in other words: I'm an electronics hw engineer. ;-)

So I have to hope for s.o. like you, Robert, Dirk,... starting to like
my idea and writing a patch :-)

> 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).
I have the feeling that any kind of rock bottom/minimum case calculation
will always require a few additional user inputs/planner preferences. I
even was very happy when I recognized that only two additional things
("problem solving time" and "SAC_rate") would be needed and all the
other info is already available in the planner.
I also hope that my proposal would be ok for most of the people. As you
are already mentioning, everybody may have his own strategy for minimum
gas calculation and this should be accepted. With my proposal one can
e.g. set "problem solving time" also to very low value or even 0min and
SAC_rate to a low value like in your example.

Your comment about lost deco gas is correct. I also consider lost deco
gas when planning a dive. But I do this differently compared to the rock
bottom/minimum gas calculation for the bottom gas. For lost deco gas I
do not plan for sharing air from one divers deco gas cylinder but I plan
for doing the deco with the remaining bottom plus deco gas of the
individual diver. I handle this at the moment by "switching off" deco
gas in the planner by entering "99m" in the "Deco switch at" field of
the gas. Then I can check the result (will I be able to do my deco?) and
can save the backup plan as a copy. This is clearly also a workaround
but its ok for me.
Automatically generating plans for lost deco  gas (do you mean this with
"automated contingency planning"?) from my point of view currently does
not fit together well with the overall concept of the Subsurface
planner. Therefore I wouldn't ask for this at the moment unless anyone
else has a good idea how to do it.

> 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 warnings.
So I would consider my rock bottom/minimum gas proposal only for the
bottom gas cylinder (last used). For the deco gas cylinders one could
keep whatever is there already.



Best regards
Stefan

-- 

Stefan Fuchs
Banzhaldenstr. 66
70469 Stuttgart

E-Mail: sfuchs at gmx.de <mailto:sfuchs at gmx.de>

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