Minimum gas calculation

[Stefan Fuchs]

Hello Willem,


Am 18.10.2017 um 06:06 schrieb Willem Ferguson:
> On 17/10/2017 22:37, Stefan Fuchs wrote:
>>
>> Now about the "5bar" result from your first plot. Yes, this makes no
>> real sense. Root cause is that my implementation currently always
>> does the calculation for the last manually entered data point and
>> this in your case is at 22m. This is so close to the possible gas
>> change at 21m that almost no gas volume is needed to go there. It
>> would be correct in this case to do the calculation for the second
>> last entered data point at 40m. But implementing this is not so easy
>> because there could be also the use case of a multi level dive with
>> some bottom time at 60m and then some more bottom time at 45m. Here
>> we would really need to check at which point of the dive (deeper
>> depth vs. longer bottom time!) the situation is most critical.
>> For your simple example I have an easy proposal: Remove the last data
>> point and let the planner do the complete ascend starting from 40m.
>> For your "plan2" the results look almost good and I even tend to
>> remember that I did use bottom SAC and deco SAC according to your
>> proposal:
>> Ascend: 27bar*2 = 54bar
>> Problem solving: 1min*20l/min*2*9=270l ~ 15bar
>> Total would be 69bar, printed is 67bar... hmm, is this the
>> compressability thing once again or just comes from rounding - I have
>> to double check this.
>> Ah, you did look at the green delta value again - I have to think
>> about the green value! ;-)
>>
>>
>> Please feel free to provide more examples and comments! I will think
>> about possible improvements.
>>
>> Best regards
>> Stefan
>
> Hallo Stefan,
>
> Let me just make sure, the green value is the minimum cylinder
> pressure when starting the ascent. This takes into account the minimum
> gas as well as another externally-specified minimum cylinder pressure.
> If it gives the green value as 159 bars and the minimum gas as 5 bars,
> where do the other 154 bars come from? There is a strong case for good
> documentation here.
>
No, much easier:
The value in black is the minimum gas result based on the calculation
described above. Let's assume it's 90bar in another example.
The value which currently is printed in green or red with the "Delta"
symbol in front in simply the delta between the 90bar and the cylinder
pressure you have left at the same moment. Let's assume based on the
cylinder pressure calculation you have 110bar left. So the Dealt is
+20bar. That means for you, you could plan a even longer bottom time. If
the Delta is negative you should reduce bottom time.

Well, ok, I put on my ToDo list:
- Change syntax at least slightly (maybe change the color thing)
- DOCUMENTATION

> The problem comes with cave dives where the geometry of the cave
> determines much of the ascent. In these cases it is necessary to
> specify at least part of the ascent to allow sufficient time to swim
> through the cave on the way up. In my typical case the cave entrance
> is at 15m where it opens into open water so this does not give the
> dive planner a lot of scope to do any useful calculation. In some
> perverse sense one may describe this as a multilevel dive. I attach a
> profile where these intermediate steps are pretty evident and where
> the ascent was specified to 15m in the dive plan. However, in this
> case the minimum gas calculation would be more meaningful if it were
> applied to the end of the bottom section and not at the point of
> exiting the cave at 15m. The problem then is, as you note, to
> distinguish between a multilevel dive and one that is not intended as
> multilevel. I wonder whether it would be possible to identify a
> "bottom gas", maybe the one that has the lowest fraction of oxygen
> (FO2) and do the minimum gas calculation for that gas mix. For an
> intended multilevel dive (say 10 min at 45m, 20 min at 30m), it would
> give the worst case minimum gas applicable to the deepest part of the
> dive.
Understood. I will think about a possible solution for this. But this
may clearly involve also switching off minimum gas calculation in cases
where the result is meaningless. And this will come after 4.7 :-)


Your feedback helps me a lot to understand user needs and to improve
things. I already yesterday identified one use case for the cylinder
handling where I missed s.th. Next small PR will follow...


Best regards
Stefan

-- 

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

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