Dive planner VPM-B problems

[Stefan Fuchs]

Hello Willem,

I will try to answer to both of you mails from today concerning the
planner gas handling and minimum gas items. I also re-attached your
screenshots - sorry for this but otherwise we may get completely lost
with all these discussions.


Am 17.10.2017 um 18:33 schrieb Willem Ferguson:
> I have been trying out a lot of things around dive planning today this
> afternoon to test your patches within my dive context. I could not
> find any problem. Attached one of the dives I worked with. I loved it
> when changing the gas of the first segment of the dive, the cylinder
> table changed to put the gas first used at the top. I added segments
> at the bottom of the dive planner points table and these were placed
> at the start of the profile if the runtime was set appropriately. I
> really messed around, deleting cylinders, changing cylinders and
> adding cylinders and everything remained coherent and stable. I also
> see that all the cylinders used in the dive planner points table are
> marked as non-removable from the dive. Ok, Good.
>
Great! I also like the "Planner: Autom. move first datapoint gas to
first gaslist position" thing best. I hope nobody will come around and
say he hates this because it breaks s.th.

Also good for the moment that the issue with the pressure graph inside
the planner disappeared. My answer would have been the same as Roberts:
Tried to reproduce it but works here.
One note: We know that it still can happen with "old" (old means created
with planner before one fix which was merged into master just some hours
ago!) dives from xml that they show strange vertical pressure lines
because they have these wrong pressure info for some specific samples.
Only workaround is to either recreate the dives from scratch or to fix
the xml with a text editor.

For the pressure graph in the merged dive: I wrote some comments but I'm
not the expert for that part.

> About minimum pressures, you can see that the minimum pressure is
> given for the final bottom gas. As planned, there are 34 bars of
> bottom gas required to from the start of ascent complete the dive. If
> I double the deco SAC (SAC factor = 2) I would need 41 bars of back
> gas for the ascent. If I took one minute for problem solving I would
> need 30(litres/min=2*deco SAC)*9(atm) = 360 litres of air = 11 bars of
> D12 cylinders. That is 45 bars in total. But the problem solving would
> have been at the bottom depth (80m) before ascent, therefore the
> problem solving need is possibly better defined in terms of the bottom
> SAC, not the deco SAC (i.e. 15 bars and not 11 bars). But, I am very
> slowly getting to understand the argument.
Regarding the minimum gas I think you meanwhile recognized that there
was a misunderstanding on your side in your first mail. You were talking
about a value of "169bar". But this was in fact the new "delta" value.
The real minimum gas result was "5bar" which is very, very low but comes
from the setup of the dive -> next topic.
Brings me to the question: Do we need a different format for printing
this. Now we have:
 — Minimum Gas (basierend auf 4.0xAMV/+4min at 60m): 3494ℓ/150bar/Δ:+14bar
The first results in black font is the real minimum gas result and the
last number in green is the delta.
Do we need some improvement here or just documentation?

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

-- 

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

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