[PULL REQUEST] VPM-B

[Rick Walsh]

Hi Jan,

On 17 August 2015 at 20:59, Rick Walsh <rickmwalsh at gmail.com> wrote:

>
>
> On 17 August 2015 at 19:46, Jan Darowski <jan.darowski at gmail.com> wrote:
>
>> > I think that currently first_stop is being calculated for each stop (so
>> > isn't actually first_stop) rather than just the first.  That's what it
>> > looked like from my limited running under gdb with a break every time
>> > first_stop is calculated, and printing the depth variable.
>> >
>> > To calculate the first stop for every cva iteration we need to be more
>> > clever than what my patch does, but I'm not sure you'll get a very
>> different
>> > resulting plan. From my understanding, cva is significant for short
>> deco /
>> > no deco dives, whereas Boyle's law compensation is significant for long
>> deco
>> > dives.
>>
>> I can't reproduce this. I've checked, there are only two places setting
>> this
>> variable, the only place it's used is passing it to the boyles_law().
>> Printing it
>> there gives the same value several times...
>>
>
> My bad - I read the code incorrectly and the examples I tested gave the
> impression that it was run at every stop because there were 3m depth
> difference between the depth variable at the breakpoints.
>
> Your explanation is correct - it was the different CVA iterations
> calculating different first stop depths.
>
>
The problem remains with different profiles being calculated when compared
> to other programs.  The same dive in the Fortran VPM-B program:
>

I think I've worked out at least one difference in the programs'
algorithms, and sorry I doubt you'll like it.  Subsurface calculates
required stops considering the ascent rate and the time to reach the stop.
The Fortran program calculates the 'instantaneous' ceiling, i.e. the depth
corresponding to the tolerated ambient pressure according to the current
allowable pressure gradient.  More fun!

Which approach is more justified?  Debatable.  The method used by
Subsurface should be 'better', but when the depth of the first stop/ceiling
is given a special significance thanks to the Boyles law compensation
process, I'm not so sure.  The 'instantaneous' ceiling method is more
conservative, and, without having modified the code and tested, I'm
guessing would produce deco schedules more consistent with other VPM-B
programs.

R
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