[PATCH] Re: [PULL REQUEST] VPM-B

[Rick Walsh]

Hi Robert,

On 18 August 2015 at 02:43, Robert C. Helling <helling at atdotde.de> wrote:

>
> On 17 Aug 2015, at 18:41, Jan Darowski <jan.darowski at gmail.com> wrote:
>
> In my opinion we shouldn't leave this as a preference, it's to
> technical and complicated to explain to most of users. We have the
> conservatism levels already, so users can manipulate how aggressive
> their schedule is.
>
>
> It’s just that I have a problem with doing things that are obviously
> stupid. At least first make sure that doing this resolves the differences
> in schedules with the original fortran program.
>

Putting aside the, "we should aim to get similar results to other VPM-B
implementations" idea, I agree that there's nothing wrong with starting to
ascend to a level deeper than the current ceiling, so long as by the time
you get to that level the ceiling is shallow enough that you don't break it.

BUT, we need to be very careful when it comes to calculating the
first_stop_pressure variable that is used for the Boyle's law compensation.

Effectively, the Boyle's law compensation adjusts the conservatism (via the
allowable N2 and He gradients) according to the depth (ambient pressure) of
the first stop.  I am a geotechnical engineer, definitely not a
biophysicist or expert in decompression theory, but I don't believe there
is anything magical about the ambient pressure at the first stop.  However,
it is a proxy for how 'big' a dive is.  I understand that the Boyle's law
compensation was implemented because it was found that the VPM was too
aggressive for long/deep dives, i.e. people were getting bent on big dives.

The problem with calculating first_stop_pressure according to the ascent
rate is twofold.  Firstly, it is more aggressive than other implementations
(at least the Fortran code, but it can be verified for non-public code by
seeing that the first deco stop is frequently calculated as deeper than
Subsurface currently does).  Secondly, a diver could reduce the ascent
rates so that they skip the deepest stops, thinking this would be more
conservative, but with the side-effect that the shallower portion of the
deco schedule is calculated with substantially more aggressive gradients.

To avoid more aggressive allowable gradients than intended by the original
VPM-B method, and regardless of whether we considering the ascent rate when
calculating deco stops, I suggest that the Boyle's law compensation should
always be calculated according to the initial (instantaneous) ceiling,
rather than the first stop.  To remove confusion, the variable
first_stop_pressure could be renamed initial_ceiling_pressure.

If we calculate the Boyle's law compensation relative to the initial
instantaneous ceiling, I think that calculating the deco stops considering
ascent rate or not will not be so significant.

Cheers,

Rick
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