Graph documentation [was: Re: Meaning of GF settings]

[Jan Schubert]

Thx Robert for detailed solving of your formula, really appreciate this!


On 01/10/13 11:24, Robert C. Helling wrote:
> On Thu, 10 Jan 2013, Jan Schubert wrote:
>
>
>> Can we check our understanding of GFlow? I remember you saying GFlow
>> refers to deepest point in dive!? I thought GFlow defines the 1st deco
>> stop (current GF should be <=GFlow).
>
> OK, you are right, I had remembered this wrong but this is what Baker
> states. Note that this moves the point where GF_low applies up which
> implies that at all shallower depth (i.e. during deco) a lower GF
> results which makes the deco longer rather than shorter.
>
> Of course, "GF_low" applies at first deco stop is a but tricky since
> the depth of the first deco stop again depends on GF_low, i.e. there
> is another equation to solve. You can do this by inverting equation
> (1) for the ambient pressure and use GF_low as the gradient factor.
> This yields
>
> amb = (b * M_value_corrected - GF_low * a * b) / ((1-b) * GF_low + b)
>
> I also attached a patch that implements this and you can switch
> between the to versions by setting the macro GF_LOW_AT_MAXDEPTH to
> true or false for you to play around with this. (It has the effect of
> changing the meaning of gf_low_pressure_this_dive from
> max_ambient_pressure to max_ceiling_pressure).

I do not see much (any?) difference setting it to 0 or 1.

> [...]
> Let me add a final remark: Deco is an art, not a science.

If we speak about implementing Bühlmann I tend more to science than art.
Both of my computers - OSTC and Shearwater Predator - are equal nearly
to the second when it comes to deco calculation while subsurface shows
noticable differences. The differences between the units might mostly be
related to the fact, that my Shearwater uses live sensor data for pPO2
while the OSTC uses a fixed setpoint. OK, to be fair: As the Shearwater
is most likeley a copy of the OSTC (and vice versa in the newer days)
the similarity when it comes to deco calculation might not be that
surprisingly. But I doubt they use exactly the same math for
calculation. And as we also use code from HW in subsurface this might
even be quite similar!?

The reason I stick to this topic is that I'd really like to be sure
about the outcome of the planning part of subsurface (which right now
shows extreme differences to other dive planning software).

> Thus I would not worry at all if two different models (or
> implementations) vary in their prediction for your stops by one or
> even two meters.

I'm not worrying about the differences in depth (as it is quite obvious
where this comes from) but the much longer deco in the shallow part. In
my example it is about 12min (!) difference on 6m and most likeley even
more for the 3m stop. The only thing I like to track down is where this
difference is coming from (I'm not saying this has to be equal!).

> That said, I find the precision at which the smooth ceiling matches
> your actual profile and what your dive computer says very very
> impressive (say from a depth of 50m). I expected much worse.

Please note that the profile is cut, I expect the calculated deco
ceiling to be much longer.

> Acutally, I would call that spot on, in particular given that your
> actual depth during deco stages also has fluctuations of the size of
> at least one meter. Which, as was mentioned in an earlier email is
> also the order of magnitude of surface pressure variations due to
> weather. And those you also do not take into account.

As we use real profile data the real depth should be taken into account.
And as my dives are done at much less than 1013mbar ambient pressure
this might result in even more difference...

> The mouse over value ("1m") is actually rounded to integer multiples
> of meters. If you want more precise values, find the word "Calculated"
> in profile.c and in the snprintf replace depthvalue by entry->ceiling.
> Then the ceiling is reported in mm and you will see that at the end of
> your dive it is even less than a meter I would expect (just don't
> commit that change).

It's about 1080mm at the end of the dive and 920mm at the very right end
(when the profile already surfaced).

> Was this the explanation you were looking for?

For understanding your math implemented in subsurface: yes, thx!
I just like to understand where the difference between subsurface and my
dive computers comes from. I'll do some more tests by my own and come
back if there is anything worth to tell...

Thx,
Jan