Meaning of GF settings

[Dirk Hohndel]

"Robert C. Helling" <helling at atdotde.de> writes:

> On Jan 8, 2013, at 6:45 PM, Jan Schubert wrote:
>
>> - It (still) seems to use the ceiling-at-this-very-specific-moment
>> approach, not considering a ascent which explains the ceiling deeper and
>> earlier in the descent phase of the dive. Just a note, it's more
>> important that we get this right for the dive planning part of
>> subsurface. And if we (would) have it there, it should not be that
>> tricky to apply this to this part as well!?
>
> This would complicate the algebra even more and at least that would
> not match my understanding of the word "ceiling".

That has always been my point. Our goal is not to match what the
divecomputer (ANY divecomputer) shows you, but what the algorithm
calculates for you.

> The ceiling really matters when you get close to it which also means
> the difference between ceiling now and ceiling once you get there goes
> to 0.

Precisely.

>> - In case the calculated deco is not ended when the original profile
>> "surfaces" it might be helpful to extend the time for the profile shown
>> to get the calculated deco ceiling displayed completely (this would
>> allow better playing with existing dives and such parameters as GF).
>
> Good idea. Which depth would you use for the remaining off-gasing,
> surface or ceiling (in which case it becomes more akin to dive
> planning).

Well, I would do the math for the ceiling - depending on the user
setting either with the floating ceiling or the 3m-multiple
ceiling. That at least gives the user the realistic "oops, you should
have stayed there for n more minutes". And can be fun when playing with
GF values.

But I want to make sure we don't do that when the user briefly violates
the ceiling and then goes back below, right?

> BTW, that reminds me of a bug the first Aladin nitrox computers had:
> They assumed for repeated dives that in your surface interval you were
> still breathing the (last) nitrox mix which is of course rarely true
> (and is an error in the dangerous direction). Do we do this correctly
> when integrating tissue loadings of more than one dive?

Yes. Look at divelist.c in init_decompression():

tissue_tolerance = add_segment(surface_pressure, &air, surface_time, 0.0, dive);

>> - seeing the smooth profile I'm quite amazed that the non-linearities
>> vanished nearly completely, I'd not have expect this before.
>
> Could you send a screen shot of your dive with smooth ceiling as well?
> My guess would be that the differences between the dc ceiling and
> deco.c get smaller (since we always round up to full multiples of 3m).

Yes, the DR5 code seemed to do something similar to rounding up, but not
quite the same :-)

Also, some software seems to have this safety distance (where they
assume that you actually do your stop 50cm deeper than planned). We
don't do that.

/D