Allow replanning of logged dives

[Anton Lundin]

On 13 October, 2015 - Davide DB wrote:

> On Mon, Oct 12, 2015 at 4:36 PM, Anton Lundin <glance at acc.umu.se> wrote:
> >
> > Currently it doesn't do any gas-switches in its calculations, because
> > thats a quite big rabbit-hole to go down. IE, when should it switch gas?
> > At mod? 1.6, 1.5 or 1.4? Configurable?
> >
> > So when i wrote that i just made it simple. No gas switches.
> >
> > Thats why you see a jump in TTS at the gas-switches. When you switch to
> > a deco gas, the TTS drops.
> >
> >
> > There were quite a few older DC's who behaved that way. Its pretty
> > uncommon in newer DC's and some can still be configured that way.
> 
> Hi Anton,
> 
> Given that the last DC I used was an Aladin Pro, well before I started
> doing tech dives, I don't know from personal use how modern DC
> approach to TTS and gas switches.
> Actually your explanation clashes with my experience when diving with
> other divers who uses the Shearwater Petrel. Sometimes when we call
> the dive we joke about deco time betting on TTS  (my on the fly deco
> time with the deco displayed by the petrel). Usually my menmonic TTS
> differs for 10 minutes from their TTS (GF 20/85).

Shearwater and HW computers do their TTS calculations on the assumption
that you switch to your active deco gases at the configured switch
depth. There are probably other modern computers who does it that way.

For a generic dive we don't have switch depth in subsurface. That means
that we would need to make up a switch depth if we would like to have
one. Thats a deep rabbit hole.

I read one discussion on the HW forum about a diver who was confused
that the TTS didn't drop when switching to a deco gas as that was how
that persons old computer worked. I don't remember what that old
computer was but there are old computers who worked that way.


And yea, I really think accurate TTS is golden for on-the-fly
re-planning of a dive. Lots of our diving is limited by how long we can
be in the water without freezing or asses off, so we usually put up
limiting factors like X gas, X bottom time or X TTS and then we call the
dive.

> How can it be if this computer calculate TTS taking in account only
> the bottom gas? As you said, 16/60 is a pretty shitty deco gas.
> So I downloaded the Petrel 2 user manual and I read:

<snip/>

> The NOTE explains why I see an update for TTS when I switch gas: DC
> will update calculations with real scheduling and depth.
> 
> PS
> Solution you proposed is perfect for me because I'm analyzing the last
> portion of the dive.
> 

Yea, there is more than one way to cook the beef. This is what i think
is the sane one for subsurface.

If someone comes up with a sane way to guess switching depths we might
add that functionality, but until then I'd say we stick with the current
variant.

Maybe you would like to sum this discussion up into something
understandable in the manual, so others thinking about this would be
able to get their answers there?


//Anton

-- 
Anton Lundin	+46702-161604