Allow replanning of logged dives

[Davide DB]

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).
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:

-----------------------------

Time-to-Surface (TTS) The time-to-surface in minutes.
This is the current time to ascend to the surface including the ascent
plus all required deco stops.
Assumes:

  • Ascent rate of 33 feet per minute (10 meters per minute).
  • Decompression stops will be followed.
  • Programmed gases will be used as appropriate

-----------------------------

Later on:

-----------------------------

• The system will use the gases that are available in the order of
oxygen content during the Time To Surface (TTS) prediction. The system
will use the next available gas that has a PPO2 of less than 1.0 for
closed circuit diving.
• If the computer is in open circuit or is switched to open circuit
during a dive, the system will calculate the TTS based on the
configured open circuit gases that are available. It will use the next
available gas that has a PPO2 of less than 1.6 for open circuit
diving.

NOTE: These gases are used automatically only for TTS predictions. The
gas used to calculate the current tissue load and the current ceiling
is always the gas actually selected by the diver.

Automatic decisions of when to switch gases for the TTS calculation
means that it is very easy to set up your CC and OC gases. There is no
need to enter a depth or a PPO2 to switch gas. Any gases that are
turned on will be used in the decompression calculation.

-----------------------------

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.

-- 
Davide
https://vimeo.com/bocio/videos