Same gas cylinders

Dirk Hohndel dirk at hohndel.org
Mon Jul 14 18:41:06 PDT 2014


On Tue, Jul 15, 2014 at 01:08:05AM +0200, Davide DB wrote:
> > > I sadly discovered this
> > >
> > > http://trac.hohndel.org/ticket/296
> > >
> > > Any news? User comments describe perfectly the problem.
> >
> > Still thinking about it. Haven't found any good design for it yet.
> >
> > The problem is that we currently model our gas switches after how most
> > dc's handle them, you switch to a gas, not a cylinder. There are quite
> > allot of assumptions in the code that it works that way, so reworking
> > this is a non-trivial task.

My goal for 4.3 is to completely redo the event mechanism. What we have
now was simply taking the libdivecomputer events and matching them 1:1 -
which in the early days was a good way to start but it has LONG outlived
its usefulness.

So once 4.2 is out I will wrap the events inside hopefully reasonably well
design Subsurface events. Those will allow you to specify a specific
cylinder at a gas change. We still won't be able to get those from dive
computers (several I own even prevent setting up two cylinders with the
same gas), but at least in post processing you can get things right then.

> Looking at one of my oxygen gas change event I found:
> 
> *<event time="119:26 min" name="gaschange" value="100" type="25"/>*
> 
> So I guess it refers to the gas via the oxygen fraction. Humm...

Yep - that's what the vast majority of dive computers do.

> In an advanced tech/cave dives I easily have one of these scenario:
> 
> Gas switches between several identical stage bottles with the same mix.
> Gas switch from a bottom stage and back gas.
> Gas switch from my back gas to a stage bottle during descent in a PSCR dive
> (even in an arbitrary point in a cave dive).
> 
> 2nd & 3rd scenarios switch between different bottle type. 1st scenario
> switches between same bottle type so cylinder type is not enough.
> Is it really so difficult having something like bottle id in the cylinder
> record and then referring later to it by id without breaking other code?

The scenario is well understood. And I have a pretty good idea how I want
to do this. It simply hasn't made it to the top of the priority list. And
since it's rather invasive...

> By the moment the only workaround I found is having a slightly different
> mix in every stage bottle.

Correct. that's the recommended workaround.

/D


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