[PATCH] Support different salinity in planner

[Linus Torvalds]

On Thu, Nov 13, 2014 at 1:20 AM, Robert Helling <helling at atdotde.de> wrote:
>
> Of course, I don’t know about you, but I myself don’t take a yard stick to
> the water but rather used depth gauges (e.g. my dive computer) that pretend
> to measure depth but in reality measure ambient pressure (and don’t take
> into account the salinity). So what we call “depth” in diving is, most of
> the time, actually not really depth but “effective depth” being the pressure
> equivalent depth assuming some standard diving liquid.

Yes. From a dive-computer reported deco standpoint, this doesn't
matter, since all that matters is the pressure it actually measures.

Many dive computers can be set into fresh/salt water modes, though,
and I wonder if that actually ends up changing the depth they show.
The difference is small enough that it likely doesn't really matter,
and there are arguments for either case (most notably: I've never seen
a fresh water dive table - it would be easy to create, and for all I
know they exist, but this is an example of where taking "fresh water"
into account actually ends up being really really confusing and adding
a lot of unnecessary infrastructure).

Personally, I believe that for diving, "fresh-vs-salt water" should be
considered purely a note for the log, the way "boat vs shore" is, and
not really change the calculations. It only adds room for confusion
and error, and there are no real upsides to taking it into account.

The problem is exactly that you don't know where the salinity has
actually been used and whether you're actually correcting for
something or not.

So if a dive computer actually gives us a salinity value, we do use
that for pressure calculations, since we assume that the dive computer
has used it for the depth conversion. But I really don't know if dive
computers actually do that. I guess we could check with the OSTC.

                       Linus