[PATCH] Support different salinity in planner

[Anton Lundin]

On 13 November, 2014 - Linus Torvalds wrote:

> 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.

Wen we plan dives we usually plan based on a depth that comes from sonar
equipment, which as far as i know is salinity-independent in its depth
measurements.

This came out of a discussion with a frend who had his Uwatec 330 show
him a depth of 72 meters while his Liquivision Xen showed him 69 meters.
This was due to that his Xen was set in salinity mode, while Uwatec 330
always shows a depth calibrated against fresh water.

Based on those dives to 69 meters, with 25 min bottom time, the
deco-schedules differers 6 minutes (7% of total time, 18/40, 50%, 100%). 
Not that deco schedules are a exact science, but that a thing that
confuses divers. They would like there numbers to line up.

> 
> 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).
> 

I don't think that there are that many divers around the world that dive
fresh water. Its probably way more who dives salt water, and thats why
the tables are salt-water-based. I dug around in Gue Deco Planner, and i
couldn't find a single reference to which salinity they use to calculate
the deco profiles. I found one reference to MSW/FSW in there gas
MOD/EAD/END calculator, thats it.


And i wouldn't say 17 loc is alot of 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.
> 

I for one can't say that you can cut 3min on 50% and 3min 100% from your
deco schedule and say that it doesn't matter.

> 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.
> 

As far as I've read the code, we actually use it correctly in all places
already.

> 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.
> 


I think correctness is preferred. If the rest of you thinks another
spinbox complicates the planner ui too much or that 17 loc is to complex
infrastructure for this feature, fine, don't take the patch.


//Anton


-- 
Anton Lundin	+46702-161604