Help needed decoding data

[Jef Driesen]

On 2014-07-02 20:03, Linus Torvalds wrote:
> On Wed, Jul 2, 2014 at 10:39 AM, John Van Ostrand <john at vanostrand.com> 
> wrote:
>> The computer has external contacts and can sense
>> between fresh and salt water. I may try diving the computer in a 
>> zip-lock
>> bag full of salt water aquarium water to see how it logs salt water 
>> dives. I
>> suspect that the dive start structure will have a salt/fresh water 
>> flag so
>> the vendor software can display depth.
> 
> Interesting, I don't think I've seen (or been aware of) dive computers
> that sense the salinity automatically.

I'm also not aware of any dive computer that supports this. The water 
contacts are usually only used to activate the dive computer's dive 
mode. Technically, it might be possible to measure the salinity by 
measuring the conductivity of the water. But as Linus already explained, 
the dive computer doesn't need this information for its decompression 
calculations. So why even bother to measure it? Dive computers have 
other and much more important things to do.

> Personally, I think salinity sensing is kind of stupid, actually. It
> has no serious meaning, since the actual deco calculations do not care
> at all. The sensor senses pressure, and that's the only thing that
> really matters. The conversion to depth is "pointless" - it has no
> real meaning for diving, except if you happen to have other depth
> measurements that you want to match with, which is very very rare.
> 
> So the only reason to give depth at all is that it's what people
> "expect", but since people don't actually care about the roughly 3%
> "real absolute difference" in depth in any normal situation, why even
> bother changing the calculations?  Even if you know the depth of the
> dive site, on most diving things like tides etc tend to be more than
> the 3% difference.
> 
> So I think "salt vs fresh" is interesting perhaps from a logging
> standpoint - the same way that it can be interesting to differentiate
> a boat dive from a shore dive. But I think it's over-engineering to
> take it into account for depth calculations.

I think the only case where a difference of a few percentages might 
matter somewhat is when doing (extreme) deep dives with a dive computer 
in gauge mode and tables. In that case you do use the calculated depth 
for the tables, and not the pressure measured by the sensor.

Jef