On Sun, Apr 15, 2018 at 12:07 AM, tormento <turm...@gmail.com> wrote:
>
> Henry's Law is based on (partial) pressures and not depths. I dunno how
> diving computer talks to subsurface. If giving depths instead of pressures
> it would be a pity.

Almost all dive computers only give depth.

Some give salinity, some don't. And some only give the flag (sweet vs
salt) rather than the salinity value they actually use to calculate
depth.

In other words: do not EVER play games with salinity. You don't know
what it was, and you will get it wrong.

The whole notion of "user should set salinity" is broken garbage. A
dive computer shouldn't even have that setting. Because that setting
cannot possibly matter, because the only thing that matters - and the
only thing the dive computer measures - is actual pressure, and all
you can do with salinity is make a "correction" to the depth
calculation that you can get wrong.

So in my not-so-humble opinion, a dive computer should always just
show depth as "salt water equivalent depth", with no way to screw up,
and no complications. If you dive in a lake, you'll get the depth
wrong by a few percent, but nobody cares since it's not a really
meaningful value anyway, it's just a user-friendly approximation for
the value that matters: pressure.

Giving the users just the absolute ambient water pressure migth be
*technically* the right thing to do, but it's such a user-hostile
datapoint that it's completely wrongheaded. It only moves the
possibility for error into another place (ie the UI during diving, or
the UI during later logging).

So what you should do is:

 - set your dive computer to salt water (if it has a setting), and
forget about it. Don't ever touch the setting, all it can do is cause
confusion.

 - think of "depth" as "equivalent depth in salt water" and be happy

 - mark your lake dives as such in the dive log tags if you care, the
same way you mark boat dives and buddy names.

The sweet-vs-salt water thing has absolutely zero meaning outside of
informing yourself, and has exactly the same relevance as the name of
your dive buddy: nice to know, but not relevant for any other meaning.
Don't give it any relevance that will only confuse you and get things
wrong.

Because anything else is just a disaster waiting to happen. You *will*
get it wrong, and you will only confuse yourself. Trying to correct
things later is just going to make things worse.

                    Linus

PS. Yes, there are dive computers that don't report depth at all, and
report the actual hydrostatic pressure that their sensor gives. Before
you say "that's what everybody should do", let me just say that (a)
they are a tiny minority and (b) even they aren't consistent (ie is it
absolute pressure, or relative to surface pressure?)

So subsurface logs what the vast majority of dive computers give you:
depth. In fact, we don't even see the pressure, because the conversion
will have been done by libdivecomputer. So as far as we're concerned,
no dive computer gives us pressure, but technically you can get the
water pressure from the Atomic Aquatics Cobatl, from the
Heinrichs-Weicamp OSTC and from the Reefnet Sensus Ultra. (That last
one isn't actually a dive computer, it's just a data logging device.
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