Dive Computer Ramblings

[Linus Torvalds]

On Wed, Sep 10, 2014 at 12:27 AM, Robert Helling <helling at atdotde.de> wrote:
>
> So let’s combine that information: It seems to me what you want is not an
> air integrated computer but in fact a data logger that is in a small box
> that sits on the first stage of your regulator like the transmitters do.

So I don't think that's _crazy_, but you'll find that it has several problems.

For example, high-pressure ports on the first stage are very limited.
Many first stages have just one, in fact. Yes, two is pretty common,
but I don't think I've ever seen three. So to connect something to a
HP port, it needs to add a fair amount of value.

And for the logging to be useful, it needs to be correlated with the
actual dive. And that means that the data logger needs to not have
just a real-time clock, but something else to synchronize with the
dive computer. I'm pretty anal about my clocks, and I keep them fairly
closely synchronized (probably more so than most), but some dive
computer timekeeping is less than wonderful. I notice that my dive
computers drift apart between dive trips, quite a bit. And
realistically, the only thing to synchronize with is having a depth
pressure log too.

So now your electronics need to have a real-time clock, a USB
interface to read the data off and set the clock with, a cylinder
pressure sensor, and depth. The battery to power it all, and the
CPU/memory to do the logging. You're pretty  close to having a dive
computer there. Sure, you're missing the display and the actual deco
calculations.

And the lack of display turns out to be a big thing: if you cannot
read the cylinder pressure data from the logger during the dive, that
really limits you.

Remember the HP port scarcity. If it doesn't have a display, you now
absolutely *have* to have two HP ports. Yeah, I'm sure we all have
that on our primary first stage, but still..  And even with two HP
ports, it means that you've effectively limited it to just a single
port, so now your SPG has no backup, and so you don't even really have
much of a choice - you'll almost certainly have to use an analog SPG.
Yeah, they break too (I've certainly have that happen - people throw
those things around), but my real point is that you're limiting
yourself. You're basically saying "I'll never use a dive computer with
air integration", because I will only use the logger.

And with a display, you'll have a harder time verifying that it's all
working when you're on the boat. Yeah, maybe you can have just a
low-power LED light that turns on when it senses cylinder pressure and
everthing is ok. If it's truly low-power, it's going to be really hard
to see in the tropics during the day. I know, because I've had that
problem: the newer versions of the Suunto wireless pressure
transmitters have exactly that, and they are completely impossible to
see.

So a dive computer with air integration is actually *so* much better.
The integration with the dive computer really solves all these
problems. Thanks to the display, you can see when the logger isn't
working. Thanks to the integration, there's no question about
synchronizing the cylinder pressure with the actual dive. Thanks to
the display, the thing now acts as a backup SPG (or, more likely, your
_primary_ SPG, with an analog backup that you never have to look at).
Or, you might be willing (like I am) to just have two different of
these somewhat unreliable wireless dive computer interfaces - I've had
the wireless fail a lot of times, but I've not so far ever had two
independent and different wireless connections fail at the same dive.
And if that ever were to happen, I'd be ok with it - aborting the dive
is in the end the right thing to do if you really have equipment
issues, I just want to make sure it's really really rare.

So I like the logger idea as a concept, but realistically speaking, I
just like the concept of air integrated computers much more. The
display really turns the logger into so much more. And the
air-integrated computer is, I think, a much larger market. You can
sell air integration to "normal divers", and in fact that is currently
the main market, because so many serious divers scoff at it (for all
the wrong reasons, imho). The existing scuba market is pretty clear
about that market existing and being sustainable. In contrast, a very
specialized logger? It doesn't have the same kind of wide market
appeal.

Good wireless air integration also allows one dive computer to see
multiple wireless transmitters. Yes, it's rare. And yes, the
transmitters tend to be expensive enough that maybe you don't want to
put them on your deco bottles. But I'd like to have the *option*. But
again, with a wireless transmitter, that's possible and more
reasonable with a dive computer than with a display-less logger. So
the dive computer with a wireless cylinder pressure sensor is just a
more flexible and complete solution (that also allows for other usage
scenarios, like seeing your buddy's air, or seeing students' air data
etc etc).

I'm not saying the multi-transmitter case is all that common, but I've
had cases where I would have liked it.

                Linus