dealing with unused tanks

[Dirk Hohndel]

Jef Driesen <jefdriesen at telenet.be> writes:
> On 28-12-12 09:20, Jacco van Koll wrote:
>> I would like you to remember that there are 1st stages, who 'leak'  air/gas
>> on purpose. The longer the dive is, the more air/gas leaked away. So, in
>> certain circumstances, this could lead to measurable differences in
>> pressure, letting software think that the tank is used.
>
> The gas mixes and tank pressures are two different things. At the 
> libdivecomputer level we treat them as completely independent because we usually 
> don't have enough information to match gas mixes with the correct tank. For 
> example if you have an air integrated model, you usually get tank pressures for 
> just one tank (because there is only one pressure sensor). But the device can 
> still support multiple gas mixes, and report gas switches.

This is an area that Linus and I discussed yesterday while driving to
our class... right now our code muddles that distinctions a tiny bit
(and that's mostly my fault as I made some assumptions based on the way
the Uemis SDA handles gas switched). But what Jef says is correct and is
the way to think about it:
a) you have an active gas. and you switch between active gasses. And all
the gas mixtures that are active at some point are the set of "used
gases" (for some reasonably definition of "gas used on a dive").
b) you have tank pressure information. On most dive computers you have
none (not air integrated or no pressure sensor connected) or data for
one tank only, even if the dive computer supports more tanks. That's the
case with the air integrated Suuntos for example. Or the Atomics
Cobalt. So you only ever have tank pressure information for one gas,
even if you switch away from that tank to a different tank (or, even if
you never switch to that tank in case you have the pressure sensor
connected to one of your stage tanks). The Uemis SDA is one of the
exceptions as it can indeed reed from three wireless transmitters and
can track pressures for the three different gases that it supports -
assuming you have three transmitters. I am still waiting to get at least
a second one so I get a better idea of what data it actually stores in
that case...

> Dirk and I were talking about the gas mixes here. In your scenario there might 
> be small changes in the tank pressure (assuming their is a pressure sensor for 
> that tank), but no gas switches because you didn't really "use" that tank. So 
> for the gas mixes, that particular mix would be considered unused.

Yep.

> If you dig into the details, there are some devices that have a number of gas 
> mixes defined, together with a flag to mark a gas active/inactive (e.g. can be 
> switched to during the dive). Currently libdivecomputer doesn't take into 
> account such flags. This is in fact one reason why libdivecomputer returns bogus 
> percentages, because some devices uses an invalid percentage (for example a 0xFF 
> byte) to mark a mix as inactive.

And some of them don't record this in the downloaded data at all. The
current firmware of the OSTC is an example for that (but based on
discussions with Jef and me, H&W may fix that in a future firmware
version).

> But even if a gas mix is marked active, it's not necessary used (as in switched 
> to that gas) during the dive. That can be because the device always stores the 
> full set of gas mixes, regardless of whether they are actually used or not (this 
> is very common and almost all devices work like this), or it might be that the 
> diver carried a tank with that gas, but never switched to it. The problem is 
> that we can't tell the difference between those two cases. If we decide to only 
> report mixes that are really used during the dive (because there are gas 
> switches for it), then that works well for the first case, but not the second 
> case. In the second case the unused gas mix would get dropped. The question is 
> now whether this is acceptable or not? If I'm not mistaken, this is exactly the 
> issue Dirk wanted to raise too (@Dirk: correct me if I'm wrong on this).

Absolutely correct.

My logic here is that this will be correct for the vast majority of
cases; I would bet that there are a thousand times more recreational
divers that have a computer that reports gases that they may have
configured when playing around with their computer, or that may even be
configured by default, but who will never ever dive with more tank, than
there are tec divers who carry an extra stage tank that for some reason
they never switch to. This of course doesn't take CCR divers into
account (who bring bail out tanks to every dive and hope never to have
to switch to those except for training purposes). But again, the ratio
of recreational divers to CCR divers is quite huge...

> I believe this isn't a major problem, because I assume the gas mixes that are 
> actually used during the dive are the most important ones.

I completely agree. So this is the direction I think I want to go with
that...

The question is: should I simply rely on libdivecomputer doing this, or
should Subsurface do that itself?

/D