dealing with unused tanks

[Jef Driesen]

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.

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.

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.

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).

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.

Jef