dealing with unused tanks

Dirk Hohndel dirk at hohndel.org
Fri Dec 28 22:57:57 PST 2012


Jef Driesen <jefdriesen at telenet.be> writes:
> I believe some Oceanics even have a feature to monitor tank pressure from your 
> buddy. In that case the tank pressure has no relation with your gas mixes at 
> all. I don't think I have ever seen data from such a device.

The Uemis can do that, too, but looking at their file format I don't
think they store that data. Once I finally have the second tank sensor
Linus and I can test the various scenarios...

> For the multiple pressure sensors, I think most devices only record a single 
> tank pressure value in the samples. Thus you only get data for the active tank. 

That is certainly true for the Uemis. If you switch gases (and have only
one sensor), then it gives you 0 pressure readings if you switch away
from the tank with the sensor.

> If you switch tanks, you no longer get any pressure data from the previous tank. 
> Of course there might be devices that store pressure data from all sensors 
> simultaneously, but I don't think I have ever seen that so far.

I'm sure that will come. With flash being so cheap and at least some
vendors switching to reasonably fast transfer methods...

> It makes sense to record the tank pressure this way. In practice there are very 
> few cases where you are actually using multiple tanks at the same time (e.g. 
> buddy breathing).

True.

>> 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?
>
> Well, I think the active/inactive flag (if available) should definitely be taken 
> care of at the libdivecomputer level. I'm pretty sure everyone will agree on that.
>
> The used/unused part is another story. Jan already pointed out a legitimate 
> use-case where dropping unused gases would be undesirable. If we move the 
> used/unused logic into libdivecomputer, then unused gases are permanently 
> dropped, and there is no way to get them back. If it's done at the application 
> level, then the user or applications still has a choice here.

Very good point. I think it would be better to have that as an
application choice (as Jan's case clearly shows). So what I think would
make sense is this:

if you have active/inactive information, libdivecomputer could drop
inactive tanks (unless they were switched to during the dive - which
means that while they may have been inactive at the end of the dive,
they were active at some point during the dive), but should report all
other tanks, regardless of whether they were used or not. And Subsurface
can then have a settings "show all defined gases / only used gases"
(with the default being "only used gases" and the whole thing hidden in
the tec preferences.

> So I'm not sure what's going to best way. Maybe we can treat CC dives different 
> here? I guess OC tec divers are less likely to carry "spare" mixes, only extra 
> gas volume for emergency situations. But I could be wrong on that.

No, I think we should treat CC and OC the same here.

/D


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