On 2014-05-20 09:29, Dirk Hohndel wrote:
Correct. And and important factor here is "how do dive computers report
this". I think Jef commented on this a little while ago - no one
appears
to have seen a dive computer that actually can report data on two
cylinders. Do you have data from CCR divers that shows how this is
done?
A small correction: a dive computer that actually can report data on two
or more cylinders *simultaneously* per sample. I know that's what you
meant, but others people may miss this subtle but very important detail
:-)
Please explain more what you think could destroy this relationship?
Right
now we have the frustrating reality that most dive computers cannot
deal
with multiple cylinders using the same gas (and some don't even allow
you
to enter more than one cylinder with the same gas) because they don't
report switches between cylinders but switches between gases. We
adopted
that model from libdivecomputer and therefore have some flexibility
regarding the association of a pressure graph with the corresponding
cylinder, but I would actually like to modify this specifically in
order
for people to have more than one cylinder with the same gas and to
switch
between them (this is a common request we get from side mount divers,
for
example). So whatever new design we come up with needs to keep that
direction in mind as well.
For a dive computer it makes sense to manage the tank information
separately from the gas mixes. Obviously, from a practical point of view
each gas mix needs to be stored in some tank. But for a dive computer,
that's pretty much irrelevant in most cases.
First of all, for the decompression calculations, only the available gas
mixes really matter. The tank sensor is basically only used to show the
tank pressure and estimate your remaining airtime. In theory a dive
computer could use it to influence the decompression to some extend
(e.g. automatically propose a gas change when a tank is almost empty),
but I doubt this is something that is done in practice.
Last but not least, many dive computers do not have any air integration
at all. And the ones that do, do not necessary have a tank sensor
attached to all the tanks you're carrying. For example a hosed dive
computer (Cobalt, Cobra, etc) can support only a single tank pressure
(e.g. the one connect to the cable), although many of them do support
more than one gas mix. The opposite is also possible. There are systems
that support monitoring the tank pressure of your buddy. In that case
you have tank pressure, but no gas mix data. And it's not difficult to
imagine other scenario's...
The important message here is that there is not always a nice one to one
mapping between tanks and gas mixes. Logically this mapping always
exists, but in practice the dive computer simply doesn't provide us
enough data to figure this out correctly. And you can't really blame the
dive computer, because it simply doesn't have (or need) this kind of
data.
That's why libdivecomputer doesn't even try to do this. You can probably
get some of the common and easy cases right (e.g. one tank and one gas
mix), but everything else is very tricky. The only one that can do this
correctly is the end-user.
Jef
_______________________________________________
subsurface mailing list
[email protected]
http://lists.hohndel.org/cgi-bin/mailman/listinfo/subsurface
