Help needed decoding data

[Linus Torvalds]

On Mon, Jun 30, 2014 at 2:20 AM, Jef Driesen <jef at libdivecomputer.org> wrote:
>
>
>> I've extracted what appears to be the mbar
>> increment data bits and graphed them. They closely, but not exactly,
>> follow
>> a dive profile from the vendor software. There is detail missing and the
>> computed depths are a few feet off. I may not have mbar to feet conversion
>> right or there is a calibration I need to consider.
>
> They might take into account atmospheric pressure and/or salinity. If there
> is a fixed absolute different, then there is a different atmospheric
> correction. If it's a fixed scale factor, then that's likely a different
> salinity factor.

Also, how sure are you (John) that the sample data is actually in millibar?

It could easily be in actual depth - in cm. The pressure of one cm of
water is very close to one mbar, so the two are almost
interchangeable, but it would be a difference of a couple of percent -
so a foot or two off depending on depth. And from what I've seen, most
dive computers do tend to sample actual depth, not pressure, if only
because that is what they show on the screen. Logically, "pressure" is
what the sensors give you, and what really matters for deco
calculations, so converting to "depth" is fraught with problems
(surface pressure, salf-vs-fresh, yadda yadda), but it's (a) what
people want to see and (b) in the end, the errors of a percent or two
are not really material, so ..

But yes, as Jef says, it could be any number of other factors too. The
difference between salt/fresh water is a few percent too, and quite
frankly, having seen the mess some dive log software make of things,
just being *wrong* can be a few percent too (ie not actually using
proper calculations at all, ie just using the rough "1 atm is 10 m of
depth", or just mixing up atm and bar). We've had those kinds of
errors too, and judging from what I've seen, we've definitely had
_less_ than some.

                Linus