On Mon, Jun 30, 2014 at 2:20 AM, Jef Driesen jef@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