O2 sensor values on CCR dives

Davide DB dbdavide at gmail.com
Fri Dec 1 06:57:12 PST 2017

On 30 November 2017 at 13:08, Jef Driesen <jef at libdivecomputer.org> wrote:
> On windows (and linux too), I already have a build of dctool available which
> uses native bluetooth communication. This should be a bit more reliable then
> the serial emulation. (On the Petrel, I've noticed that a low battery also
> causes download issues, probably because the bluetooth chip draws more
> power.) The bluetooth build is available here:
> http://libdivecomputer.org/builds/experimental/windows/dctool-bluetooth.exe
> Instead of the serial port, you need to pass the bluetooth mac address as a
> hexadecimal number (e.g. prefix with 0x, and remove the colons).

Ok, I will try it if needed. Generally 70% of my Petrel battery is
drawn for BT download.

>> I haven't gotten around to it. Jef might be interested in looking at it
>> to, so I'll ping him.
> Do we have some ground truth to compare against? Last time I checked
> Shearwater Desktop didn't show the ppO2 values. And if they do, do they show
> the average ppO2 or the individual sensors?

Yes, what do you need exactly?
I this thread I shared the dctool raw data file and some screenshots
of Shearwater desktop and Subsurface of the same dive.
I can share the Subsurface xml file and maybe an export from
shearwater desktop if exist.
Eventually I will send you everything off list.

> The average ppO2 is stored directly in the data, so that shouldn't match
> perfectly. But the individual sensors is another story. In the data there is
> only the millivolt, and libdivecomputer converts this into a ppO2 value
> based on the calibration values. This conversion is reverse engineered, so a
> mistake there is not impossible. Note that libdivecomputer no longer reports
> the average ppO2, only the individual sensors.
> Is the resulting ppO2 wrong for the entire dive, or only during the deco
> stop? The calibration does a simple multiplication with a constant value, so
> if there is something wrong there, I would expect to see the same kind of
> error throughout the entire dive.

Yes, it's wrong for the entire dive. I noticed the error looking at
the deco part just because Subsurface signalled me being at over 1.6
for all the 6 meter stop. But looking at O2 values in the deep part,
Subsurface seems to report always 0.1, 0.2 more than the real value.



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