O2 sensor values on CCR dives

[Anton Lundin]

On 10 December, 2017 - Davide DB wrote:

> On 7 December 2017 at 15:12, Jef Driesen <jef at libdivecomputer.org> wrote:
> > So all three have and ADC offset of -1, although there are CCR dives
> > present.
> >
> >> Regarding the question if sensor #2 was voted out. I don't think so. When
> >> one or more sensors are voted out their pO2 value is clearly displayed in
> >> yellow. I never saw something strange going on in water. I guess even the
> >> SD software should report a similar event which is not.
> >> From the Petrel manual I see that a sensor is voted out for a percentage
> >> difference >= 20%. In the last part of my deco I see a max difference of
> >> 10% I think.
> >
> >
> > Well, what I see is the following. The patch I attached in my previous
> > email, logs the millivolt (for each of the 3 sensors) and ppO2 values
> > (respectively the stored average, the calculated average, the delta between
> > the two, and also the 3 ppO2 after the conversion). To make it easier to
> > filter on ppO2 values with a large delta between stored and average, those
> > lines are marked with an extra "Voting" prefix. So you get something like
> > this for each sample:
> >
> > Status 00: gasswitch=0 extppo2=0 setpoint=0 sc=0 oc=0
> > mV: 54 55 54
> > ppO2: 98 114.1 16.100 113.4 115.5 113.4
> > Voting: 98 114.1 16.100 113.4 115.5 113.4
> >
> > (I've attached the full output for your dive.0001.bin file.)
> >
> > In this case there is a delta of 16.1 between the stored avgppO2 value (98)
> > and the calculated value (114.1). But the values of the three sensors are
> > reasonably close together. So that means this delta is clearly not due to a
> > sensor being voted out. They all have the same amount of error from the
> > stored avgppO2 value, and also every sample has this kind of large delta. So
> > this is most likely an error in the conversion from millivolt to ppO2.
> >
> >
> > If you compare this with the deltas I see in some of the other datasets
> > where larger deltas occur, they look different:
> >
> > Status 00: gasswitch=0 extppo2=0 setpoint=0 sc=0 oc=0
> > mV: 68 67 69
> > ppO2: 123 132.2 9.213 121.9 149.3 125.4
> > Voting: 123 132.2 9.213 121.9 149.3 125.4
> >
> > Again a large delta 9.213, but this time we have one ppO2 value (149.3) that
> > is much larger than the other two (121.9 and 125.4). If we assume that
> > sensor got voted out, and calculate the average over the remaining two, then
> > we get an average value (123.65) that is close the the stored avgppo2 (123).
> >
> > This is a sample from the petrel.stevewilliams dataset. And there the voted
> > bit for sensor 1 is indeed zero! I haven't checked all samples, but the few
> > I checked were all similar.
> >
> >
> > Now, the interesting part is that all three datasets show the same problem.
> > What they have in common is not only that ADC offset of -1, but also the
> > calibration value of 2100. Based on the other datasets, that appears to be
> > some kind of factory default value. And the more I think about it, the more
> > I believe that's the value we should be looking at. I assume you have
> > calibrated your sensors at least once, right? In that case I would expect to
> > see some different value from the default. None of the other datasets with
> > CCR dives and extppo2 monitoring enabled have the default calibration value.
> >
> > If I do a simple linear regression (on your dive.0001.bin data) between the
> > millivolt values and the stored avgppo2, then I find these calibration
> > values for each sensor:
> >
> > Sensor 0: 1861
> > Sensor 1: 1765
> > Sensor 2: 1874
> >
> > On average that's a difference with a factor 0.873!
> >
> > Jef
> 
> I calibrate my two Petrels ALWAYS when I assemble my unit as part of
> my checklist and ALWAYS the same day I dive and. No calibration No
> party.
> I made other dives and all of them show the same error. Shearwater
> desktop tell me that everything is ok (like my Petrel underwater)
> while Subsurface reports pO2 over 1.6.
> Thank you for the detailed explanation.
> Where do we go from here?

Is this a divecan unit or something?

I'm just guessing that it might affect how/where the cal factors are stored.


//Anton


-- 
Anton Lundin	+46702-161604