On 25-09-17 19:33, Linus Torvalds wrote:
I still think that the most likely reason (by an absolutely _enormous_
margin) is simply calibration issues with the pressure sensor.
All the dive computers I've ever seen basically consider a 1m depth to
be "surface", probably partly because you might be bobbing at the
surface with your arm in the water, but almost certainly also simply
due to pressure sensor calibration issues (which might not be about
the sensor, it could be about simply dropping atmospheric pressure -
going from a high-pressure system to a low-pressure one - but rapidly
falling pressure of that magnitude really is "major storm is here or
imminent").
The dive computers that produce negative depths are the ones that store the
depth as absolute pressure (e.g. reefnet and uwatec/scubapro). In order to
convert to a depth value in meters, you subtract the atmospheric pressure, which
is also recorded with the same sensor. Now, for a common pressure sensor like
the Intersema MS5541C (which is used by many dive computers), the accuracy is
-50 to 20 mbar.
Near the surface you subtract two nearly identical values, each with some error
margin. The result will be close to zero, but not exactly. You end up with
errors in both positive and negative direction with an order of magnitude of the
accuracy of the sensor. So there is nothing special about those small negative
values.
Dive computers that store the converted depth value in meters instead of the
absolute pressure probably clamp the result to zero, and thus you won't see them
there.
I assume most dive computers measure the atmospheric pressure by continuously
sampling their pressure sensor while in surface mode (probably at a much lower
frequency to save battery power). And once they detect a dive has started, they
just use the last measured value (or a few ones ago) as the atmospheric
pressure. In that case there is really no need to start with a surface pressure
at about 1m depth.
Jef
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