interpolating depth values

[Jef Driesen]

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