interpolating depth values

Linus Torvalds torvalds at linux-foundation.org
Mon Sep 25 10:33:42 PDT 2017


On Mon, Sep 25, 2017 at 9:25 AM, Lutz Vieweg <lvml at 5t9.de> wrote:
> BTW: It should be fairly easy to try out what your dive computer
> says about negative water depths:
>
> Next time you go into a bath tub or swimming pool take a (big)
> transparent measuring cup with you, and somehow fix your dive
> computer to the bottom of that measuring cup with the display
> readable from the outside.
>
> Completely fill the cup with water, submerse it in the pool/tub,
> turn it such that the opening points downwards, then drag the cup
> above the surface, along with the water and dive computer in it.
>
> Would be interesting to know what different dive computers display
> in that situation.

I agree that you can force a "less than atmospheric pressure".

But 20cm of negative water depth really is a surprisingly big negative
atmospheric pressure. The difference between 1atm and 0.98atm sounds
miniscule, but it really is something like 200 meters of air. You are
not there, but you're literally getting into the order of magnitude
where you would need to care about "altitude exposure after diving"
rules (or put another way: I think people generally consider 300m the
point where you start using altitude dive tables when diving in
lakes).

So I do think that if your dive computer shows a depth of -0.2 m, not
only do you have to be at the surface (because if you're even just one
foot below water, you're now trying to make up for not 0.2m, but 0.5m
of water pressure), you have to be at the surface in fairly extreme
circumstances.

Your "diving in an airtight cave with huge and sudden tidal
differences" may well do it. I do think you'd likely have bigger
issues.

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").

                              Linus


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