Fwd: Re: [PATCH] Fix up SAC calculations for ATM/bar confusion

[Lutz Vieweg]

On 02/25/2013 10:04 PM, Dirk Hohndel wrote:
> So let's assume we take whatever temperature reading we have at the time
> during the dive into account, plus the non-linearity of compression. And
> the mixture of gas in the tank. Is that all it takes to get the correct
> volume of gas inside the tank?

I guess that's the kind of question a physicist could never answer with
a final "yes" - but the residual error between the model and reality
may be very small.

Examples for further theoretical sources of errors:

- We cannot know whether the middle pressure stays the same all the
   time, thus some gas may or may not be lost for breathing in hoses
   and the like.

- As SAC is relative to "surface pressure", we would have to know
   that surface pressure, but weather influences alone would render
   this futile.

- We don't know how rigid the cylinder walls are. They will bend
   outward due to the pressure of the gas, and may bend inward
   due to the outside water pressure while descending. Thus the
   cylinder volume is not constant.

- We don't know the orientation of the cylinder and the location of
   the pressure sensor - so when turning upside down during the dive,
   the pressure readings will change slightly. (Subject to
   the not precisely known strength of the local gravitation field,
   of course.)

- Some gas may dissolve in (and possibly later evaporate from)
   the cylinder material.

- A small part of the gas molecules will consist of unstable isotopes
   that happen to break apart just during the dive.

- We don't know for sure yet how the expansion of the universe will
   influence the volume of our cylinder or the gas molecules.
   And oh, if there is really "dark matter" or "dark energy" existing...

- Some prominent therories claim that protons are not stable.
   Depending on the length of the dive, some could vanish and
   steal some valuable gas molecules. Luckily, this will only have
   relevant effects for dives in the range of 10^60 years or so...

There's certainly a lot of effects I forgot to mention. :-)

Regards,

Lutz Vieweg