cuft_to_l

[Lutz Vieweg]

On 08/01/2012 01:19 AM, Linus Torvalds wrote:
> Yeah, I don't know how to adjust for anything like that.  I just accept
> the fact that most of my dive profiles show red for the momentary SAC
> rate at the beginning of a dive. How much of that is because the tank
> cools down and thus the pressure goes down even if I weren't
> breathing, and how much of that is because I actually *do* use more
> air at the beginning of the dive for things like initial buoyancy
> adjustment and just calming down? Who knows?

Yes, it is probably not worth the effort trying to adjust for the
cooling, as the probably most influential observable in the calculation
would be the temperature of the gas before the dive, and that is
not usually measured. (Even if the dive computer does measure temperature,
it is likely to measure something very different from the gas temperature,
either the skin surface temperature of the person wearing the D*,
or some environment temperature where dive gear is stored - away
from the tanks that may well be exposed to the sun etc.

One thing that could be done: Disregarding those samples for
"average dive SAC computations" that were measured before
any available temperature measurement settled. But even that
is flawed in that you don't know how quickly the DC cools down
in comparison to the gas in the tanks.


> Anyway, the confusion about non-metric tank sizing is absolutely so
> big that it's not even funny. [...]

Incredible, I didn't know there was _that_ much confusion.

> The Uemis
> dive computer, for example, can be set for cuft. And if I recall
> correctly, it then does the SAC calculations (and exports cylinder
> sizes in liter) using a fixed 200 bar working pressure (which is
> pretty close to right for the normal AL80 model that has a 3000 psi
> working pressure), but it's just incredibly confused.

Well, Uemis went out of business already once, so I wonder whether
anyone will ever fix this in their firmware.
I wonder whether they still allow downloading of dive data only via their online
service, which is really a complete no-go when you are away on a boat for
weeks or months in regions where you might not have network access at all.
(And their DCs, while pretty colorful, were oversized, and consuming
way too much power for their small battery.)

Regards,

Lutz Vieweg