cuft_to_l

[Linus Torvalds]

On Tue, Jul 31, 2012 at 3:51 PM, Lutz Vieweg <lvml at 5t9.de> wrote:
>
>> (What is *not* very well specified is what "pressure" means.
>
> I guess the next biggest inaccuracy regarding SAC measurements
> would result from the cooling of the tanks that usually happens at the
> start of a dive when (usually warmer) tanks get cooled by (usually colder)
> water. But that's probably not worth trying to adjust for, too.

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?

The color coding of the air use by SAC rate is still interesting. I
can definitely see "Oh, there I had to actually start swimming" spikes
too, so whatever effect the tank cooling has, it's at least not always
the dominant effect ;)

Anyway, the confusion about non-metric tank sizing is absolutely so
big that it's not even funny. Even if the cylinder is a "HP100", and
thus generally should be 100 cuft of air at 3442psi, in practice the
actual details between manufacturers differ. If you go and download
the actual spec sheets from the manufacturer, you'll find that the
tank is probably actually really 97 cuft or whatever.

And that doesn't even *begin* to address the confusion about "plus
rating". Because generally manufacturers specify the tank size using
the plus-rated (ie 10% higher than actual spec working pressure)
pressure, because that way the tank obviously looks bigger. In fact,
they pretty much always seem to do it for LP cylinders (2400 psi with
plus-rating means that the size is computed using 2640 psi), and for
HP cylinders they may use 3442 psi or 3500 psi or whatever.

Never mind that the size is then "rounded" anyway (where "rounding"
may not be to a really round number, but rather to a common size).

If I recall correctly, the HP119 cylinder that Dirk usually dives is
the same size as a LP85, and when we checked the manufacturer specs it
was actually something like 122 cuft at the rated pressure of 3442
psi.

The take-away from this all: cylinder sizes in cubic feet should
always be considered approximate. You might be off by 10-15% due to
rounding and the whole "is it for the plus-rated pressure or not"
issue. Which is why I decided I didn't care in the least about the
0.5% error "is the working pressure absolute or gauge pressure" kinds
of details.

So it's a complete mess. The european wet size is much simpler and
obviously doesn't have the "at what pressure?" issue.

But I'm pretty sure subsurface gets these things generally correct.

Which is a *lot* more than I can say for anything else I've seen. The
amount of confusion I've seen about cylinder sizes from both dive log
software and actual dive computers is absolutely horrendous. 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.

Other dive log software seems to get really confused in this area.

                          Linus