SAC terminology
Linus Torvalds
torvalds at linux-foundation.org
Thu Feb 13 09:50:23 UTC 2014
On Thu, Feb 13, 2014 at 8:33 AM, Tommi Saviranta <wnd at iki.fi> wrote:
>
> I'd show the actual units where possible, be it psi/minute,
> litres/minute or something else. With the units visible it's easier to
> understand what the figures mean, even if your understanding of SAC/RMV
> would differ from that of mine. Point being, in my opinnion "SAC: 9.7"
> isn't obvious, but "SAC: 9.7 psi/min" and "SAC: 16 l/min" are. (I hope I
> didn't get my numbers all wrong.)
Imperial is ft³/min, so generally you'd see "0.43 cuft/min" or "12 l/min".
The problem with units is that in some places they can be very
annoying. Notably in the divelist, where we often want to show a lot
of things densely.
But in the info/statistics window I agree with you - show units.
> In a way I prefer RMV because of "minute" and "volume" in it, but I also
> dislike it because it's not immediately obvious if it means surface
> consumption or not. On the other hand SAC is clear about this, but it
> gives no clues about used units.
Some course material I've seen (I think it was the SDI tech one) tried
very hard to make SAC mean the "base breathing volume" - literally
breathing *on* the surface, while sitting still - while they try to
make RMW mean the "SAC equivalent while actually diving". So your
"SAC" is some personal pseudo-constant that doesn't change, and then
you estimate your RMW based on that and the difficulty of the diving
(so on an easy drift-dive, maybe your RMW would be 1.1x SAC, while on
some technical diving in cold water and work it might be 2.0x your
SAC).
But quite frankly, while that might make sense conceptually, it really
doesn't make much sense in practice. Rather than compare your air use
against sitting on a couch, any sane diver will compare it to their
previous dives. Nobody ever says "ok, this is going to be a tough
dive, so I'll use a 2.5x multiplier to my couch-air-use". It may make
sense for teaching, but not for any actual diving.
Besides, I personally refuse to use RMW just because it's a completely
unpronounceable TLA compared to SAC.
Linus
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