SAC terminology

Tommi Saviranta wnd at
Thu Feb 13 08:33:25 UTC 2014

On Thu, Feb 13, 2014 at 16:58:02 +0200, Willem Ferguson wrote:
> I am doing a technical qualification with IANTD and they use SAC as
> the rate of decrease per minute in the pressure in the gas cylinder
> (units=bar/minute). -- [SAC/RMV] Is this something worth worrying
> about for the SAC rate given on the dive info panel?

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.)

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. That said, I've learnt to measure my
(surface) gas consumption in litres/minute. In Finnish/local scuba
jargon we often refer to this with "pintakulutus", which nicely
translates to "surface consumption". In fact the first time I heard of
RMV (and using pressure per time) was just six months ago.

When I was going through IANTD's material I found that having gas
consumption in pressure/time caused me lots of unnecessary work. Apart
from trivial calculations in which using psi/minute required only a
single division, I always ended up calculating everything in
volume/time. That was obviously because of my fixation with methods I
was familiar with, but IIRC also because that allowed me to avoid using
IANTD's comprehensive collection to conversion and gas matching tables.
Besides, with proper units, the calculations were actually very simple.

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