Issues with SAC terminology

[James Cialdea]

On Tue, Jul 18, 2017 at 2:14 PM, Dirk Hohndel <dirk at hohndel.org> wrote:

> Remember, we always show the unit (l/min or cuft/min), so it's clear that SAC isn't a pressure drop.

Side note - In the mobile app, I see only "cuft", not "cuft/min".
Clearly, the numbers being shown are "cuft/min" - I definitely consume
more than .47cuft of air on a 49min at 60ft OC dive. :)

I agree that the terms are a bit fuzzy in their colloquial usage, but
I do not recall SAC being used in any documentation referring to
volume per time.  Sure, people talk about "consumption" and sometimes
improperly use the term "SAC", but people also use "good" where they
should use "well" - it doesn't make it right.  I was surprised to see
"SAC" associated with volume per time when I started using Subsurface,
but the displayed unit does make it clear what is being discussed (at
least on the desktop application).

The way SAC has been explained to me is that it is tank specific, but
depth adjusted.  This makes it useful for "how much time can I spend
at X depth with Y tank?" but not for "was I breathing hard?"  For that
question, I've used the RMV (Respiratory Minute Volume) term, the
calculation of which is neither tank nor depth specific, as it uses a
volume that is normalized to the ambient pressure at depth.  I've also
heard RMV called SCR (Surface Consumption Rate) or SACR (Surface Air
Consumption Rate or SAC Rate).

For reference, the SSI definition of SAC (imperial units) is:

SAC = [( PSI / T ) * 33ft] / [D + 33ft]
where:
PSI = Gas consumption (PSI)
T = Time (min)
D = Depth (ft)
SAC => PSI/min

This neat NAUI calculator is using a similar formula, but I couldn't
find an official reference:
http://www.scuba.com/scuba-gear-120/058048/NAUI-Scuba-Dive-Calculator-Surface-Air-Consumption.html

I wasn't able to find an official reference for PADI, either.

Here's an SDI/TDI article using the same formula:
https://www.tdisdi.com/air-management-and-the-importance-of-learning-surface-air-consumption-rate-sac/

Those bodies aside, Google searches for calculating SAC seems to line
up with the formula above (and its metric equivalent).

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