SAC (surface air consumption) does not have units defined with it. It
merely means your gas consumption rate normalised to 1 ATA. Just because
the DAN website references SAC in pressure units and RMV in volume units
doesn't make it authoritative.
Expressing gas consumption in pressure units is a very limiting
measurement. It's sort of like saying my average running speed is 2.5
body heights per second. My actual running speed is useless unless you
know my height, and you can't use the same units to compare with someone
else of a different height.
If you use pressure units to measure SAC, your measurement has no
consistency across different tanks. I have never used anything but
liters per minute to calculate my SAC, and have never met anyone who
uses bar/minute.
Think about the dive statistics graph. I can look at my SAC rate (in
liters per minute!) across whatever cross section of dives I've done,
whether I've used an Al80, a steel 10L, twin 7s, twin 12L. Doesn't
matter. They all run on the same scale. Try and do the same thing using
an SAC metric that based on pressure units. It's a meaningless data set!
------ Original Message ------
From "matt wilbur via subsurface" <[email protected]>
To [email protected]
Cc [email protected]
Date 23/08/2024 3:32:52 PM
Subject Re: bug fix - SAC -> RMV
that makes sense. I kind of figured.
but I would be pretty amazed if there were a single credible reference
anywhere (us navy, Dan, dive agencies) that express sac in cuft per
minute. that is RMV. it's not SAC rate. :-)
I can keep doing the maths myself. silly to have an app do it for me :)
matt
---- On Thu, 22 Aug 2024 22:01:39 -0700 [email protected] wrote ----
These terms were discussed quite a few times in the last ten-plus
years. Definitions of the terms are not universally the same, we ended
up picking the one that seemed most useful - surface pressure adjusted
consumption in liters or cubic feet. Doing a tank pressure based
measurement is meaningless unless you always dive with the same tank.
Similarly, we ended up agreeing on the term SAC to indicate that
measurement. And yes, people have complained about that choice before
- but it is what it is and I am extremely resistant to changing it ten
years into the history of it being used...
/D
On Aug 22, 2024, at 20:50, matt wilbur wrote:
the subsurface website indicated requesting bugfixes via the mailing
list is ok.. if not or if this is an old discussion I apologize!!
Currently subsurface says it reports SAC for gas consumption - but
SAC is generally in psi or bar per minute. what subsurface is
showing, labelled as SAC, is respiratory minute volume or RMV,
typically expressed as volume per minute (like 1 cuft/min)
DAN has a great article on the difference here
https://dan.org/alert-diver/article/estimating-your-air-consumption/
I'd LOVE it if we could show sac and rmv (or set as an option) but if
the app just calculated actual SAC it would be awesome!!
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