There's clear ambiguity in the definitions. You say SAC doesn't have
units defined with it, but I can find references that show volume / min
(my PADI self reliant instructor guide) and many more, like from TDI
(and on my shearwater) that express it as PSI/min.
PSI/min has some value- how much air do i gulp in a dive :) If I dive
the same HP100s (I do), its a very useful number.
It's cool if you want to say pressure/min has no value- but clearly
other people besides me disagree with you :)
I'd recommend there be a setting in "Tech setup" that lets us decide how
"SAC Rate" is expressed. The math is easy.
Matt
On 8/23/24 12:32 AM, Mark Stiebel wrote:
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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