My definitions (maybe not everyone's):
SAC is first and foremost, "Gas Consumption".
As such, it goes up when you're working hard, and is low when you're relaxing. Since we've got a fixed supply, it's in our interest to minimize our consumption, hence the endless discussions about trim, streamlining, skip breathing and the like.
Since gas consumption volume does not change due to gas type, as long as you're not breathing a hypoxic mixture, then Air or Nitrox or Trimix, it's all "SAC."
Wherever you measure it, it is useful to convert that volume to a Surface Value, so you can extrapolate for depth and compare with your buddy.
Now, the variable practice steps in.
For
me, SAC is the psi drop per minute, from a known tank at a steady level of exertion at a constant depth, which is then converted to a
surface value by dividing by atmospheres. Shearwater does this for you on the fly, so that when I look down at my wrist and see 17psi/min with an AL80
at any depth, I know that I'm exerting more than my usual effort and need to relax a bit if I want my dive to last, or plan on a tank-limited, rather than NDL-limited dive.
Having already gas planned my dive based on an expected SAC, and added any desired reserves, knowing my SAC on the fly helps me predict whether my dive will last, or I need to modify my plan. That (to me) is better than just waiting for some Rock Bottom Pressure to appear on my gauge and know that I have to suddenly start up.
For me to compare SAC with others diving with different tanks (or doubles), it is necessary to convert this from a pressure which varies by tank, to a volume which can be applied to any tank. Using "tank value" (cu ft/psi) I arrive at an RMV (Respiratory Minute Volume) in cfm which I can compare with my buddies. Best example is
@scubadada 's thread
Average Gas Consumption
While at a constant effort, my SAC or RMV
as defined above does not vary with depth, tank gas used obviously does. At 100 feet, my AL80 is exhausted twice as fast as at 33 feet. I presume that this is obvious, since this is the Advanced Forum.
@60plus , if you are willing to work with my definitions, or explain yours, perhaps there is more to discuss. I understand that prior experience obviates the need for formal gas planning for dive #27 of a dive you have made before, perhaps even with light deco. I would dispute that dive planning is unnecessary for ANY diver making his first (or tenth) deep or decompression dive. You have received such pushback because, 1) I think our definitions vary, and 2) yours is just bad advice to a new diver extending his or her skills, no matter what you have personally gotten away with in the past.