RMV math…
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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.
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.
Surely you are aware that one must take into account at least two things on all dives, namely your nitrogen status (via NDL, for example) and gas status (do I have enough to finish this dive?).
Your example is about nitrogen status. Fine, glad you are doing that.
But what about your gas status? Why does that seem to be less important to you than your nitrogen status? After all, DCS is rarely fatal. Running out of gas is almost always fatal.
Perhaps you could present a scenario more to the point, one in which gas usage and sufficiency are important?
I think even on a NDL dive, it can be useful to an extent to know your SAC as you could happen to choose a different cylinder size so you can stay the time you want.
Also in some places you need to return to the shot line meaning that your gas planning need to make sure you have enough gas for you and your buddy’s SAC rate
(that’s the case if you dive close to a shipping lane)
Also in some places you need to return to the shot line meaning that your gas planning need to make sure you have enough gas for you and your buddy’s SAC rate
(that’s the case if you dive close to a shipping lane)
LOL. He will respond that he always uses the same cylinder, and that his dives are never near shipping lanes.I think even on a NDL dive, it can be useful to an extent to know your SAC as you could happen to choose a different cylinder size so you can stay the time you want.
Also in some places you need to return to the shot line meaning that your gas planning need to make sure you have enough gas for you and your buddy’s SAC rate
He is NOT planning a dive; he is repeating a dive he has already done. Then he will give some example that is off topic....
I'm really glad this thread is no longer in Basic; it is hard to imagine worse advice to a new diver.
EFX
Contributor
You know, after being a regular on SB for a number of years we seem to always have to define SAC. I wish we and the diving industry would change the acronyms to SACP for surface air consumption pressure and SACV for surface air consumption volume. RMV is not intuitive.
Belzelbub
Contributor
True. However, regardless of the term, if the units are included, it becomes pretty apparent which we are talking about. But, it's just been either Pressure or Volume. In all the years I've been familiar with SAC (RMV is relatively newer for me), it's never been referred to as a diver constant until earlier in this thread.
L13
Contributor
If units are given it is easy. Even without, you can usually determine which it is:
If you are in a metric using region of the world, it is most likely to be SAC(V).
SAC(P) should always have a cylinder size specified, even if units aren't. (but you can't count on this since people are lazy)
In a metric world, if the number is less than ~10 it is probably SAC(P) (though metric folks don't usually use it), otherwise it is probably SAC(V).
In a non-metric world, the rule is reversed. If the nmber is less than ~10 (usually <1) it is probably SAC(V), otherwise it is probably SAC(P).
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