SAC Rates

[triton94949]

I wouldn't get too wrapped around the axle over whether you choose to express SAC in CF/min or PSI/min. Bob is correct that the generally accepted expression is in PSI/min, as that's usually the way it's introduced in OW training.
"RMV," however, is borrowed from the medical community, where it is generally expressed in Liters/min, which in Scuba in the States isn't particularly useful, and so we in Scuba usually use CF/m. Still, there's room for confusion.
Personally, I prefer the term SCR (Surface Consumption Rate) - a purely Scuba term, with the generally accepted expression in CF/min.
Rick

There is always room for confusion!

SCR gets confusing since this also represents Semi-closed Circuit Rebreather.

I use RMV-SCR when I am talking about the respiratory minute volume surface consumption rate, and SCR when I am talking about a Semi Closed Rebreather, and CCR when I am talking about a Closed Circuit Rebreather.

But you are right, its nothing to get wrapped around the axle about! (Funny expression! Denotes "wrapped too tightly" and that normally says it all.)

 

[triton94949]

...Getting back to the original question ... you will ALWAYS consume more gas in colder water than warmer. The reasons are several ...

- you are wearing heavier exposure equipment and more gear, so you work harder
- cold makes your body metabolize more oxygen for heat generation ... so you breathe harder
- dark water causes subconscious changes in your breathing pattern (human nature) which cause you to breathe more rapid and shallow (less efficient)

The last is also why if you track your consumption rate on all your dives, you'll notice that you have a higher SAC rate on deeper dives than on shallower ones. It's not for physical reasons ... it's because your psychology causes you to breathe differently.

... Bob (Grateful Diver)

Excellent explanation!

Spending a few minutes at the surface relaxing before beginning your descent can lower your CO2 initial loading and slow down your breathing. Then you will notice your RMV-SCR improving.

 

[Rick Murchison]

I still did this right didn't I?

Yep.
Rick

 

[donacheson]

Just for funsies . . . a SAC fromula that accounts for fill volume:

(((Pb-Pe) / Pb) x ((Pb / Pw) x V) / T) / ((D/33)+1)

Where:
Pb = Beginning Pressure (SPG)
Pe = Ending Pressure
Pw = Nominal Working Pressure of Tank
V = Nominal Volume of Tank at Nominal Working Pressure
T = Time of Dive in Minutes
D = Average Depth of Dive

Needless to say, the kicker is determining your average depth of dive.

Have fun !!!

Your equation reduces easily to: (((Pb-Pe)/Pw)xV/T))/((D/33+1), which is depends only on the difference in beginning and ending pressures, not the beginning pressure.

Additionally, being able to compute SAC accurately from average depth assumes that SAC is independent of depth - not necessarily true!

 

[The Kraken]

The "Pb/Pw" fraction is in there to clarify the thought process.
The result gives the average SAC rate for the specific dive.
Not exactly rocket science, but it gives the diver a pretty good idea of how the air management aspect of his/her diving is progressing.

Besides that, for me it's just fun to play with the numbers.

God, I've got to get out and dive more !!!!!!!!!!

 

[Rick Murchison]

SCR gets confusing since this also represents Semi-closed Circuit Rebreather.

Well, to remove confusion, inasmuch as "SCR" predates semi-closed circuit rebreathers by (in my experience) at least one and probably two decades, (to my knowledge the Draeger was the first on the open market) and "CCR" means Closed Circuit Rebreather, I vote for "SCCR" for "Semi-Closed Circuit Rebreather" and let "SCR" remain the Surface Consumption Rate it's been since the '70's.
Rick

 

[triton94949]

Well, to remove confusion, inasmuch as "SCR" predates semi-closed circuit rebreathers by (in my experience) at least one and probably two decades, (to my knowledge the Draeger was the first on the open market) and "CCR" means Closed Circuit Rebreather, I vote for "SCCR" for "Semi-Closed Circuit Rebreather" and let "SCR" remain the Surface Consumption Rate it's been since the '70's.
Rick

Thats a good idea, but will it catch hold?

 

[MXGratefulDiver]

Or ... as I told my AOW student last week ... "I don't care what you call it as long as you understand the concept" ...

... Bob (Grateful Diver)

 

[triton94949]

Or ... as I told my AOW student last week ... "I don't care what you call it as long as you understand the concept" ...

... Bob (Grateful Diver)

I am surprised that it is normally first taught as a psi concept. That makes it tank-specific, and more confusing, in my view. I prefer to teach it as a volume-consumption concept, and adjust for pressure at depth. That takes out the issue of the tank size and simplifies things slightly.

No matter how you teach it, whether from the perspective of SAC psi per minute or RMV cubic volume per minute, math is math, and many students glaze over once you get into the math. You have to tailor the instruction to the student. And for many students, math is simply not their forte. To say nothing of physics!

The SUUNTO dive computers, and others, that give average depth per dive, make the RMV-SCR much easier to determine impirically. This gives you incredible precision as well, and you can track RMV-SCR for every dive, and compare the differences. You soon learn that active dives (lots of kicking etc) give higher consumption rates while inactive dives (hovering and flowing with the current) give lower rates. Since the primary objective of the scuba experience is immersion, then less is definitely more, when it comes to activity versus air/nitrox/trimix supply.

 

[Rick Murchison]

Thats a good idea, but will it catch hold?

Nope... folks are too lazy.
Rick