SAC & It's Importance?

[sarita75]

Good morning everyone. I read people discussing SAC rates on here and I am a bit curious about what it is and its importance, good rates, etc. I know that SAC means surface air consumption, but beyond that, I am miffed. Anyone?

Thanks and have a great weekend!

 

[Steve50]

http://home.flash.net/~table/gasses/sac.htm

 

[The Kraken]

Sarita,

Your SAC rate can be used to determine the amount of time you can dive on a given tank/tanks at a given depth.

Let's just say you have an average SAC rate of 0.45 CFM.

If you were diving a 100 cf tank at 66' (3 ATA) you could get a maximum of 74 minutes dive time . . . 100 cf / (0.45 SAC x 3 ATA) . . . 100/(.45 x 3) . . . 100/1.35 = 74+.

Or if you were doing a 100' dive (~ 4 ATA) you would get about 55 minutes of gas time.

the K

 

[Daner]

I am no way an expert on SAC rate but wouldn't you want to measure SAC in cubic feet per minute instead of pounds?

Edit: In reference to the link in the first response.

 

[The Kraken]

Daner,

Yeah, for some reason this always generates some argument.

Some people like to express consumption in PSI, others Cubic Feet per Minute.

All I know is that when I did a treadmill tests a long time ago the doctors measured our surface air consumption in cubic feet per minute.

the K

 

[sarita75]

Is there any range that a diver should shoot for in terms of SAC rate? I have been a bit concerned about my air consumption on my last few dives, and would like to explore that a bit.

 

[The Kraken]

Well, one doesn't really "shoot" for a specific SAC rate.

One's SAC rate will decrease over time with experience, dives, comfort levels and so forth.

One's SAC rate is predicated upon numerous variables, one of which, and not the least, is the size of the individual.

Others being, type of suit one is wearing, weight of the body, temperature of the water, currents, whether or not one smokes, streamlining of the rigs . . . . ad infinitum.

the K

 

[Steve50]

I am no way an expert on SAC rate but wouldn't you want to measure SAC in cubic feet per minute instead of pounds?

Edit: In reference to the link in the first response.

Yes, as PSI/min would vary with tank size.

 

[crpntr133]

Good question sarita75. I think Kraken has pretty much answered your question. To give you some idea of how far a SAC rate can vary here are a few.

Dry, with lp85, diving with someone that is diving wet in 48 degree water, 1.0
Dry with lp-85 diving with someone that is also dry in 40 degree water 0.8
Wet, 3mm farmer john, lp-85 in 74 degree water 0.5

None of these were stressful dives. If anything during the wet dive I would have been more nervous since it was my first time to 135'.
I am sure that the dry dives are a bit high on the sac rate but with time I am sure that they will come down a bit.

 

[ClayJar]

Well, one doesn't really "shoot" for a specific SAC rate.

I would second this comment. If you try to "breathe less", you'll likely end up with a CO₂ headache (or worse).

SAC is a nice measure to have, as long as you never worry about it. For example, mine showed quite clearly the process of getting used to a new backplate and wing -- it shot up significantly on the first dive, and by about four dives on, it was back to showing my normal relaxed rate. I could then correlate that with what I was feeling on the dive.

Watching your SAC is like marking your height on the wall. You can be uncomfortable and stretch to try to get a taller mark, but that's missing the point. Just relax, improve your diving, and watch the marks change.