SAC Rates??

[tombiowami]

Here's one for the masses that I have not seen...
I am interested in Surface Air Consumption rates of our members here. My interest was piqued while reading a Naui book and it used 1cubic foot/min. as a basis. That seemed really high to me. Please include some basics like weight, physical shape, number of dives and/or experience. Thanks.

Mine is usually .54/cu.ft./min though on my last dive that lasted an hour and a half and included a good bit of slow moving time I got it to .49. I weigh about 180 and have 24 dives under my belt. My buddy (similar weight and phys.condition, though with 100s of dives and an instructor) is at .43 and it hit .39 on one of our recent dives. I have a long way to go...


Thanks, Tommy

 

[Rick Murchison]

1 CF/m SAC is indeed pretty high - that's 39 psi/min on an AL 80. I think the beginner's average is more like 25 psi/min (.65 CFM), with an experienced diver's average being down around 18-20 psi/min (about .5CFM). Instructors typically get on down to the .4CFM range, and I once dove with a female instructor in Bonaire who just didn't use any air at all! (Probably less than .3CFM)
I instruct in Alabama and North Florida, though, where we don't do much cold water diving, so my experience may be a bit different that those diving less benign climes.
Rick

 

[Dee]

Rick.....I'm very mathmetically challenged. Could you post the formula you use to figure cf/m SAC rates?

I've been using the SAC rate that my online log book program figures for me but it gives me SAC rate, SAC volume, Volume used and Vol/min. Is cf/min the same as vol/min?

 

[Greg G.]

Useless academics in my humble opinion.

 

[roakey]

Originally posted by Greg G.
Useless academics in my humble opinion.

It's *extremely* important for planning gas consumption on decompression dives.

It has little use in the recreational arena.

Roak

 

[The Chairman]

EXCEPT as a benchmark. I try to hit around .45 and get it consistently. I can tell when I am WAY out of shape by what my sac is. But as I am fond of pointing out... I am in shape, 'cause round is a shape!!!

:tease:

 

[tombiowami]

I am a recreational diver also and find it very important The old axiom of plan your dive and dive your plan to me is centered around having enough of that beloved gas that keeps us alive. On every dive there is some limiter, whether it be nitrogen time, oxygyen tox, depth, dive boat leader, or enough air. In being a good buddy it is needed to know how much air your buddy is using. For his sake because he may come a knockin when he is out, or vice versa if you run out or have a serious malfunction, your buddy is your backup equipment. I want to know how much air is in my tank and in my buddys in general.

Wow, that instructor with a .3, on a shallow reef dive she could pretty much go forever!

I do agree though that it is something fun (in my opinion) that we can talk about since we can't dive all the time, have to work to suport it, eh?

It is also a good measure for me to see how stressed on the dive I was, and is integral in logbook keeping.

So long..

Tommy

 

[sharpenu]

Greg is somewhat correct. Comparing your SAC with others proves little because there are too many variables. Water temp, conditions, current, size of the diver, experience, gender, gas mix and what type of dive (drift, wreck, cave, etc.) all play a role. I personally use it to guage my own fitness and comfort level. Competing against myself causes me to improve more rapidly.

To answer your question, I am a Master Diver, I have been diving for 11 years, I weigh 215 lbs and my SAC is around .52

 

[Rick Murchison]

Dee says:
Rick.....I'm very mathmetically challenged. Could you post the formula you use to figure cf/m SAC rates?
----------
Sure...
A standard AL 80 holds 80 CF at 3100 psi (77.4 CF at its working pressure of 3000 psi). Dividing 3100 psi by 80 CF yields 38.75 psi per CF, so if we're using an AL 80 and have calculated our SAC rate at 21 psi/min, for example, we divide 21 psi/min by 38.75 psi/CF and get .54 CF/min for our Surface Consumption Rate (SCR).

The chief advantage to using CFM vice psi/min [or l/min vice bar/min] is that psi/min is only good for one type tank, where CFM can be used with any size tank(s).

If you prefer, you can also use liters/min by starting with bar/min. In the example above, .54 CFM is equivalent to about 15 l/min.

[For example, if you have an 11 l tank and calculate your SAC at 1.4 bar/min, then your SCR is 1.4X11 or 15.4 l/min]

Rick

 

[Dee]

Thanks Rick. I use the same tanks HP tanks at home but often on a trip have to use alum 80's.