How to figure your SAC.

[Dark Wolf]

This is probably a kinda lame question for some of you more experienced divers but here goes.

I have been reading about Surface Air Consumtion Rates, but the book that I have does not go into any detail about how to figure it.

Could anyone help me with this? Any help would be much appreciated.

Thanks,

Dark Wolf

 

[cbborromeo]

SAC Rate = (DCR x 33) / (Depth + 33)

Check these sites out for more information:

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

[SIZE=-1]www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf[/SIZE]

 

[fppf]

Ok
First take the working tank PSI and divide that by the tank volume in Cubic Feet.
This gives you how many PSI per cuibc feet there are.

Then take your start pressure and your end pressure and subtract them, getting how many PSI you used. Then divide that by the PSI per cubic foot number.
This gives you how many cubic feet you used.

Then take your average depth and divide that by 33 and add 1 to it. This will give you avg atmospheric pressure in atmospheres absolute. Most guys I know use half there max depth as an approximation. If you use a computer it can tell you what your true avg depth is.

Take your cubic feet used and divide it by your atmospheric pressure to get how many cubic feet you would have used if on the surface. Then divide that number by your dive time in minutes.

End result is surface cubic feet per minute, SAC.

As always your milage may vary.
Remember that SAC rates are just rough numbers to give you an idea. It will change given the conditions of the dive, currents, stress, depth, and so on.

I would not use the first link posted above becuase that is just how many PSI you would use. If you ever use a different volume or pressure tank your numbers will NOT work.
The seconded link post is more true but SAC should be in Cubic Feet per minute, or some volume per minute, not pressure.

 

[Dark Wolf]

Great, thank you both for answering my question! I appreciate it greatly.

Dark Wolf

 

[spectrum]

First of all there are several ways to get at a value.

The tricky piece of data is your average depth which can come from 2 basic methods.

If you are diving with a computer you can just take the average depth reading along with the runtime of the dive. This will provide a comprehensive value based on all of the dives activities. You will then need your starting and ending tank pressures, more on that latter.

If you do not have a computer then you need to get to a location where you can swim at a consistent depth. With your pressure gauge on a clearly read value, like 2500PSI swim at a steady depth at a typical rate and log how long it takes to get down to another clearly read pressure like, 2000.

Before we go on let's take a look at your cylinder. I'll assume an AL80 which holds 77.4 cubic feet of air at a rated pressure of 3000 PSI. Division tells us that each 38.75 PSI represents a cubic foot of air.

Now lets assume that in either method you were at 33 feet for easy figuring.

Using the no computer method lets' say that you swam for 10 minutes while you went from 2500 to 2000 PSI. 500 PSI divided by 38.75 PSI/CF = 12.9 cubic feet of air was used.

You dove for 10 minutes so 12.9 /10 = 1.29 cubic feet per minute were used.

You were at 2 atmospheres of depth, that is 33 feet plus the surface so 1.29 /2 = .64

Your SCR was .64. You can apply that to a future depth to predict air usage per minute and calculate that and other profile segments against cylinder contents (not simply capacity) to plan future dives.

If you use the computer method just use the starting and ending pressures, runtime and average depth such as:

Starting with 3000 PSI and ending with 1500 PSI you used 1500 PSI

1500 PSI / 38.75 PSI/CF = 38.70 cubic feet of air

With a runtime of 30 minutes you used 1.29 cubic feet per minute

Since at 33 feet you were breathing at 2 atmospheres 1.29 / 2 = a SAC of .64

Pete

 

[ron63]

Okay, I just tried computing my SAC rate for my last dive. If my numbers are correct and I did everything right my SAC rate came out at .52. Is this a good number or a bad number?

Ron Shephard

 

[spectrum]

.52 is a fairly typical male.
New divers often start out in the .75 range
When gas planning for a panicked diver 1.0 is commonly used.

My personal best is .36.
Cold water diving wet is around .45
In my drysuit a little over .50

Pete

 

[H2Andy]

Okay, I just tried computing my SAC rate for my last dive. If my numbers are correct and I did everything right my SAC rate came out at .52. Is this a good number or a bad number?

Ron Shephard

that's good, about average, i would say

i'd be happy with anything below .6

anything above 1.0 sucks big beach balls

be aware that your SAC will increase with effort and stress, so take that into consideration

(notice how i cleverly avoid revealing my horrible .78 ex-smoker's SAC)

 

[ron63]

Thanks Pete and Andy. I think I will spend a little more time in the pool and condition a little more. See if that brings down my numbers a bit. My wife's SAC must be around .35 or so. She thinks I'm an air hog!

Ron Shephard

 

[H2Andy]

women kick our butts, dude

smaller lungs, slower metabolisms

ack...

one of my regular buddies has a .42 SAC ... blast her