Sac Rates
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Really it isn't something to worry about. Your SAC rate is merely a value for consumption that you'll use for gas planning. It will change over time. It reflects many different factors. It isn't something you would want to use to 'rate' yourself in comparison to another diver with a SAC rate higher or lower than yours. Genes play a role, conditioning and comfort play a role, how relaxed you are can play a role, water temp can play a role, age can play a role, ...many different factors. What is 'normal' for you may not be 'normal' for someone else.
The real benefit of what your Vytec is showing you is that you can use that value to calculate gas consumption rates for future dives that will be valid for you, and you alone.
The real benefit of what your Vytec is showing you is that you can use that value to calculate gas consumption rates for future dives that will be valid for you, and you alone.
H2Andy
Contributor
SAC is an expression of your surface air consumption, expressed in pounds per minute or cubic feet per minute.
the lower the number is, the less pounds of air or cubic feet of air per minute you are using.
here's a calculator for figuring out SAC:
http://www.spearfishing.org/bruces_tips/java/sac.html
the lower the number is, the less pounds of air or cubic feet of air per minute you are using.
here's a calculator for figuring out SAC:
http://www.spearfishing.org/bruces_tips/java/sac.html
Sure. SAC stands for Surface Air Consumption, although more properly it might be termed "gas" rather than air, as it will work for nitrox mixes as well. (Consumption may vary for helium mixes (trimix), as helium breathes easier, but lets not get wrapped over that.) Thus, SAC = a ratio of gas consumed over time, expressed in psi gauge per minute.
To do gas planning, divers must determine their personal gas consumption rates. All divers should know their Respiratory Minute Volume (RMV) in cubic feet per minute (cfm). To find RMV, you must first find your SAC. One way is as follows:
1. Descend to a depth of 33 fsw/10 msw and record cylinder pressure on a slate.
2. Swim at a comfortable pace with a relaxed breathing rate for exactly 10 minutes.
3. Document your cylinder pressure at the end of the 10 minutes, and subtract this value from your initial reading.
4. Divide this figure by the depth in atmospheres absolute (ATA) (which, in this case, would be 2).
5. Divide this figure by the swim time (in this case, 10 minutes).
It would be expressed like this:
SAC = Total Gas Consumed (psig/bar) -:- Depth (ATA) x Time (minutes)
If your values were 400 psi consumed in 10 minutes it would be:
SAC = 400psig -:- (2 ATA x 10 minutes) or SAC = 20 psi/min.
[According to Lee Somers (IANTD 2000)] your SAC will vary with cylinder size and working pressure. Consequently, you should convert the SAC to RMV in order to plan dives using different cylinders. To find your RMV you factor in the tanks you're using:
1. Divide the rated cylinder working pressure by the rated volume;
2. Divide your SAC rate by this value (from step 1) to find your RMV.
RMV = SAC -:- [Rated Cylinder Working Pressure -:- Rated Cylinder Volume]
The RMV essentially provides a consumption value that measures "cubic feet per minute". Then, to plan your gas consumption - regardless of which set of doubles you might be using - you divide the total cubic feet of gas at your disposal by your consumption rate in cubic feet per minute to estimate your gas consumption during the dive.
Of course, it all depends on things going smoothly! Anxiety and Stress can do lousy things to most people's gas consumption rates!
The point is that, yes, a lower rate is a 'better' rate, all other things being equal. But it will vary for you over time - depending on how hard you are working down there, how hard the currents may be, how cold it is, etc. Its an estimate. You use your consumption rate to plan dives, not as a 'ranking' of how skilled a diver you are.
Besides, consider a team of 3 divers - the entire team turns when the first diver hits his turn point (e.g. the guy with the highest consumption rate). So...diving as a team, your consumption rate really doesn't matter individually - except as it impacts team planning.
Hope this was helpful.
To do gas planning, divers must determine their personal gas consumption rates. All divers should know their Respiratory Minute Volume (RMV) in cubic feet per minute (cfm). To find RMV, you must first find your SAC. One way is as follows:
1. Descend to a depth of 33 fsw/10 msw and record cylinder pressure on a slate.
2. Swim at a comfortable pace with a relaxed breathing rate for exactly 10 minutes.
3. Document your cylinder pressure at the end of the 10 minutes, and subtract this value from your initial reading.
4. Divide this figure by the depth in atmospheres absolute (ATA) (which, in this case, would be 2).
5. Divide this figure by the swim time (in this case, 10 minutes).
It would be expressed like this:
SAC = Total Gas Consumed (psig/bar) -:- Depth (ATA) x Time (minutes)
If your values were 400 psi consumed in 10 minutes it would be:
SAC = 400psig -:- (2 ATA x 10 minutes) or SAC = 20 psi/min.
[According to Lee Somers (IANTD 2000)] your SAC will vary with cylinder size and working pressure. Consequently, you should convert the SAC to RMV in order to plan dives using different cylinders. To find your RMV you factor in the tanks you're using:
1. Divide the rated cylinder working pressure by the rated volume;
2. Divide your SAC rate by this value (from step 1) to find your RMV.
RMV = SAC -:- [Rated Cylinder Working Pressure -:- Rated Cylinder Volume]
The RMV essentially provides a consumption value that measures "cubic feet per minute". Then, to plan your gas consumption - regardless of which set of doubles you might be using - you divide the total cubic feet of gas at your disposal by your consumption rate in cubic feet per minute to estimate your gas consumption during the dive.
Of course, it all depends on things going smoothly! Anxiety and Stress can do lousy things to most people's gas consumption rates!
The point is that, yes, a lower rate is a 'better' rate, all other things being equal. But it will vary for you over time - depending on how hard you are working down there, how hard the currents may be, how cold it is, etc. Its an estimate. You use your consumption rate to plan dives, not as a 'ranking' of how skilled a diver you are.
Besides, consider a team of 3 divers - the entire team turns when the first diver hits his turn point (e.g. the guy with the highest consumption rate). So...diving as a team, your consumption rate really doesn't matter individually - except as it impacts team planning.
Hope this was helpful.
Well said, Doc Intrepid!
I agree with others. It's not a matter of good or bad but a matter of good or bad for you.
You'll notice that you're sac or RMV is lower on the dives that you're comfortable, confident and relaxed. Those are all good dives and that's your good sac.
You'll notice that you're sac or RMV is lower on the dives that you're comfortable, confident and relaxed. Those are all good dives and that's your good sac.
I wasnt trying to see if mine were better than anyone else's which is why i didnt post the numbers. I was just trying to get a feel for what the numbers actually represented for myself. Maybe some bad wording on my part but thanks for all of the great information.
Hmm, what do you know? My I'd noticed that my SAC was getting alot lower than it used to be... I just thought it was part of getting older.
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