I learned to dive when I was a young man, in the cool emerald waters of Cape Cod. I've noticed though, 20 years later that my SAC has dropped quite a bit. I've been wondering... is it because I'm getting older, or is it because it's so derned hot?
Sac
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Does your SAC go back up in cold water?
Is there anything about your health that might cause a change in your SAC? How often do you measure your SAC?
Is there anything about your health that might cause a change in your SAC? How often do you measure your SAC?
Its the freakin heat! 
My SAC rate is pretty low whether in cold or warm water...but in hotter waters its real low as if I am dozing--lol.
My SAC rate is pretty low whether in cold or warm water...but in hotter waters its real low as if I am dozing--lol.
This will give you a quick idea.
"Surface Air Consumption Rate........SAC 101
The compressibility of gasses is also an important consideration for divers due to its affect on how long a diver can stay underwater. Scuba regulators are designed to deliver air to a diver at the same pressure as the surrounding water pressure, at ambient pressure. That means that when a diver fills his lungs at a depth of 33 feet, he is taking in the equivalent amount of air as two breaths at the surface. Obviously then, a tank will only last half as long at 33 feet as it would at the surface. And tank that would last 1 hour at the surface would only last 1/3 as long, or 20 minutes, at a depth of 66 feet, etc.
It can be beneficial to be able to estimate how long a scuba tank might last at a given depth when dive planning. To determine this, it is first necessary to determine a divers Surface Air Consumption (SAC) rate. For example, if you are diving at 33 feet, and use 500 lbs of air in 10 minutes, it is easy to determine that you are using 50 lbs per minute. This is only true for this depth however. How much air would you use at 66 feet, or 99 feet?
The first thing we must do is calculate SAC rate. In the preceding example, a diver using 50 pounds per minute at a depth, would use 25 pounds per minute at the surface. His surface air consumption rate is 25 pounds per minute. With our SAC number of 25, it is easy to calculate our consumption rate for depths of 33, 66, 99 feet etc. We know we are under 3 times the pressure at 66 feet, so we would use 3 times as much air, or 75 pounds per minute at this depth.
The process becomes slightly more complex if depth consumption rate (DCR) is determined at a depth that is not in even atmospheres. (Not at 33, 66, 99 feet etc.) For this situation we use a formula that is simply an adaptation of Boyle's Law to determine our SAC rate:
SAC Rate = (DCR x 33) / (Depth + 33)
Let's look at an example. Suppose you did a 50 foot dive for 25 minutes and used 1700 pounds of air. This would mean our DCR is 1700/25 or 68 pounds per minute. Using this in our formula we get:
SAC Rate = (68 x 33) / (50+33)
or: SAC Rate = 2244/88 or 25.5 pounds per minute.
We can then turn the equation around to determine our DCR for any depth.
DCR = SAC Rate x (Depth + 33)/33
Let's assume our SAC Rate is 25 and we want to know how fast will we use 2000 pounds of air at a depth of 75 feet.
Dropping our numbers into the equation we get: DCR = 25 x (75 + 33)/33 or DCR = 25 x 108/33 or DCR = 81.81
This means at a depth of 75 feet, we will use 81.81 pounds of air per minute. Dividing this into the 2000 pounds, we see this amount of air would last 24.4 minutes.
It is important to note that SAC Rate takes into account the assumption that you are exerting the same amount of energy at any given depth, and you are using the same size tank as you used when calculating your DCR.
For example, under strenuous diving conditions, you can consume air 4 times faster than when sitting still taking pictures. Also it is obvious that a 50 cubic foot tank would not last as long as an 80 cubic foot tank, even if they were both filled to the same pressure."
Texdiveguy got it right but here are a few more things that should be mentioned.
1. SAC is an exression of air consumption in psi per minute. That is obvious but oten misued as people will talk about SAC and give it in terms of volume (.60 , .81, etc)
2. What is less obvious is that consequently SAC rate is tank dependent as the internal volume and hence the psi making up one cubic foot per tank varies for each model of tank. Your SAC rate for an AL 80 for example will differ from your SAC rate with an FX100 or LP 95.
3. People use the term "SAC" and work in units of volume to in get around the limitations of #2. However that is more correctly referred to as "Respritory Minute Volume" or "RMV. It is just a SAC converted to a volume measurement with an additional step.
Assuming in the example in the previous post that you got a SAC of 25.5 psi per minute and were using an AL 80, an AL 80 holds 77.1 cu feet at 3000 psi.
A. You divide the 3000 psi by 77.1 cu ft to determine that 38.9 psi = 1 cu ft.
B. Then you divide 25.5 by 38.9 to covert your SAC to an RMV, in this 25.5/38.9= .6555 which rounds to .66 or .7 depending on how far up you wish to round.
4. A whole bunch of terms and acronyms get thrown around with relation to SAC. For example a Depth Consumption Rate (SCR) used to be and stil is called a Depth Use Rate, and RMV is often expressed in metric terms using liters and don't forget that metric divers already have the advantage of working in bars, but what ever the terms used the concept is identical.
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As for SAC dropping with age - everything drops with age.
1. SAC is an exression of air consumption in psi per minute. That is obvious but oten misued as people will talk about SAC and give it in terms of volume (.60 , .81, etc)
2. What is less obvious is that consequently SAC rate is tank dependent as the internal volume and hence the psi making up one cubic foot per tank varies for each model of tank. Your SAC rate for an AL 80 for example will differ from your SAC rate with an FX100 or LP 95.
3. People use the term "SAC" and work in units of volume to in get around the limitations of #2. However that is more correctly referred to as "Respritory Minute Volume" or "RMV. It is just a SAC converted to a volume measurement with an additional step.
Assuming in the example in the previous post that you got a SAC of 25.5 psi per minute and were using an AL 80, an AL 80 holds 77.1 cu feet at 3000 psi.
A. You divide the 3000 psi by 77.1 cu ft to determine that 38.9 psi = 1 cu ft.
B. Then you divide 25.5 by 38.9 to covert your SAC to an RMV, in this 25.5/38.9= .6555 which rounds to .66 or .7 depending on how far up you wish to round.
4. A whole bunch of terms and acronyms get thrown around with relation to SAC. For example a Depth Consumption Rate (SCR) used to be and stil is called a Depth Use Rate, and RMV is often expressed in metric terms using liters and don't forget that metric divers already have the advantage of working in bars, but what ever the terms used the concept is identical.
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As for SAC dropping with age - everything drops with age.
Gee, I'd like to think that above all your comfort and technique have improved. Warm water and a declining metabolism can't hurt.
Charlie99
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Have you started inflating the "middle age spare tire" yet? YMMV, but my resting metabolism dropped like a rock somewhere around age 40. Lower resting metabolism leads to lower resting SAC.
IF (and that's purposefully a BIG "if") you are wearing proper thermal protection, then water temperature shouldn't have much effect on SAC. Most people underdress and therefore burn up lots of calories just to maintain body temp. That results in more CO2 and more air exchange to remove it. This is true even in what most people consider to be warm water -- normal skin temp is in the lower 90's, so water much lower than 90F is going to be sucking out heat.
Just like smokers adapt to high CO2 levels, some frequent divers adapt to high CO2 levels and no longer feel the need to breathe as much as others. If you find yourself consciously increasing your breathing rate when you speed up your swimming, then this probably applies to you.
IF (and that's purposefully a BIG "if") you are wearing proper thermal protection, then water temperature shouldn't have much effect on SAC. Most people underdress and therefore burn up lots of calories just to maintain body temp. That results in more CO2 and more air exchange to remove it. This is true even in what most people consider to be warm water -- normal skin temp is in the lower 90's, so water much lower than 90F is going to be sucking out heat.
Just like smokers adapt to high CO2 levels, some frequent divers adapt to high CO2 levels and no longer feel the need to breathe as much as others. If you find yourself consciously increasing your breathing rate when you speed up your swimming, then this probably applies to you.
jpsexton
Contributor
I've only been diving a few years but I have noticed that as I work on buoyancy and skill my SAC rate continues to get better. This summer I consciously worked on just hovering and being completely still in the water and the by product of that is my SAC rates got even better. I think it's all interrelated. You're probably at a better skill and comfort level than you've ever been.
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