Know Your Air Consumption: Calculating Your SAC’s
If you’ve moved beyond the basics of Scuba diving, and have started planning your own, independent dives, you should know you Surface Air Consumption, or SAC.
It could save your life.
It’s happened to me more times than I like to admit; running out of air during a dive. OK, it has actually only happened once where it was critical, but I have had a few close shaves in addition to this. The one time was during a dive in Egypt, where we needed to exit through a small cave.
But I had over-extended myself on the dive going longer than I should before turning around, so as we were suspended on a safety stop line inside the cave, my manometer mercilessly ticked away towards zero.
Luckily, I had my buddy close by, and I was able to use his octopus while we finished our safety stop and surfaced.
After that, I learned my SAC’s
Surface Air Consumption is an expression of your air consumption during dive, with the depth of the dive taken out of the equation. As we all know, air consumption increases with depth due to the pressure increases the density but reduces the volume of the air we breathe (as per Boyle’s Law).
So simply looking at our manometer doesn’t tell us the whole story, as we can consume more air during a 30 minute dive to 35 meter as we do during 1 hour dive to 8 meters.
When we know our SAC, we can make estimations as to how much air we will likely use during a dive to any given depth, allowing us to factor air consumption into our dive planning.
Calculating your SAC isn’t that difficult, and is part of most solo diving and technical diving courses.
Do some measuring
First, do a number of dives at different depths and conditions. For each of these dives, note down the depth, time, gas used, and cylinder size.
Ideally, include a dive where you were really strained due to currents, or swim for 5 minutes at maximum effort during a dive. Then make note of the air consumed in those 5 minutes, to also have an idea of your air consumption while working under strain.
Do the math
Once you have these numbers, you can start your calculations. While there are a number of internet sites where you can simply punch the numbers into boxes and get an answer, I find it useful to be able to do the calculations yourself.
The calculations go as follows:
VT x VC / T / P = SAC
Where VT is Total Volume of the cylinder used in liters, VC is Consumed Volume in bars during the dive, T is duration of the dive, P is the pressure in bars of the average depth of the dive (or maximum depth if you stayed at the same depth the entire dive), and SAC is the Surface Air Consumption in liters per minute.
An example: You do a dive to an average of 20 meters with a 15 liter tank that is filled to 200 bars at the start of the dive. You finish the dive with 150 bars after 20 minutes of diving. The equation would then look like this:
VT = 15 liters
VC = 50 bars
T = 20 minutes
P = 3 (20 meters of water equals 3 bars of pressure)
So... 15 x 50 / 20 / 3 = 12.5 liters per minute
Now I know that I consume 12.5 liters per minute at the surface. For any given subsequent dive, I can now simply time this number with the intended depth and time to see how much air I’ll need for that dive.
For the maximum strain test, do the same calculation, with 5 minutes as the dive time.
A few examples:
- A dive of 45 minutes to 25 meters: 12.5 x 45 x 3.5 = 1969 liters of air
- A dive of 1 hour to 10 meters: 12.5 x 60 x 2 = 1500 liters of air
- A dive of 30 minutes to 35 meters: 12.5 x 30 x 4.5 = 1687.5 liters of air
Needless to say, in all three of these examples, I should be returning to the surface with at least 50 bars of air left in my cylinder, so I would need to add that to the number.
So to take the first dive as an example, before jumping in to do the dive, I should make sure that I have a cylinder with at least 2019 liters of air in it. For a 12 liter cylinder, that I would consume 169 bars of air. So to do this dive safely, I would need a full 12 liter cylinder, filled with 200 bars of air.
For the imperial system, the formula gets a little trickier, but still easy, once you the grasp of it. It goes as follows:
R x PsiC / WP / T / P
R is the cylinger rating in cubic feet, PsiC is the Psi consumed, WP is for Working Pressure, the rated pressure that the tank operates under, T is the time in minutes, and P is the pressure at the average depth of the dive, or maximum if you’ve stayed at the same depth the entire dive.
An example: I’ve done a dive to 66 feet for 35 minutes, with an 80 cubic feet cylinder with a working pressure of 3000 Psi (both can be found on the cylinder), I have consumed 2000 Psi of that air. So the numbers would be:
R = 80 cubic feet
PsiC = 2000 Psi
WP = 3000 Psi
T = 35 minutes
P = 3 (pressure at 66 feet)
Or... 80 x 2000 / 3000 Psi / 35 / 3 = 0.5 psi of air pr. minute
Of course, the SAC rates above are based on a single dive, and seeing as a number of factors can influence air consumption.
It's best to do a series of dives, as mentioned, and calculate the SAC for each, then taking an average of those dives. And by doing the strain test mentioned earlier as well, you will have your SAC both for general diving and for strenuous conditions.
So redo the calculation every now and again. I typically do it once a year, or anytime I haven’t gone diving in a while.
Know you SAC’s, and never risk running out of air again!