Charlie99:
I use "turn pressure" and "rock bottom" to mean slightly different things.
Me too, but I may decide my turn pressure based on rock bottom, especially if turning the dive means ascending directly, and not swimming back.
shrswnm:
How is SAC determined? I assume it is air consumption, but what does the S stand for?
The S stands for Surface. Surface Air Consumption is a calculation of the volume of air you use per minute while diving at 1 atmosphere of pressure.
In order to get it, descend to a depth (lets say 33 ft... that's 1 additional atmosphere of water pressure, so it makes the math easy) and note your current pressure. Swim normally at that depth for 10 minutes, and note your new pressure. Subtract your end pressure from your start pressure, and you'll know how much air you used in psi. Let's say for the sake of simplicity that you used 1000 psi. We want to know how much you used in cubic feet though, so there's some easy math involved.
Find the volume of your tank, and it's service pressure. For example, an AL80 holds 77.4 cubic feet of air when filled to 3000 psi. Divide the volume by the service pressure, and you'll have the volume of air represented by each psi. In this case, 77.4/3000=.0258.
If we multiply air you consumed in psi by the volume each psi represents, we get the volume of air you consumed. In this case, .0258*1000=25.8 cubic feet consumed in 10 minutes at depth. If we divide that by ten minutes, we can see that you consumed 2.58 cubic feet per minute at depth.
Now, we need to know the actual pressure in atmospheres (ATA) of the depth at which you were swimming. We get that by dividing depth by 33 and adding 1 (for the earth's atmosphere). In this case, (33/33)+1=2 ATA.
If we divide the volume per minute at depth by the depth in ATA, we get your SAC. In this case, 2.58/2=1.29. Your SAC would be 1.29 in this case.
Using that number, you can determine how much gas you will use at a given depth in a given amount of time. If, for example, you wanted to know how much gas you'd use if you stayed for an extra 5 minutes at 110 feet, you could figure out that (110/33)+1=4.34 ATA for that depth, and multiply that by your SAC and the number of minutes to see that you would use an extra 4.34*1.29*5=28 cubic feet of air.
You can use it to figure out what size tanks you need to do a dive, what duration/depth dive you can do with tanks you have, rock bottom gas requirements, or even just to monitor how much less gas you've been using since giving up smoking.
