Thursday, August 9, 2007

Calculating your air consumption or SAC rate

Ok, the other day I had a discussion with a couple of dive friends about SAC or air consumption rates. SAC is the acronym for “Surface Air Consumption” and is a variable rate that helps technical divers compute how much air they will likely use at depth. It’s usually calculated in two forms; “working rate” and “resting or deco rate”. The formula for calculating your rate is actually fairly simple:

The calculation is performed by calculating the volume of gas used (PSI used * ( Tank volume rating / Tank pressure rating )), dividing this by the time at depth, then dividing this result by the pressure at the average depth ((D/33)+1). This result is the Surface Air Consumption (SAC).

The procedure for finding your SAC rate is to first identify your tank’s working pressure; that’s the pressure stamped on the tank eg. 2450 psi. Second is to find your tanks official volume at that pressure. This is a little harder, since many tanks don’t specify it on the tank so you may have to visit the manufacturer’s website and identify your tank by size and pressure. Fortunately some tanks are labeled. So, if my tank is rated at 2450 psi at 80 cubic feet I can easily calculate that at the surface each cubic foot is equivalent to 30.6 psi. (2450/80).
Now since the air is compressed at depth, but I still need the same volume for each breath; my consumption increases as I either breathe air that is more dense (depth) or faster (rate). What the SAC does is to determine a baseline at the surface which I can use to calculate at a specific depth my usage based on my prescribed rate. We isolate those variables by performing the following exercise. First to find your working rate you need to dive to a specific depth; most often we use 60’ saltwater or 60 fsw. Once at 60 feet we record or pressure as accurately as possible (helps to have a digital pressure gauge). We then swim at a steady depth (60 fsw) for 15 solid minutes, recording the pressure at each 5 minute interval When we’re done we now know how much air we consumed in psi for a 15 minute working swim. Let’s say we use 1000 psi out of the tank when we’re doe.

Since the air is denser at depth we need to calculate the affect in atmospheres (remember that 33 ft salt water per atmosphere from Open Water Class). So we have to figure in the equivalent by taking our 60 ft dividing by 33 for each atmosphere, and then adding our 1 atmosphere for the pressure exerted by the air at sea level.

I now have everything I need to finish the calculation. I would take my psi used times the volume rating for the tank divided by the tank’s pressure rating (1000*(80/2450)) = 32.65. I then divide by my time at depth 32.65/15 = 2.17 I need my pressure at 60 fsw which is (60/33)+1) or 2.81 atm. And 2.17/2.81 = .774! Or, it means I breathe at a working rate .774 cubic feet of air a minute on the surface. Now all I need to do to figure out how much air I would breathe at 99 fsw is to multiply my SAC rate by the atmospheres of that depth (4) which would be 3.09 cu ft each minute.

I can then do the same exercise for a resting rate, and figure how much air I would use during my safety or deco stop.


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