One of the fundamental calculations we rely on as technical divers is the SAC, or surface air consumption rate. We care because that is the primary means for determining the volume of gas we need to take along to make a given dive.
The formula looks like this: SAC(cfm) = ((PSI used / working pressure) X cylinder capacity) / ((depth in feet + 33) / 33), all divided by the number of minutes. Granted that’s pretty hard to read, but I didn’t feel like figuring out how to better display math in a blog post. It’s beside the point anyway.
The point is that stumbled across a pretty cool SAC Rate Calculator online created, or at least put on the web by, the South Florida Spearfishing Club. It’s at: http://www.spearfishing.org/bruces_tips/java/sac.html
I calculated my own SAC this morning during a deco hang at 10 feet in water a chilly 44 degrees. I didn’t use this calculator to figure it out this morning, but calculating manually and using the SAC calculator gave me the same number, which is .2. That’s pretty good, especially considering that I was getting pretty cold at the end of the dive.
With SAC, it’s nice to see low numbers (less costly for gas, smaller cylinders, greater safety margin, etc.) but don’t let that tempt you to “fudge” the numbers. It’s not a contest, and you need to know you are carrying enough gas to make the dive, while meeting any deco obligations and reserve requirements.