If I remember correctly, I was taught that 20-25 was average. I just needed a quick refresher.
That depends entirely on the size and pressure rating of the cylinder you are using. The numbers you quote are most certainly for an AL80.
If you use a different style cylinder and breathe exactly the same, the psi per minute number will be different.
A more useful number is RMV (respiratory minute volume), which is your SAC converted to cubic feet per minute ... because this number represents the actual volume of air you are breathing, and remains the same regardless of the size cylinder you are using.
For example, if you are using 22 psi per minute with an AL80 cylinder (with a working pressure of 3000 psi), you can convert to RMV by first dividing the rated volume of your cylinder by its working pressure ... in this case, 80/3000 ... which is 0.0266 cubic feet per psi. This is called your baseline ... and is unique to the cylinder you are using.
Then multiply your psi per minute (22) by the baseline, which will give you the actual volume of air you breathe on the surface in one minute ... which is your RMV. In this case it's 0.572 cubic feet per minute. Not a bad consumption rate.
If you were breathing exactly the same using a steel LP95 cylinder, which has a baseline of 0.036 cubic feet per psi, you would discover that you're only using 16 psi per minute.
So ... 22 psi per minute in an AL80 is the same volume as 16 psi per minute in an LP95. So whenever quoting your SAC, it's important to also include the type of cylinder you are making the calculations from.
... Bob (Grateful Diver)
