10-2 Air Depth Calculation

The partial pressure of nitrogen in a NITROX mixture is the key factor determining the diver’s decompression obligation. Oxygen plays no role. The decompression obligation for a NITROX dive therefore can be determined using the Standard Air Tables simply by selecting the depth on air that has the same partial pressure of nitrogen as the NITROX mixture. This depth is called the Equivalent Air Depth (EAD). For example, the nitrogen partial pressure in a 68% nitrogen 32% oxygen mixture at 63 fsw is 2.0 ata. This is the same partial pressure of nitrogen found in air at 50 fsw. 50 fsw is the Equivalent Air Depth.

Equivalent Air Depth Calculation.
                    The Equivalent Air Depth can be computed from the following formula:

        

Where:
          EAD = equivalent depth on air (fsw)
          D = diving depth on mixture (fsw)
          O2% = oxygen concentration in breathing medium (percentage decimal)

For example, while breathing a mixture containing 40 percent oxygen (O2% =
0.40) at 70 fsw (D = 70), the equivalent air depth would be:

Note that with NITROX, the Equivalent Air Depth is always shallower than the diver’s acual depth. This is the reason that NITROX offers a decompression advantage over air.