Equivalent Air Depth Notes

The equivalent air depth is the depth at which an air mixture has the same nitrogen partial pressure as a Nitrox mix for a given depth or put another way its tells you what depth you would have to dive to on air to receive the same nitrogen exposure. It is used as a way of approximating the decompression requirements of a Nitrox dive using standard air tables.

What is the equivalent air depth of a 27 meter dive using Nitrox 36?

First we need to work out the absolute pressure at 27 meters. Assuming one atmosphere exerts a pressure of 1 bar and 10 meters of water is equivalent to 1 atmosphere. The absolute pressure will be 

(27 / 10) + 1 = 3.7 bar

now we need to adjust the absolute pressure by the relative reduction in nitrogen

3.7 * (0.64 / 0.79) = 2.997 bar

Next we need to convert this pressure back into a depth:

10 * (2.997 - 1) = 19.97 meters

These steps can be expressed in a single formula as:

Equivalent Air Depth = ((( Depth / 10)  + 1) * (Nitrogen Fraction / 0.79)) -1 ) * 10