Finally, convert this air-equivalent absolute pressure to
an equivalent air depth.
Example: What is the equivalent air depth for a diver
diving with EAN32 to a depth of 24 meters (80 feet)?
Step 1: Find the absolute pressure at 24 meters (80
feet) using the formula from Chapter 2:
or
Pbar = D msw +
1
Pbar = 24 msw +
1
In Imperial/U.S. measure:
or
atm
Pbar = D bar +10
msw
Pata = D fsw + 33
fsw
atm = 3.42 ata
Pata = D fsw +
1
fsw atm
33 /
Pata = 80 fsw +
1
fsw atm
33 /
Step 2: Apply your nitrogen “credit.” Since you are
diving with EAN32, the nitrogen fraction of your mix is
0.68 (= 1.0 - 0.32). Your air-equivalent absolute pressure
is set by the ratio between the fraction of nitrogen
in your mix and the fraction of nitrogen in air, or:
0.68
Pata ( air equiv .) = ×
3.4 ata = 2.93 bars (ata)
0.79
Step 3: Convert 2.93 bars (ata) to an equivalent air depth:
D msw = (Pbar - 1 bar) x 10 msw / bar
D msw = (2.93 bar - 1 bar) x 10 msw / bar = 19.3 msw
or in Imperial/U.S. measure:
= 64 + fsw.
D fsw = (2.93ata −1atm)×33 fsw/ atm
You would use 21 meters (70 feet) on your air dive
tables.
The equivalent air depth can also be determined using a
single formula:
⎜ ⎞
⎝
or in U.S. equivalents
( ) EAD fsw D fsw fsw FN 33 fsw
⎜ ⎞
⎝
33 2 ⎟ −
Using the above example:
= ⎛ + × =
⎜ ⎞
⎝
24 10 0.68 ⎟ −
or in U.S. equivalents
= ⎛ + × =
⎜ ⎞
⎝
80 33 0.68 ⎟ −
Since, the nitrogen fraction is “one minus the oxygen
fraction,” you can even add even another twist into
the formula by substituting (1 - FO2) for FN2, and the
formula becomes:
⎜ ⎞
⎝
or in the U.S. formula
10 1 2 ⎟ −
⎜ ⎞
⎝
33 1 2 ⎟ −
USING THE OCEANX TO ESTABLISH
EQUIVALENT AIR DEPTH
To find equivalent air depth with the OCEANx
calculator, simply dial-in the oxygen percentage into the
upper window. The long window immediately below
D fsw = (Pata −1atm)×33 fsw/ atm
fsw atm
33 /
bar = 3.4 bars
msw bar
1 /
bar
msw bar
10 /
msw bar
10 /
( ) EADmsw msw msw 10 msw 19 + msw
0.79
⎠
( ) EAD msw Dmsw msw FN 10 msw
10 2 ⎟ −
0.79
⎠
= ⎛ + ×
( ) EAD fsw fsw fsw 33 fsw 64 + fsw
0.79
⎠
0.79
⎠
= ⎛ + ×
( )( ) EADmsw D msw msw FO 10 msw
0.79
⎠
= ⎛ + × −
( )( ) EAD fsw D fsw fsw FO 33 fsw
0.79
⎠
= ⎛ + × −
NAUI Nitrox Diver
60 Dive Tables and Dive Computers
FIGURE 5-6: THE OCEANx CALCULATOR ALSO SIMPLIFIES FINDING
EQUIVALENT AIR DEPTH DURING DIVE PLANNING