5
62 NAUI Nitrox Diver
your exact maximum depth is not on the table, use the
next greater depth. Move to the left along that row to
the column at the far left. The equivalent air depth that
you should use with air dive tables is shown in the left
column. For example, for 32% oxygen, any dive between
26 meters and 29 meters would use 24 meters on an air
table, and any dive between 86 feet and 98 feet would use
80 feet on an air table for dive planning. Contingency
depths for which the PO2 is between 1.4 and 1.6
atmospheres have been shaded darker.
Example 1: On the first dive of the day, your
planned depth is 24 meters (80 feet), and your cylinder is
filled with EAN38. What is your maximum dive time for
the dive?
Solution: Enter the equivalent air depth table at the
38% column and move down. Twenty-four meters (80
feet) does not appear on the table, so continue down to
the next depth – 25 meters (85 feet). Moving to the left,
the equivalent air depth for this dive is 18 meters (60
feet). Using standard NAUI Dive Tables for air, you will
find that your maximum dive time for a 18 meter (60-
foot) dive is 50 minutes.
Example 2: Your actual dive time for the dive in
Example 1 is 35 minutes. Two hours later, you re-enter
the water to return to the same location. You are diving
with EAN32. What is your adjusted maximum dive time
for this repetitive dive?
Solution: Determine your exiting letter group for
the first dive. With an EAD of 18 meters (60 feet) for the
first dive and a 35-minute dive time, you emerge from
the water as a “G” diver on the NAUI (air) Dive Tables.
Two hours later you are a “E” diver. Since you are now
diving to a depth of 24 meters (80 feet) with EAN32, your
equivalent air depth from the EAD table is 21 meters (70
feet). An “E diver” diving to 21 meters (70 feet) has an
adjusted maximum dive time of 15 minutes on the NAUI
Dive Tables.
Calculating Equivalent Air Depth (EAD)
As with MOD and best mix, equivalent air depth can
be calculated in discrete steps, or the procedure can be
combined into a single formula. As a step-wise process,
you would first determine the absolute pressure at the
depth to which you will be diving. Second, apply the
nitrogen “credit” that your nitrox blend gives you.
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:
Pbar = 2 4 m s w + 1 bar = 3.4 bar
1 msw/bar
In Imperial/U.S. measure:
Pata = D f s w + 1 atm
33 fsw/atm
Pata = 8 0 f s w + 1 atm = 3.42 ata
33 fsw/atm
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:
Step 3: Convert 2.93 bar (ata) to an equivalent air
depth:
D msw = (P bar − 1 bar) × 10 msw / bar
D msw = (2.93 bar −1 bar) × 10 msw/bar = 19.3 msw
or in Imperial/U.S. measure:
D fsw = (Pata −1 atm) × 33 fsw / atm
D fsw = (2.93 ata −1 atm) × 33 fsw/atm = 64 fsw
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
Using the above example:
or in U.S. equivalents
Pata = D f s w + 3 3 f s w
33 fsw/atm
or
Pata (air equiv.) = 0 . 6 8 × 3.4 ata = 2.93 bar (ata)
0.79
(D msw + 10 msw) × EAD msw = ( FN2
) − 10 msw 0.79
(D fsw + 33 fsw) × FN2
EAD fsw = ( 0.79
) − 33 fsw Pbar = D m s w + 1 bar
10 msw/bar Pbar = D bar + 10 msw
or 10 msw/bar
=( (24 msw + 10 msw) × 0.68
EAD msw 0.79
) − 10 msw = 19 msw =( (80 fsw + 33 fsw) × 0.68
EAD fsw 0.79
) − 33 fsw = 64 fsw