4
52 NAUI Nitrox Diver
Step 2: To find the depth at which the absolute
pressure is 3.9 ata, use the formula from Chapter 2:
D fsw = (Pata − 1 atm) × 33 fsw / atm =
(3.9 ata −1 atm) × 33 fsw/atm = 95 fsw
(rounding down), or
D fsw = (Pata × 33 fsw/ atm) − 33 fsw =
(3.9 ata × 33 fsw/atm) − 33 fsw = 95 fsw
Using S.I./metric measurements with a value of 10
msw/bar, the depth is 29 meters.
The two step process can be combined into a single
formula by replacing the total pressure in Step 2 with its
equivalent “PO2 limit / FO2” that you used in Step 1:
Or, if finding the absolute pressure first and then
subtracting the number for fsw in one atmosphere:
Using the first formula, the above example becomes:
Using the NAUI MobileTM Application to
Determine MOD
The NAUI MobileTM Application MOD feature allows
divers to quickly calculate maximum operating depth
and several other essential values (Figure 4-3). The
MOD calculator can calculate nitrox mixes from 21% to
40% O2 with partial pressures of oxygen (PO2) ranging
1.2 to 1.6 ata. From the NAUI MobileTM App, open the
Dive Tools menu. Then use the slide-bars to select the
appropriate FO2 and PO2. For example, the MOD for a
FO2 of .32 (or 32% O2) and PO2 of 1.4 ata is 33 meters
(111 feet).
CHOOSING YOUR “BEST MIX”
The standard nitrox mixes of 32% and 36% oxygen are
sufficient for most recreational oxygen-enriched air
diving. The NAUI EAN32 and EAN36 dive tables, which
will be presented in the next chapter, have a tolerance of
±1% and are therefore useable for mixes of 31% to 33%
and 35% to 37% oxygen. Yet there may be times when
a diver wants to maximize no-required-decompressionstop
dive time for a specific depth.
To do this the diver needs to determine the highest
fraction or percentage of oxygen (up to EAN40) that
can be used at the target depth. The filling technician
can then blend the specific mix for that dive. This mix is
known as the “best mix.” As with maximum operating
depth, best mix can be determined from a table, from a
formula, or by using the NAUI MobileTM App calculators.
Best Mix by Table
Figure 4-4 presents oxygen percentages (up to 40%
maximum) that will provide oxygen partial pressures
from 1.2 to 1.6 atmospheres at various depths. Except for
contingency purposes, the diver should select a PO2 of
1.4 or less. Some extra-cautious divers always use a PO2
of 1.3 as their personal maximum. After selecting the
PO2, move down the column to the row that shows the
planned depth, rounding up if the exact depth is not on
the table. The intersection of column and row shows the
best mix oxygen percentage for the dive.
Calculating Best Mix
Calculating best mix is similar to the calculation for
maximum operating depth in reverse.
First, convert the target depth into the absolute
pressure. Next, determine the fraction of oxygen in the
mixture that will produce the maximum acceptable
PO2 at that absolute pressure. This can be done in two
D fsw = ( P O 2 l i m i t − 1 a tm) × 33 fsw / atm
FO2
D fsw = ( PO2 limit
× 33 fs w / a tm) − 33 fsw
FO2
( 1.4 atm
D fsw = − 1 at m) × 33 fsw / atm = 95 fsw
0.36
Figure 4-3 NAUI MobileTM App Dive Tool Calculators