can be set on the OCEANx. With the appropriate oxygen
percentage dialed into the window at the upper edge of
the wheel, the calculator shows the actual depth for partial
pressures of oxygen from 1.0 to 1.6 atmospheres.
These actual depth values appear in the “AD fsw” window
at the bottom. The OCEANx also displays the equivalent
air depth for actual dive depths up to a PO2 of 1.6
and other oxygen exposure information, such as NOAA
single dive oxygen exposure time limit. Equivalent air
depth will be used in the next chapter’s discussion of
using standard air dive tables with nitrox.
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
Chapter 4 - Choosing the Best Nitrox Mix
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 OCEANx calculator.
BEST MIX BY TABLE
Figure 4-3 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
Choosing the Best Nitrox Mix
47
Oxygen Percentages for Best Mix
PO2 1.2 1.3 1.4 1.5 1.6
fsw msw
40 12 40% 40% 40% 40% 40%
45 14 40% 40% 40% 40% 40%
50 15 40% 40% 40% 40% 40%
55 17 40% 40% 40% 40% 40%
60 18 40% 40% 40% 40% 40%
65 20 40% 40% 40% 40% 40%
70 21 40% 40% 40% 40% 40%
75 23 37% 40% 40% 40% 40%
80 24 35% 38% 40% 40% 40%
85 26 34% 36% 39% 40% 40%
90 27 32% 35% 38% 40% 40%
95 29 31% 34% 36% 39% 40%
100 30 30% 32% 35% 37% 40%
105 32 29% 31% 33% 36% 38%
110 33 28% 30% 32% 35% 37%
115 35 27% 29% 31% 33% 36%
120 36 26% 28% 30% 32% 35%
125 38 25% 27% 29% 31% 33%
130 40 24% 26% 28% 30% 32%
FIGURE 4-3: BY SELECTING THE PLANNED MAXIMUM DEPTH
AND THE MAXIMUM OXYGEN PARTIAL PRESSURE
(USUALLY 1.4 ATA), THE BEST OXYGEN PERCENTAGE
FOR THE NITROX MIX IS EASILY ESTABLISHED.
ALTHOUGH PERCENTAGES HIGHER THAN 40% ARE
OBTAINED BY CALCULATION, THIS TABLE SHOWS
EAN40 AS THE HIGHEST OXYGEN MIX USED IN
RECREATIONAL NITROX DIVING.
FIGURE 4-2: OCEANX CALCULATOR