Chapter 4 - Choosing the Best Nitrox Mix
Choosing the Best Nitrox Mix
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oxygen partial pressure of 1.4 atmospheres, but it can be
calculated for other oxygen partial pressures as well.
When a cylinder is filled, the mixture in a cylinder is
analyzed, logged, and a contents label is placed on the
cylinder. At that time, the MOD should be written prominently
on the cylinder’s contents label. If the reference
limiting PO2 is other than 1.4 ata, this should be noted
on the cylinder too.
Maximum operating depth can be established using
a table (see Figure 4-1: Maximum Operating Depth), or
it can be easily calculated using the concept introduced
in Chapter 2 that “the part is a fraction of the whole.”
The OCEANx calculator also can be used to find maximum
operating depth for a range of nitrox mixtures.
MAXIMUM OPERATING DEPTH BY TABLE
To determine the maximum operating depth, go to
the column in either the metric or imperial MOD table
that contains the percent oxygen in your mix. If your
mix is not on the table, use the next greater percent.
Move down the column to the row that is your chosen
maximum oxygen partial pressure (probably 1.4). The
maximum operating depth in meters or feet of seawater
is shown at the intersection of column and row.
Example 1: You are given a cylinder of EAN36. To
find the MOD, read across the top row to the column
marked 36%. Then read down that column to the row
for 1.4 ata. Your maximum operating depth is 28 meters
Maximum Operating Depth (in fsw) for Various Nitrox Mixes
Percent O2 21% 22% 24% 26% 28% 30% 32% 34% 36% 38% 40%
PO2
1.0 ata 124 117 104 93 84 77 70 64 58 53 49
1.1 ata 139 132 118 106 96 88 80 73 67 62 57
1.2 ata 155 147 132 119 108 99 90 83 77 71 66
1.3 ata 171 162 145 132 120 110 101 93 86 79 74
1.4 ata 187 177 159 144 132 121 111 102 95 88 82
1.5 ata 202 192 173 157 143 132 121 112 104 97 90
1.6 ata 218 207 187 170 155 143 132 122 113 105 99
FIGURE 4-1a: TABLE SHOWS MAXIMUM OPERATING DEPTH FOR BREATHING GASES FROM AIR TO EAN40. FOR INTERMEDIATE OXYGEN FRACTIONS,
USE THE NEXT RICHER MIX. FRACTIONAL DEPTHS HAVE BEEN ROUNDED DOWN TO THE NEXT SHALLOWER WHOLE NUMBER. DEPTHS
GREATER THAN 130 FSW ARE SHADED DARK. MODS FOR THE RECOMMENDED MAXIMUM EXPOSURE OF 1.4 ATA ARE LIGHTLY SHADED.
Maximum Operating Depth (in msw) for Various Nitrox Mixes
Percent O2 21% 22% 24% 26% 28% 30% 32% 34% 36% 38% 40%
PO2
1.0 ata 38 35 31 28 26 23 21 19 17 16 15
1.1 ata 42 40 36 32 29 26 24 22 20 18 17
1.2 ata 47 44 40 36 33 30 27 25 23 21 20
1.3 ata 52 49 44 40 36 33 30 28 26 24 22
1.4 ata 57 54 48 43 40 36 33 31 28 26 25
1.5 ata 61 58 52 47 43 40 36 34 31 29 27
1.6 ata 66 63 57 51 47 43 40 37 34 32 30
FIGURE 4-1b: TABLE SHOWS MAXIMUM OPERATING DEPTH FOR BREATHING GASES FROM AIR TO EAN40. FOR INTERMEDIATE OXYGEN FRACTIONS,
USE THE NEXT RICHER MIX. FRACTIONAL DEPTHS HAVE BEEN ROUNDED DOWN TO THE NEXT SHALLOWER WHOLE NUMBER. DEPTHS
GREATER THAN 40 MSW ARE SHADED DARK. MODS FOR THE RECOMMENDED MAXIMUM EXPOSURE OF 1.4 ATA ARE LIGHTLY SHADED.