NAUI Nitrox Diver
58 Dive Tables and Dive Computers
Equivalent Air Depth (in fsw)
Percent O2 26% 27% 28% 29% 30% 31% 32% 33% 34% 35% 36% 37% 38% 39% 40%
EAD fsw
40 44 46 47 48 49 50 51 53 54 55 57 58 60 61 63
50 55 56 58 59 60 62 63 64 66 67 69 71 72 74 76
60 66 67 69 70 71 73 75 76 78 80 81 83 85 87 89
70 76 78 80 81 83 84 86 88 90 92 94 96 98 100 99
80 87 89 90 92 94 96 98 100 102 104 106 108 105 102
90 98 100 101 103 105 107 109 112 114 116 113 109
100 108 110 112 114 117 119 121 123 122 117
110 119 121 123 126 128 130 132 127
120 130 132 134 137 139 137
130 141 143 145 148 143
MOD/1.4 ata 145 138 132 126 121 116 111 107 102 99 95 91 88 85 82
MOD/1.6 ata 170 162 155 149 143 137 132 127 122 117 113 109 105 102 99
FIGURE 5-5a: EQUIVALENT AIR DEPTH FOR VARIOUS NITROX MIXES. A DIVER DIVING UP TO THE DEPTH SHOWN IN THE CORRECT EANx COLUMN
WOULD USE THE DEPTH IN THE LEFT-HAND COLUMN OF THE SAME ROW WITH AN AIR DIVE TABLE. SHADED AREAS HAVE A PO2
BETWEEN 1.4 ATA AND 1.6 ATA
the pressure gradient between the inspired nitrogen partial
pressure and the tension of nitrogen dissolved in the
tissues. If the inspired nitrogen partial pressure is less,
the pressure gradient is less, and nitrogen moves into
the tissues more slowly. And, according to Henry’s Law
the total amount of nitrogen that will dissolve in the
tissues over time is directly proportional to the nitrogen
partial pressure.
If you find this difficult to conceptualize or understand,
here is a concrete example. If you were breathing
a mixture that is 36% oxygen, then the nitrogen percentage
would be 64%, and the nitrogen fraction would be
0.64. When you dive with this mixture, you expose yourself
to 64/79ths the nitrogen partial pressure that you
would encounter if breathing air. Therefore, you can
consider your depth to be 64/79ths (roughly 80%) of the
absolute pressure that you would encounter at your
actual depth if you were breathing air. If you were
breathing EAN40, your nitrogen partial pressure would
be 60/79ths (about three-quarters) of what it would be if
you were breathing air. You must relate your nitrogen
advantage to absolute pressure and then convert that
absolute pressure to a depth. If you were to try to relate it
immediately to a depth, you would be neglecting the one
atmosphere of air pressure at the surface. You should
remember this from your beginning class in which you
learned about pressure at depth and the volume of air in
a flexible container.
Equivalent air depth is especially useful if you want
to change blends from one dive to the next, if you are
requesting a custom blend based on “best mix,” or if for
some reason you find yourself diving with a mix that is
not standard EAN32 or EAN36. If you know equivalent
air depth for your mix, you can use that EAD with the
NAUI standard Air Dive Tables (or any air dive tables) to
plan your dive or to plan repetitive dives. By determining
the EAD prior to each dive, you can easily change mixes
to either richer or leaner oxygen content from one dive
to the next. You can find equivalent air depth by using a
table, by calculating EAD, or with the NAUI OCEANx calculator.
Let’s begin with using a table.
EQUIVALENT AIR DEPTH BY TABLE
The Equivalent Air Depth Table relates the percentage
of oxygen in your nitrox mix to an actual dive depth
to show an equivalent air depth. The actual dive depth is
not usually a round number because it is relating to a
round-number equivalent air depth that will be used on
the dive tables.