NAUI Nitrox Diver
46 Choosing the Best Nitrox Mix
or 95 feet. If you dive deeper, you will exceed 1.4 ata
PO2.
Example 2: You receive a cylinder that contains
EAN33, and you choose to dive to a more conservative
oxygen partial pressure of 1.3 atmospheres. To find your
MOD, read across the top row to the column marked
34% (rounding up from 33%). Then read down the column
to the row for 1.3 ata. Your maximum operating
depth for your chosen oxygen partial pressure is 28
meters or 93 feet.
CALCULATING MAXIMUM OPERATING DEPTH
To calculate the maximum operating depth for any
nitrox mixture, begin by finding the total pressure that it
takes to produce the maximum acceptable oxygen partial
pressure. Then convert this total pressure to a depth.
You can do this in two separate steps, or the two steps can
be blended into a single formula, as will be shown later.
Example: What is the maximum operating depth
for EAN36 (using 1.4 ata PO2 as your acceptable limit)?
Step 1: Find how many total atmospheres of pressure
will produce your target PO2. To help you remember
the formula, use the mnemonic phrase: “The part is
a fraction of the whole” or the diagram that was presented
in Chapter 2:
The basic formula is:
Pg = Fg x Ptotal
Or, since you know the target partial pressure and the
fraction of gas in the mix:
Ptotal = Pg / Fg or Pata = PO2 limit / FO2
Remember to convert the oxygen percentage to
a fraction.
Pata = 1.4 atmospheres / 0.36 = 3.9 atmospheres absolute
Step 2: To find the depth at which the absolute pressure
is 3.9 ata, use the formula from Chapter 2:
=
D fsw = (P ata −1atm)×33 fsw/ atm
(3.9 ata -1 atm) x 33 fsw/atm = 95 fsw
(rounding down), or
=
D fsw = (Pata ×33 fsw/ atm )− 33 fsw
(3.9 ata x33 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:
⎞
⎛
= −
2limit × ⎟ ⎟⎠
atm fsw atm
PO
⎜ ⎜⎝
D fsw 1 33 /
FO
2
Or, if finding the absolute pressure first and then subtracting
the number for fsw in one atmosphere:
⎞
⎛
= ×
2limit − ⎟ ⎟
fsw atm fsw
PO
⎜ ⎜
D fsw 33 / 33
FO
2
⎠
⎝
Using the first formula, the above example becomes:
D fsw = ⎛ atm −
1atm 33 fsw/ atm = 95 fsw
⎜ ⎞
⎝
1.4 ⎟×
0.36
⎠
USING THE OCEANX TO ESTABLISH MOD
The OCEANx calculator is a wheel-type tool that
allows the oxygen percentage of the mix to be dialed in
(See Figure 4-2). Oxygen percentages from 25% to 40%
Partial
Pressure
Total
Pressure
Gas
Fraction