4
Chapter 4 Choosing the Best Nitrox Mix 51
The partial pressure of oxygen is the limiting factor
for all diving with oxygen-enriched air. If you are to limit
your exposure to a PO2 of 1.4 atmospheres (1.6 ata as
a contingency), then you must be able to establish the
maximum operating depth for the gas mixture with
which you are diving. On the other hand, if you know
your planned maximum depth in advance, you should
be able to pick the best mix for that dive from the range
available to you (air to 40% oxygen) and request the
optimal blend of oxygen and nitrogen when you have
your cylinder filled by the blending technician.
MAXIMUM OPERATING DEPTH
The maximum operating depth (MOD) is the
maximum depth that should be dived with a given nitrox
mixture. You will have to know your MOD if you are
only able to obtain a particular nitrox blend, such as
EAN32. Regardless of the blend, you need to establish
the maximum depth to which you can dive using that
cylinder. If you are able to obtain a custom blend to your
specifications, you can use the more flexible “best mix”
and ask for an appropriate oxygen percentage in your
mix.
For normal recreational nitrox diving, the MOD
should be derived from the recommended maximum
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 (Figure 4-2), or it can be easily calculated using
the concept introduced in Chapter 2 that “the part
is a fraction of the whole.” The NAUI MobileTM App
calculators 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 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:
Partial
Pressure
÷ ÷
Total
Pressure
Gas
Fraction
X
The basic formula is:
Pg = Fg × 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