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48 NAUI Nitrox Diver
account for the most potentially serious physiological
concerns facing every diver on every dive.
Oxygen
You learned in your Open Water Diver Course, oxygen is
the component of the breathing gas that we metabolize.
Keep in mind that although oxygen is necessary to
sustain life, too much of a good thing may lead to
problems.
At the NAUI Nitrox Diver level, you will be typically
breathing one of three mixes, normal air (21% oxygen),
Nitrox 32 (EAN32, NOAA Nitrox I , 32% oxygen) or
Nitrox 36 (EAN36, NOAA Nitrox II, 36% oxygen) and
depending on your other certification levels may be
conducting dives to a depth of 40 meters (130 feet),
providing you stay within appropriate oxygen limits. In
this Nitrox Diver course, you learn toxicity tracking and
safe depth limits for various nitrox mixtures. The two
pieces of information you will need to satisfy and check
off this portion of the planning worksheet are a central
nervous system (CNS) limit and a total oxygen exposure
limit (OTU). If these numbers are within acceptable
ranges, the oxygen portion can be checked off.
Decompression
This section of the dive plan centers around your
breathing gas and nitrogen elimination that was covered
in your entry level Scuba course. You will need to know
the planned depth of your dive and the time you are
planning. This is only slightly different at the Nitrox
level as you still have various methods to ensure you
stay within the no decompression limits. Since you are
diving nitrox as part of your course, you may be using
a nitrox blend of 32% or 36% oxygen, and can use the
published NAUI Nitrox tables (standard NAUI Nitrox or
NAUI Nitrox RGBM,) your Nitrox capable personal dive
computer, or standard NAUI air tables using your EAD
to find this information.
Inert Gas Narcosis
The inert gas narcosis section, is the portion of the dive
plan where you consider the potential narcosis exposure
that you might encounter on your planned dive. Nitrogen
narcosis was covered in your earlier training. As a
NAUI Open Water Diver, your training was limited to
a maximum depth of 18 meters (60 feet), which means
that it may take some dives and time extending your
experience to reach deeper depths where you will learn
how nitrogen narcosis affects you.
Nitrogen narcosis involves other factors besides
depth. Exertion (carbon dioxide buildup), anxiety, cold
water and poor visibility play an active role and should
be considered when planning a dive. Evaluate all of these
to determine if you or other members of the team think
narcosis may occur. Some nitrogen narcosis exists on
every dive, although the effects may not be noticeable
by you or your buddy, especially at shallow depths.
Your NAUI Instructor can help you further understand
narcosis.
Prudent divers know their limits from experience.
Awareness must be keen in order to make wise decisions.
If nitrogen narcosis is a significant concern for the
planned operation, adjust your maximum planned depth
or wait until conditions improve. The important concept
is to be aware of nitrogen narcosis, the signs of it, and
what to do if you begin to feel it effects. Remember,
nitrox is not a narcosis-reducing gas, and you should feel
the same level of narcosis at a given depth whether diving
air or a nitrox mix.
Gas Management
The boat captain or dive leader tells you to be back on
the boat with no less than 34 bar (500 psi). The next
question a new diver may ask would be: “How exactly do
I accomplish that?” The best method of doing this is to
utilize a "gas rule." A gas rule is simply a predetermined
procedure that allows the diver to return to the point of
origin with the appropriate amount of breathing gas to
exit the water safely, usually 34 bar (500 psi). A practical
method for the open water diver in normal conditions is
the half plus 14/(200) rule: Divide your cylinder pressure
in half and add 14 bar (200 psi). This amount then
becomes the minimum pressure in your scuba cylinder at
which you turn the dive and start making your way to the
exit point.
Metric example: If you started with 200 bar in
your cylinder:
1. 200 bar ÷ 2 = 100 bar
2. 100 bar + 14 bar = 114 bar turn pressure
Imperial example: If you started with 3000 psi in your
scuba cylinder:
1. 3000 psi ÷ 2 = 1500 psi
2. 1500 psi + 200 psi = 1700 psi turn pressure
An additional method that lends even greater
conservatism is to use the rule of thirds. The three steps
of the rule of thirds are:
1. Round your starting pressure down to a number
easily divisible by three.
2. Divide that number by three.
3. Subtract this amount from your total pressure.
Metric example: If you started with 200 bar in your
scuba cylinder:
1. 200 bar; round down to 180 bar
2. 180 bar ÷ 3 = 60 bar
3. 200 bar − 60 bar = 140 bar turn pressure