LEARNING GOALS
In this chapter, you will learn:
• About Maximum Operating Depth and
determining it for any nitrox mixture.
• How to use the OCEANx Calculator.
• What “Best Mix” means and how to calculate it.
Up to this point, you have learned theory and
concepts related to diving with oxygen-enriched air.
This included such topics as gases, gas mixtures, partial
pressure, physiology, decompression sickness, oxygen
toxicity, and recommended maximum oxygen partial
pressures.
With this chapter, you will begin learning how to
use this knowledge to plan and execute safe dives using
nitrox. This and the remaining chapters of this book are
more practical in nature.
For instance, you already know that you should
limit the partial pressure of the oxygen that you breathe
at depth in order to minimize the risk of oxygen toxicity.
In this chapter we will present how to determine the
maximum depth to which you should dive with any
given nitrox mix. We will also show how to approach
this from the other direction, that is, how you can optimize
your nitrox benefit by choosing the best fraction of
oxygen for your blend if you are planning to dive to a
known depth. In later chapters, you will learn how to
use nitrox dive tables in your dive planning as well as
how to use standard air dive tables when diving with
nitrox, precautions that you must use when handling
oxygen and oxygen rich mixtures, and how oxygenenriched
air is prepared.
Actually, diving with enriched air nitrox is similar
to diving with air. In all diving, we plan our dives using
dive tables or dive computers to monitor and control our
nitrogen loading. We dive with an awareness of and
respect for the environment and the limitations of our
training and ability. We are responsible divers–responsible
for our own safety, responsible to our dive partner,
and responsible in our attitude and impact on the ocean
or aquatic environment we are visiting. The added difference
is that with nitrox there are more variables
involved because we are now in control of what we are
breathing. Also, using oxygen means that we must be
aware of certain special precautions regarding its use. It
is sometimes said: “Once you strap the tank on your
back, diving with nitrox is just like diving with air.” To
a large degree this is true–if you have properly prepared
and planned your dive and follow your plan.
ENRICHED AIR NITROX MIXTURES
Historically, there are two standard nitrox mixes,
32% oxygen and 36% oxygen, also called NOAA Nitrox I
and NOAA Nitrox II. Many basic nitrox courses teach
only the use of these two mixtures. In this NAUI course,
however, you will be qualified to plan dives using any
oxygen enriched air mixture from air up to 40% oxygen
(the upper limit that does not require special cleaning of
all equipment–see Chapter 6).
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
NAUI Nitrox Diver
44 Choosing the Best Nitrox Mix