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Chapter 4 Choosing the Best Nitrox Mix 49
Imperial example: If you started with 3000 psi in your
scuba cylinder:
1. 3000 psi
2. 3000 psi ÷ 3 = 1000 psi
3. 3000 psi − 1000 psi = 2000 psi turn
around pressure
What if I have 2900 psi in my scuba cylinder?
1. Round down to 2700 psi (rounding down
to the next number easily divisible by three)
2. 2700 psi ÷ 3 - 900 psi
3. 2900 psi − 900 psi = 2000 psi turn pressure
These turn pressures are viewed as minimums,
depending of course on the environment being dived.
A safe dive plan builds in conservatism where it makes
sense. You may decide to turn your dive sooner due to
the fact you are diving at an unfamiliar dive site, the
current is strong, or the team is becoming tired or cold.
Get back “home” to the exit point with enough gas and
time to make a safe ascent and proper safety stop before
you surface.
Thermal
Thermal considerations are environmentally driven,
but are a concern in all but the most ideal conditions
encountered when diving. Several factors come into
play when choosing your exposure protection for the
dive. These include water temperature at the surface
and at depth, the dive time, air temperature and
personal comfort. Gather this information and decide
what thickness wetsuit or perhaps a drysuit is the most
appropriate thermal protection for your planned dive.
Wetsuit diving options:
• Wetsuit thickness or dive skin
• Vest
• Hood
• Foot protection
• Gloves
• Active (electric) heating
• Layering
Drysuit diving options:
• Drysuit material
• Foot protection if not built in
• Undergarment thickness
• Hood
• Gloves (wet or dry)
• Active heating
• Layering
Your thermal status and your decompression
efficiency go hand in hand. A cold diver doesn’t
eliminate nitrogen as efficiently as a comfortable diver.
Plan to be comfortable during all parts of your dive.
Plan your exposure protection to last for the entire dive.
Being cold on a dive is a distraction, is uncomfortable
and may predispose the diver to potential decompression
sickness issues. Cold may turn your dive into an exercise
in endurance, which takes the fun out of the experience.
Ideal body temperature is the goal, and a diver who is too
warm is just as problematic as a diver who is too cold.
Mission
The mission is the primary objective for the dive. The
mission may be a simple one, such as a local dive to
continue your dive education or take some pictures or
video with your new underwater camera. Perhaps you
are planning to travel internationally to explore World
War II wrecks or to share the water with massive pelagic
animals. Does diving off of a sunny, warm beach sound
like the perfect dive trip to you? When choosing your
mission remember to consider the training, experience
and safety mindset of the dive team. The goals you have
may require further training and experience. The NAUI
course offerings can help you achieve your diving goals.
Contact your local NAUI affiliated dive center or NAUI
Headquarters who can help find an instructor who is
right for you.
Logistics
With the mission in place you can begin working on
the logistics — the planning, implementation and
coordination of the details of the whole mission/dive.
This may require long-term as well as short-term
planning. Areas of concern will include:
• Long Term Planning
• General Logistics
• Diving Logistics
• Short Term Planning
• Personal Logistics
Creating a thorough checklist can help ensure that
you remember the details. At the nitrox level, ensuring
that your chosen dive destination provides nitrox and
supports nitrox diving by having appropriate mixing,
service and analysis capabilities may be all you need that
is different from a facility to typically provides air only.
See Appendix for a sample checklist.
ENRICHED AIR NITROX MIXTURES
Historically, there are two standard nitrox mixes, 32%
oxygen and 36% oxygen, sometimes 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).