LEARNING GOALS
In this chapter, you will learn:
• Precautions that must be taken when around
oxygen.
• What the fire triangle is and how it applies to
nitrox use.
• What oxygen cleaning is and what scuba
equipment must be oxygen cleaned for nitrox use.
• About the “40% Rule.”
• How cylinders and valves are prepared for nitrox
use and how they are marked as nitrox cylinders.
• Five methods that are used to make nitrox and
which are most common.
As a recreational nitrox diver, you are not likely to
be around pure oxygen or even mixtures that are
extremely rich in oxygen. Safe handling of pure oxygen
will be the responsibility of the blending technician who
fills your cylinder. Nevertheless, if you are to dive intelligently
with oxygen-enriched air, you should have some
knowledge of the hazards of oxygen and the precautions
that must be taken when working with or around it.
Why, for instance, must your cylinder be a dedicated
enriched air nitrox cylinder, but your regulator does not
have to be a special nitrox regulator? Why does the fill
station take so long to deliver your requested nitrox blend
when, if it were an air fill, your fully-charged cylinder
would be ready in a relatively short time? Why is the
upper limit for recreational nitrox set at 40% oxygen?
Actually, you may have occasion to use pure oxygen
at some time in your diving career. As you advance your
diving skills and knowledge, an adjunct to a rescue
course or a leadership course may be certification as an
oxygen provider. This will qualify you to provide oxygen
first aid to persons suffering diving maladies such as
decompression sickness or arterial gas embolism. An
important part of that course will be the safety measures
to be taken when around pure oxygen.
OXYGEN HANDLING
As you learned in an earlier chapter, oxygen supports
combustion and combines readily–sometimes
aggressively or violently–with almost anything that is
not already oxidized. Slower oxidation can destroy a
material over time–solid iron turns into rust. Or, oxidation
can be rapid enough to produce extreme heat and
visible light, which we call burning or fire or combustion.
Sometimes this combustion can be so violently
rapid that it is an explosion. There are other elements
and materials that are reactive enough to combine
aggressively, produce extreme heat, and “burn,” but
they are not so commonly found. Oxygen, on the other
hand, is ever-present, and in enriched air nitrox it is
present in higher than normal amounts.
In the Earth’s atmosphere, with its oxygen fraction
of 0.21 ata, and at normal temperatures, materials do
not spontaneously ignite and burn. A source of ignition
(heat) is required to initiate burning. After a material
(fuel) is ignited, then the fire itself provides the heat to
sustain burning.
Firefighters use the concept of the “fire triangle.”
In order for a fire to occur or continue, three things must
be present: fuel, oxygen, and heat. If any one of these is
absent, a fire will not start. If any of the three is
removed, the fire will be extinguished. When a fire has
consumed all available fuel, burning ceases. If the fire is
NAUI Nitrox Diver
68 Oxygen Precautions and Preparing Nitrox
Fuel
Heat
Oxygen
FIGURE 6-1: THE FIRE TRIANGLE