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Chapter 6 Oxygen Precautions and Preparing Nitrox 69
LEARNING GOALS
Comprehension Objectives
Understand precautions that must be taken
when around oxygen.
Learn what the fire triangle is and how it applies
to nitrox use.
Understand what oxygen cleaning is and what
scuba equipment must be oxygen cleaned for
nitrox use.
Understand the “40% Rule”.
Learn how cylinders and valves are prepared
for nitrox use and how they are marked as
nitrox cylinders.
Understand the 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
(Figure 6-1). 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 sufficiently cooled, as by dowsing it with water,
it will go out. If a carbon dioxide fire extinguisher is used
to remove the oxygen around the fire, the fire goes out.
But as the fraction and partial pressure of oxygen
increase, many materials that do not burn under normal
conditions will burn if ignited. Also, any fuels will ignite
more easily. Materials that are of little concern in air may
become quite flammable in an oxygen-rich environment
and even more so in a high-pressure oxygen-rich
environment. Petroleum-based products and other
hydrocarbons are of special concern because they ignite
extremely easily in a pure oxygen environment. Other
materials, although they may not become flammable, will
oxidize and degrade much more rapidly in the presence
of hyperbaric oxygen. When high pressures of oxygen are
going to be present, extra care must be taken to prevent
the fire triangle from occurring.
Figure 6-1 The fire triangle.