6
70 NAUI Nitrox Diver
EQUIPMENT CONSIDERATIONS
The ways that nitrox is prepared will be discussed
later, but the most common mixing method involves
introduction of pure oxygen at pressure. All equipment
systems that will be so exposed must be specially
prepared in order to withstand the oxygen. Hydrocarbons
and petroleum-based products must be avoided. This
includes not only petroleum-based compressor lubricants
but also the silicone lubricants normally used in scuba
air systems. Neoprene, silicone “rubber,” plastic and
metal shavings, even finely divided particulate matter
all become potential fuels for a fire in an oxygenrich
environment, especially one at high pressure. A
spectacular laboratory demonstration is to burn steel
wool in a high-oxygen environment.
OXYGEN CLEANING
All equipment to be used with pure oxygen must be
cleaned for oxygen service. The equipment itself must
be oxygen clean, and oxygen compatible parts must be
used in order to minimize the risk of fire or destruction
by the oxygen. Oxygen clean means that any potentially
flammable contaminants have been removed. Oxygen
compatible means that the materials used in the various
parts in the system are not flammable or readily
oxidizable in the presence of high-pressure or pure
oxygen. This special oxygen cleaning can be complex,
involving an initial cleaning using solvents that remove
all hydrocarbons and then the use of special oxygencompatible
lubricants, o-rings, seats, seals, flexible
tubing, etc. as the system is reassembled. Even though
certain materials may not be flammable in the presence
of high-pressure oxygen, more durable materials that
do not so rapidly degrade will be substituted. “Formal
oxygen cleaning,” which is required in many industrial
and governmental (e.g., NASA) applications, requires
adherence to very strict procedures as well as careful
documentation. After cleaning, future contamination
must be carefully avoided, or the whole oxygen cleaning
process will have to be repeated. Even skin oil from
a finger can contaminate formally oxygen-cleaned
equipment and necessitate re-cleaning.
Care must also be taken to avoid sources of ignition,
the third side of the fire triangle. A sudden increase in
system pressure could elevate the temperature in the
system sufficiently to cause ignition of any contaminants.
Valves should be opened slowly to prevent a sudden
increase in pressure with an accompanying rapid
temperature rise. Even a static spark from a rug or
opening a Velcro® closure could cause ignition in a pure
oxygen environment.
The 40% Rule
When is oxygen cleaning necessary? As a recreational
nitrox diver, you will dive with an ordinary, wellmaintained
regulator, but your cylinder will have been
cleaned for designated service as a nitrox cylinder, and it
will be clearly marked as an enriched-air nitrox cylinder.
The general rule of thumb is that any equipment
that is to be used with pure oxygen or an oxygen level
that is above 40% (and at a pressure above 200 psi) must
be cleaned for oxygen service and have only oxygencompatible
parts.
This so-called “40% Rule” has gained acceptance
over time although there are no published test results
to show that 40% is the absolute limit above which
oxygen cleaning is unequivocally necessary. No
definitive industry-wide standards exist for handling
gas mixes intermediate between air and 100% oxygen.
The U.S. Navy handles gas mixes up to 40% oxygen the
same as air. The U.S. Occupational Safety and Health
Administration has set a 40% break-point above which
special cleaning is required. NAUI and the recreational
diving industry as a whole have accepted the 40% rule,
and EAN40 is the accepted upper-limit fraction of
oxygen in recreational nitrox diving. Forty percent seems
to be a valid limit in the sense that at or below this level
oxygen compatibility problems have not occurred. Above
40% one is in a gray area for which there is seemingly
no firm data. Flash combustion has occurred above
40% oxygen, so it is better to err on the side of safety.
Technical divers using higher oxygen fractions, such as
in a decompression gas mix, use scuba equipment that is
specially serviced and is fully oxygen clean and oxygen
compatible for these special applications.
Equipment Preparation
So what about your own regulator and your cylinder?
Your regulator will not encounter an oxygen level above
40%, so the rule applies. Normal maintenance service
is sufficient for your regulator. Of course, as with all of
your equipment, you should take care of your regulator,
rinsing it after dives, keeping it clean, avoiding extreme
heat, and having it serviced regularly. The same general
cautionary statement is true of all scuba equipment that
will be used with nitrox up to EAN40 – submersible
pressure gauges, BC inflators, BCs, etc. For all scuba gear,
cleaner is better, and the prudent diver takes good care of
his or her life support equipment.
Your cylinder is a different matter. The most
common way to prepare nitrox is to introduce an initial
quantity of pure oxygen into an empty cylinder, which is
then topped off to service pressure with specially cleaned
air. Every time your nitrox cylinder is filled in this
manner, it is subjected to pure oxygen at high pressure.
So, your cylinder must be “dedicated” to nitrox use