must be cleaned for oxygen service and have only oxygen
compatible 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. This
means that it will be cleaned before it is used as a nitrox
cylinder. To prepare a nitrox cylinder, the interior of the
cylinder is first washed with a solvent to remove all
hydrocarbons and particulates. All traces of the solvent
are then flushed out. The cylinder valve, through which
the oxygen must pass, is disassembled, thoroughly
cleaned, and then reassembled using oxygen-compatible
lubricants, o-rings, seats, and seals. After being cleaned
for use with oxygen-enriched air, the cylinder will be
labeled to clearly identify it as a nitrox cylinder.
The original “standard” identification for an
enriched air nitrox cylinder is a yellow cylinder with the
top painted green down to ten centimeters (four inches)
below the shoulder of the cylinder and with an identifying
EANx label stenciled onto the cylinder. (If the cylinder has
been more formally cleaned for service with higher than
40% oxygen mixtures, it will be labeled as such.)
Many cylinders that are put into nitrox service are
not yellow but some other color, so the more common
standard identification labeling is a “nitrox” decal label
that encircles the cylinder just below the shoulder. The
label has a green band four inches wide that is bordered
above and below with a narrow yellow band. An identifier
such as “Nitrox” or “Enriched Air Nitrox” is printed
in yellow on the green band.
NAUI Nitrox Diver
70 Oxygen Precautions and Preparing Nitrox
FIGURE 6-2: NITROX CYLINDERS ARE SPECIALLY CLEANDED AND
MARKED FOR EANX SERVICE