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What is Nitrox?
Nitrox is here to stay, but it's still a hot issue within the diving
community. The debate is continuing among diving professionals,
and we're still face quite a few misconceptions. We will try to
clarify some of the principles, logistic concerns and concepts e
and hopefully answer the most commonly asked questions regarding
nitrox used for recreational diving. Nitrox is nothing new. It has
been used for military, commercial and research diving for more
than thirty years. What is new is recreational diving with nitrox.
We already have numerous labels such as "Enriched Air Nitrox",
"EANx", "Safe Air", "Nitrox" and others.
Nitrox, in simplest terms, is merely a mixture of nitrogen gas and
oxygen gas. Air, therefore, is a naturally occurring nitrox for
all practical purposes. In diving, however, "nitrox" usually
means a nitrogen-oxygen mix other than normoxic air, normally with
more than 21% oxygen.
To avoid confusion over the various possible nitrox mixes one can
create, we frequently refer to a Nitrox mix with a number reflecting
the oxygen content, such as "Nitrox 36". This would mean
a nitrox mix of 36% oxygen and 64% nitrogen. The common names, "Nitrox
I" / "EANx32" and "Nitrox II" / "EANx36",
refer to 32% and 36% oxygen mixtures, respectively. These designations
were created by the U.S. National Oceanic and Atmospheric Administration
(NOAA), who chose to standardize and identify the most commonly
used mixes of Nitrox gas.
The Purpose of Nitrox
As every certified diver knows, decompression limits and the potential
for decompression illness are caused by the inert nitrogen component
of breathing air. Nitrogen also causes nitrogen narcosis at deeper
depths. Nitrox was conceived as a way to reduce the quantity of
nitrogen breathed during a dive.
By replacing nitrogen in the breathing gas with oxygen, the diver’s
body absorbs less nitrogen for a given depth and time. Since the
body metabolizes oxygen, in theory oxygen doesn’t contribute
to decompression illness, so a diver breathing EANx32, for example,
should have longer no-decompression limits at a given depth than
when breathing air. If decompression stops become necessary, a shorter
decompression schedule would be required. Alternatively, the diver
could base his decompression schedule as though he was breathing
air, with the reduced nitrogen acting as an extra safety factor.
In either case, nitrogen narcosis should theoretically be reduced
somewhat compared to breathing air at the same depth.
Nitrox is not intended for extremely deep diving. As we will see,
EANx32 becomes unsafe at depths not much greater than the recreational
maximum limit of 40 meters, with higher oxygen mixes limited to
even shallower maximum depths. Sometimes nitrox becomes confused
as a deep diving gas because commercial and technical divers frequently
use nitrox as a decompression gas following deep dives. However,
these divers don’t begin using nitrox until returning to shallower
depths.
Is it safe?
Since the start more than thirty years ago, nitrox has been used
successfully by closely supervised and well-trained military, scientific,
commercial and technical divers. Though it hasn’t had as much
formal testing as air, the nitrox safety record has been very good.
Doppler tests have been made, with low or no detectable bubble formation
when diving within table limits. In working dives, nitrox isn’t
associated with any higher DCI rate than diving with air. Used properly
there doesn’t appear to be any greater risk diving with nitrox
than diving with air.
Using Nitrox
The key words are "when used properly". Diving with nitrox
provides longer no-decompression times compared to diving with air,
but its use requires training to account for considerations not
associated with recreational diving with air. The first consideration
is that while there is only one type of air there are potentially
78 mixes of nitrox with more than 21% oxygen. (Nitrox calculations
normally assume that plus or minus 1% variation is tolerable in
a nitrox mix, so fractions of a percent are not treated as separate
mixes. Nitrox with 31.6%, 32% and 32.3% oxygen are all considered
EANx32). Each of the 78 mixes differs in no-decompression limits
and decompression schedules.
The most common way to handle this is to convert the actual depth
into an Equivalent Air Depth (EAD) that may be used with air tables.
This is accomplished by using a simple formula. For example, at
a depth of 25 meters using EANx36, the EAD would be 18,4 meters;
the diver would use the no-decompression limits from that depth
(rounded to 20 meters). Using the EAD concept, any nitrox mix can
be used with air tables. In the above example, nitrox’s decompression
advantage become immediately noticeable: 45 minutes no-decompression
time is available at 25 meters, compared to only 29 minutes using
air, depending upon the air table used. Alternatively, the diver
could plan the dive as an air dive, creating a 16-minute safety
factor.
To simplify nitrox use, nitrox tables have been developed. NOAA
released tables for EANx32 in 1978, and for EANx36 in the early
1980s; these tables are used exactly like the standard U.S. Navy
tables. In fact, these tables are simply the U.S. Navy tables rewritten
with the actual depth replaced by the calculated EAD. Besides these,
other organizations, such as various navies around the world, have
nitrox tables and finally PADI’s RDP32 and RDP36 where introduced
to recreational divers in 1995/96. In using nitrox tables the diver
must be sure that he uses the appropriate table for the particular
mix he will be breathing.
Oxygen Toxicity
Unfortunately, nitrox’s nitrogen advantages are gained only
at the price of oxygen disadvantages. When using nitrox, exposure
to dangerously high oxygen partial pressures becomes possible at
depths far shallower than with air. Nitrox mixtures with more than
32% oxygen cannot even be taken to the 40-meter depth limit for
recreational diving.
The problem is Central Nervous System (CNS) oxygen toxicity, which,
depending upon exposure duration, becomes a potential risk when
oxygen partial pressures exceed critical limits. A case of CNS oxygen
toxicity causes the diver to convulse and lose consciousness, and
may or may not be proceeded by warning symptoms. Convulsions are
not dangerous themselves, but an unconscious diver will lose his
regulator and drown.
Critical oxygen limits vary drastically with partial pressure.
A diver can tolerate .6 Ata oxygen partial pressure for up to twelve
hours without significant risk of CNS symptoms. At 1.6 Ata, which
is the maximum acceptable p.p., the time is about 45 minutes, possibly
shorter than the no-decompression limit of the equivalent air depth.
The nitrox diver monitors his p.p./time allowances to make certain
he doesn’t exceed his "oxygen clock", in much the
same way he watches no-decompression limits. Minimum surface intervals
must be noted as well.
As the oxygen percentage in nitrox increases, the depth at which
the critical 1.6 limit is reached rises. When breathing air the
diver reaches 1.6 Ata at 66.5 meters. Breathing EANx32 1.6 Ata is
reached at 40.3 meters; and with EANx36, 34,6 meters. Below these
depths, the risk of CNS oxygen toxicity goes up dramatically.
Equipment Considerations
With the increase in technical diving and nitrox use, there have
been quite a few conflicting reports about whether standard scuba
equipment is compatible with nitrox.
NOAA’s policies since the 70s have been:
- Standard air equipment, except the tank, may be used with any
nitrox mix, which has 40% or less oxygen without any modification.
- Equipment being used with mixes with more than 40 % oxygen,
OR used for oxygen mixing must be oxygen compatible.
This means that for nitrox mixes with 40 % or less oxygen, which
are the most common mixes, a nitrox diver can use the same equipment
that he uses for air diving. The tank is an exception to the 40
% guideline, and needs to be oxygen compatible regardless of oxygen
content in the nitrox. The reason the tank must be oxygen compatible
is that a common method for mixing nitrox is partial pressure mixing.
This is done by adding 100 % oxygen to the cylinder, then topping
of with air that’s filtered to oxygen compatible standards.
Even if the final mix may be less than 40 % oxygen, exposure to
100 % oxygen during mixing requires oxygen compatibility. Besides
oxygen compatibility, nitrox tanks should be clearly marked to avoid
confusion with air tanks. This is important considering that using
a mix with more than 32 % oxygen, a diver can exceed the maximum
oxygen depth limit while within the traditional 40-meter recreational
depth limit.
Nitrox also requires at least one specialized piece of equipment:
the oxygen analyzer. While the nitrox diver doesn’t need to
own one, the diver must confirm the mix’s oxygen content personally
after the tank has been filled. This safety procedure ensures use
of the correct EADs and calculation of oxygen p.p.
Mixing Nitrox
Mixing nitrox isn’t as simple as pumping compressed air. In
fact, while the air in a typical dive store meets the standards
for compressed air diving, it frequently does not meet the higher
standards required for mixing nitrox. This means that even an oxygen
compatible cylinder with oxygen already in it cannot simply be topped
off with air from a typical dive store compressed air system. Doing
so actually creates a risk of fire or explosion. To avoid these
complications, nitrox diver training emphasizes these points; the
nitrox diver learns that the typical dive store isn’t capable
of providing nitrox (yet). Specialized dive stores and operations
have and use appropriate nitrox mixing equipment. The point is that
safe nitrox mixing requires special equipment, and like nitrox diving,
requires special training.
Nitrox Courses
 Diving
with a Nitrox requires training to account for considerations not
normally associated with recreational air diving. PADI’s Enriched
Air Nitrox Course addresses these considerations and teaches
you how to benefit the most by diving with Nitrox. In addition the
PADI Enriched Air Instructor
Course teaches the PADI Instructor how to successfully organize,
plan and market Nitrox training for recreational divers.
As a certified Nitrox Diver you can enjoy Nitrox
Diving on our day
trips, adding a new dimension to your diving adventures.
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