Some divers routinely switch
between air and Nitrox, using air
on the first dive to go deep, then
Nitrox later in the day to extend
bottom time. To monitor decompression,
some switch back and
forth on an air/Nitrox computer,
while others follow the air tables all
the way. Does it make sense?
At the 2000 Divers Alert Network (DAN) Industry Nitrox
Conference (see January 2003
Undercurrent), consensus was that
when breathing Enriched Air
Nitrox (EAN or just Nitrox) mixes
with 40 percent or less oxygen, it's
not necessary to use dedicated,
specially-cleaned tanks or regulators.
Furthermore, many computers
also offer air or Nitrox dive
modes, allowing the user to preset the percentage of oxygen in the
breathing mix.
The most tangible advantage
of air is when a diver goes below
the maximum operational depth
for a given Nitrox mix. For
instance, breathing Nitrox 32
below 130 fsw puts the diver at risk
for developing oxygen toxicity,
which can cause deadly convulsions. So some divers consider air
the safer choice for deep dives. But
Tim O'Leary, director of technical
operations for NAUI, disagrees.
He told Undercurrent that "nobody
should use air on deep dives"
because of the risk of nitrogen narcosis.
Bret Gilliam, who runs
Technical Diving International,
believes the risk of narcosis is
acceptable for sport divers down to
145 fsw, whether on air or Nitrox.
His agency allows instructors to
train divers to reach 180 fsw on air.
However, Gilliam told Undercurrent that susceptibility to narcosis below
130 fsw "is so individual, it should
be individually determined, preferably
under the supervision of an
experienced technical instructor."
As an alternative to deep diving
on air, Gilliam points out that
most Nitrox stations can provide a
custom mix, such as 30% oxygen,
that will allow a diver to exceed
130 fsw. On subsequent, shallower
dives the oxygen percentage can
be increased to allow for less
buildup of nitrogen in the system.
"Why switch to air at all," he asks,
"when you can have your Nitrox
blended to match your maximum
depth on each dive?"
When a dive computer is in
the Nitrox mode, it uses a formula to calculate the potential for oxygen
poisoning. "It's a hypothetical
model, not a true measure of oxygen
buildup in the diver's body,"
says Bill Hamilton, a physiologist
with four decades of experience in
decompression and breathing
gases. Hamilton helped develop
limits for NOAA that today are the
basis for most recreational diving
calculations.
During surface intervals, the
body rids itself of excess oxygen,
but there's debate among the
experts about how rapidly this
occurs. Hamilton points out that
"Oxygen is very fickle and different
people get rid of it at different
rates." So it's best to err on the side
of caution, as the computer manufacturers
have done with their
algorithms.
Computers that handle Nitrox
display the partial pressure of oxygen
(P02) because too much oxygen
at depth is toxic to a diver.
NOAA and most physiologists
agree that a PO2 of 1.6 atm is as
high as a scuba diver should ever
breathe in the water. Many, including
Hamilton, say 1.4 atm PO2 is a
safer practical upper limit, and
that's the limit at which most
Nitrox computers begin displaying
P02 warnings.
Gilliam and the other industry experts agree that "if you're diving
a single 80 cu. ft. aluminum tank
with Nitrox and following your
computer, you'll hit your nodecompression
limit on any given
dive before you develop oxygen
toxicity." Yet it's possible you may
get a P02 warning on your computer.
If such a warning occurs
underwater, you should immediately
begin a slow ascent in accordance
with your Nitrox training,
then take a surface interval long
enough to allow the PO2 to drop
down to safe levels (usually about
two hours).
Some divers who get such
warnings after doing repetitive
dives on Nitrox switch to air to slow
down oxygen accumulation. When
doing this, it's best to keep one's
computer in the Nitrox mode, so
you can continue to track the P02
display. Some computers, such as
Suuntos, won't allow you to switch
to air mode for a day or two after
diving in Nitrox mode. If you
decide to breathe air after Nitrox,
Jim Clymer, of Suunto's Technical
Department, recommends setting
the Nitrox mode for 21 percent,
the equivalent of air.
"Just remember," warns
Hamilton, "that you are dealing
with toxicity, not accumulation of
oxygen, and the recovery period is
not really known."