Cautions when using Nitrox - Know your Maximum Depth Allowance!
Few
advances in diving have had a more profound impact during your lifetime
than the widespread availability of Enriched Air Nitrox. Nitrox
and air divers alike are typically aware of the longer bottom times
allowable while diving with Nitrox. But, I’d like to remind all
of you about the danger of oxygen toxicity associated with diving with
Nitrox.
Any percentage of oxygen can cause toxicity at a high
enough pressure. But, when diving on air, you would have to dive in
excess of 220 feet, to experience acute oxygen toxicity. The danger of
nitrox is that it brings the possibility of oxygen toxicity within
recreational diving depths.
In order to avoid exceeding the
safe depth limitations, nitrox divers are taught to use a table or
complete a maximum operating depth (MOD) formula before each dive:
(PPO2 / O2 percentage = MOD). Using this formula, a diver breathing
Nitrox 35 will have a maximum operating depth of 99 feet, or 4 atm,
when using the recreational limit of 1.4 PPO2: 1.4 / 0.35 = 4 atm, 33
feet x (4 atm - 1 atm--the one surrounding the earth) = 99 feet.
The same calculation for 40% Nitrox yields: 1.4 / 0.40 = 3.5 atm, 33
feet x (3.5 atm - 1 atm) = 82 1/2 feet.
Both the formula
and the nitrox tables are easy with proper training, and an easily
programmable nitrox dive computer can eliminate the need for
calculations altogether.
Nitrox divers should recognize that air
divers may not be aware of the MOD issue associated with using a
cylinder containing Nitrox and could get into trouble if they
inadvertently use a Nitrox cylinder.
All divers would
benefit from knowing that oxygen toxicity manifests as symptoms such as
visual changes (such as tunnel vision), ringing in the ears (tinnitus),
nausea, twitching (especially of the face), behavioural changes
(irritability, anxiety, confusion), and dizziness. If you see
such symptoms manifest in a dive buddy while at depth you may be able
to alleviate the symptoms by buddy breathing from your air source or
assisting the affected individual in a controlled ascent.