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Chapter 43
- 7
OXYGEN ENRICHED AIR
or
NITROX (EANx)
CONFUSION of TERMINOLOGY and JARGON ——— SPECIFY or DIE
40/60 = 40% O2 (EUROPE) or 60% O2 (USA)
NOAA 36% + 32% O2
—
max. 1.6 ATA O2 pressure
NURC, NC
—
max. 1.45 ATA
"
SWEDEN
—
max. 1.4 ATA
"
DAN (R. VANN)
—
max. 1.2 ATA
"
BUT O2 TOX = ?CNS, ?RESP, ?CO2. Specify which.
"EAD" = EQUIVALENT N
2
PRESSURE
Much of the technical diving now performed involves the use of nitrogen/oxygen mixtures in
which the oxygen concentration is greater than that of compressed air. Under these conditions it is
very important to specify exactly how much oxygen is being used. Such phrases as 40–60 or 60–40
are not only confusing but often misleading. In Europe 40–60 could imply 40% oxygen, whereas in
the USA it is more likely to imply 40% nitrogen.
The actual percentages used in technical diving do vary with different countries and establishments
but NOAA in the USA have chosen 32% oxygen and 36% oxygen as their two major mixes. These
should not be referred to as Nitrox 1 or Nitrox 2, as this could also be misleading.
The EANx refers to enriched air (nitrox) with the x = oxygen percentage. Thus EAN 32 should
mean 32% oxygen and not 32% nitrogen! Do not rely on jargon. Specify the exact mixture, in full.
Any EANx diving has a safe depth range less than air, due to oxygen toxicity.
The oxygen pressures that are considered acceptable vary with different authorities, and in many
cases there is confusion between the neurological oxygen toxicity (which can result in nausea,
vomiting, seizures, etc.) and respiratory oxygen toxicity, which tends to only occur with prolonged
exposure. Also, many of the pressures being quoted in the literature refer to the oxygen pressures
observed with rebreathinge quipment, when the carbon dioxide levels have not been measured –
complicating considerably the actual cause of symptoms. Most of the work carried out during
World War 2 and soon after, failed to measure the carbon dioxide levels and therefore their
conclusions regarding safe oxygen limits, are questionable.