Chapter 2-Gases & Gas Mixtures
17
We normally simplify the numbers and say that
oxygen is 0.21 or 21% of atmospheric air. We will also
include argon, as an inert gas, with the nitrogen and
say that nitrogen is 0.79 or 79% of air. The amount of
carbon dioxide varies somewhat around the world (there
is more present in the more industrialized northern
hemisphere), and while carbon dioxide has a significant
role in general human physiology as well as the physiology
of diving, at only about 350 parts per million (ppm)
of air, it does not enter into nitrox calculations. The
“others” category includes neon, helium, krypton, sulfur
dioxide, methane, nitrous oxide, etc.
SOME FACTS ABOUT INDIVIDUAL GASES:
Oxygen (O2)
Oxygen is one of the most abundant elements on
earth. It is present not only as a gas–as free oxygen in the
atmosphere and as an element in atmospheric carbon
dioxide; it is a constituent in many other compounds,
such as silicon dioxide (quartz and sand). By weight,
almost half of the composition of the earth’s outer crust is
oxygen in various compounds. Oxygen is highly reactive
and combines readily with most other elements. As a gas,
it is colorless, odorless, and tasteless. Oxygen is our lifesupporting
gas and is the essential component of all
breathing gas mixtures. It is used in respiration, combining
with glucose in our cells to produce carbon dioxide
and water. Although it is essential for body metabolism,
when it is breathed in excessive amounts under pressure,
oxygen can be very toxic. Oxygen supports combustion,
and many substances will spontaneously ignite or burn
furiously in a pure or high oxygen environment.
Much of what you will learn in this course is about
the risks of oxygen, the proper use of oxygen, setting
appropriate limits by controlling the oxygen fraction in
the nitrox you use and the depth to which you dive,
safety considerations in oxygen handling, and care of
scuba and air-fill equipment that will be exposed to
high concentrations of oxygen.
Nitrogen (N2)
Nitrogen is a largely inert gas. It is not used in
metabolism and serves essentially to dilute the oxygen
in the air we breathe. It is also colorless, odorless, and
tasteless. Although it does not combine easily with
other elements, it is a component in many organic
compounds and is in all living organisms. When
breathed at higher pressures, it has a pronounced
anesthetic effect referred to as nitrogen narcosis.
Nitrogen narcosis is one of the reasons that recreational
diving is limited to about 40 meters (130 feet).
The mechanisms of narcosis are only partly understood,
and several factors seem to be involved–including psychological
predisposition, stress, and anxiety, but hyperbaric
nitrogen is the major contributor. Narcosis can impair a
diver’s ability to function at greater depths, and severity
increases with depth. A favorite tongue-in-cheek “law” of
divers is Martini’s Law: “Breathing air, each 15 meters (50
feet) of depth is equivalent to drinking one dry martini on
an empty stomach.” The symptoms of nitrogen narcosis
also parallel those often seen in alcoholic intoxication:
inability to concentrate and multi-task, easy distraction,
loss of motor ability and dexterity, mood changes, etc.
Sensitivity to narcosis varies from person to person and
time to time. There is no certain evidence that breathing
oxygen-enriched air reduces the risk of narcosis.
Theoretically it might, but clinical studies have produced
mixed results, perhaps because the individual differences
among human subjects outweigh the physics.
Argon (Ar)
Argon makes up about 1% of air. As noted above, it
is included as part of the nitrogen component in
enriched air nitrox calculations. Argon is one of the
“noble gases” meaning it is virtually completely inert,
not combining with other elements. (The other noble
gases are helium, neon, krypton, xenon, and radon.) It
has a narcotic potency slightly over twice that of nitrogen.
Because argon has lower heat conductivity than air,
it is often used as an inflation gas for dry suits to
improve their insulating properties.
Gases & Gas Mixtures