Carbon Dioxide (CO2)
Carbon dioxide is colorless, odorless, and tasteless
in small quantities. It combines readily with water to
form carbonic acid, and at higher concentrations it has
an acidic taste and odor, which you can sometimes
sense when you sniff fizzing carbonated beverages. At
high concentrations (above about 10%), it can be
extremely toxic and cause convulsions and death.
Carbon dioxide is a natural byproduct of our respiration
and combustion of organic compounds.
The concentration of carbon dioxide in the atmosphere
actually varies depending on place and season,
and much of the concern about global warming is
due to the increasing fraction of carbon dioxide in the
atmosphere from burning fossil fuels and other sources.
The amount of carbon dioxide in the atmosphere was
about 280 parts per million (ppm) in the early 19th
century. Currently, sampling at Mauna Loa, which
because of its favorable location and altitude is considered
to be a reliable indicator, showed a carbon dioxide
content of 316 ppm in 1959 increasing to 373 ppm in
2002, an increase of 18%.
Helium (He) & Neon (Ne)
Both helium and neon are less dense than nitrogen.
They have a very low narcotic potential and so are possible
candidates to reduce both the nitrogen content and
the oxygen content of the breathing gas in deep diving.
Both are expensive; neon is very expensive. Helium is
the mixing gas of choice for diving deeper than the air
depth range. You will learn more about helium if you
venture into technical diving. “Trimix” refers to a mixture
of oxygen, helium, and nitrogen. Neon is not only
too expensive for general use; it is also very slowly
released from the body tissues and so requires long
decompression times.
HOW GASES BEHAVE
The interrelation of pressure, volume, and temperature
of gases are described by the “gas laws” or the Ideal
Gas Laws. You met them in your entry-level scuba
course. The separate laws were formulated experimentally
by many investigators over a period beginning in
the seventeenth century. Scientific laws are frequently
associated with the names of persons who first observed
and formulated them, and the rules of gas behavior are
no exception.
Although the gas laws were discovered independently,
they can actually be derived from the general
kinetic theory of gases, which is easy to visualize in a
simplified form. A gas is composed of molecules, which
are exceedingly small and separated by great distances
relative to their size. The molecules are in constant
random motion, traveling in straight lines until they
bounce off each other or their container. The pressure
of a gas is the measured impact force of its molecules
striking the container. The force of impact is related to
the speed and mass of the gas molecules. If we heat a
gas, we are adding energy, increasing the speed of the
molecules and the force of the impacts. The pressure
rises. The pressure also depends on the frequency of
impacts: more impacts means more force applied. If we
compress a gas, more molecules are in a given volume
and the density increases. As the density increases, the
frequency of impacts on a unit area increases, and the
pressure is higher.
10m / 33 ft 2 ata
30m / 99 ft
3 ata
4 ata
20m / 66 ft
1 ata
Surface
NAUI Nitrox Diver
18 Gases & Gas Mixtures
FIGURE 2-2: BOYLE’S LAW: PRESSURE AND VOLUME ARE
INVERSELY RELATED