2
Chapter 2 Gases and Gas Mixtures 27
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.
Boyle’s Law: Pressure, Volume,
and Density
The relationship between pressure, volume, and
density of gases was studied by Sir Robert Boyle in the
seventeenth century. There are four variables that can be
altered in a gas sample–pressure, volume, temperature,
and quantity. Boyle fixed the amount of gas and its
temperature during his investigations. He found that
when he changed the pressure, the volume responded in
the opposite direction (Figure 2-2).
Boyle’s Law states: “At a constant temperature, the
volume of a gas varies inversely with absolute pressure,
while the density of a gas varies directly with absolute
pressure.” Expressed as a formula, for a given gas sample:
PV = K
where P is the absolute pressure; V is the volume;
and K is a constant.
As a working formula, we usually use:
P1V1 = P2V2
where the P and V are the absolute pressure and the
volume for any two sets of conditions (the “before” and
the “after”).
UNITS OF MEASUREMENT
There are several ways to measure
pressure. In diving, our concern is with
multiples of atmospheric pressure.
One atmosphere is the pressure of the air
at sea level. This is the unit most commonly
used in this book.
In the metric system (International System
or S.I.), a commonly used equivalent is
bars. One atmosphere equals 1.013 bar.
This is not significantly diferent from an
atmosphere of pressure, and the two are
generally used interchangeably.
Other equivalents can be used:
1 atmosphere = 760 milimeters of mercury
= 29.92 inches of mercury (the "weather
report" measure)
= 101.3 kilopascals (kPa)
= 1.013 bar
= 14.7 lbs/in2 (psi)
= 10 meters of seawater (msw)
= 33 feet of seawater (fsw)
Most of these equivalents are actually
close approximations. In all calculations,
10 msw = 1 bar.
Fresh water is 2.5% lighter than seawater.
You could use 10.25 meters or 34 feet of
fresh water as equivalent to an atmosphere
of pressure. The difference has little
physiological consequence in diving, and
in practice, instrument errors and other
approximations can easily outweigh the
2.5% difference.