2
28 NAUI Nitrox Diver
10m / 33 ft 2 ata
A commonplace example Boyle’s Law is the action
of breathing. The diaphragm is a muscle that is located
just below our lungs. When we inhale, we move the
diaphragm downward, and the lungs increase in volume.
This decreases the pressure inside our lungs so that it is
less than the outside pressure. As a result, air flows into
our lungs.
There are three important aspects of Boyle’s Law for
divers:
1. Large quantities of gas can be compressed into
cylinders. If the pressure is doubled, twice as
much gas will be in the cylinder.
2. Inhaled air will be denser with an increase in
depth. When diving, the regulator delivers
breathing air at the ambient pressure. Even at
recreational depths, an alert diver can notice that
the air feels “thicker” beyond a certain depth.
3. During ascent, the reduction of pressure will
cause the gas in our lungs to expand. If you hold
your breath, not allowing the excess gas to be
exhaled, you risk rupturing your lungs–with
possibly calamitous consequences.
You may also remember the other gas laws that relate
temperature to volume (if the pressure is held constant)
and temperature to pressure (if the volume is held
constant). These have less obvious relevance in enriched
air nitrox diving, but you will get to know them well if
you take the NAUI Master Scuba Diver course.
Henry’s Law: The Solubility of Gases
Gases dissolve in liquids. Solids dissolve in liquids too.
You know that sugar dissolves in water because you can
see it happen. Most persons are less aware that gases also
dissolve, although carbonated beverages are a perfect
everyday example of gas solubility. Some gases are more
soluble in a liquid than other gases, and some liquids are
better solvents of a gas than other liquids.
When the pressure of a gas on a liquid is increased,
more of the gas will dissolve in the liquid until the partial
pressure of the dissolved gas (or gas tension) equals
the impinging gas partial pressure. In other words, the
amount of a gas that will dissolve in a liquid is directly
related to the pressure of the gas on the liquid.
This is described in Henry’s Law: “The amount
of any given gas that will dissolve in a liquid at a given
temperature is a function of the partial pressure of the
gas that is in contact with the liquid and the solubility
coefficient of the gas in the particular liquid.” According
to Henry’s law, the relationship is linear. If one quantity
of gas will dissolve at one atmosphere of pressure, then
three quantities of gas will dissolve at three atmospheres
(Figure 2-3).
Temperature also affects the quantity of a gas that
will be absorbed by a liquid. The solubility of a gas is
inversely related to the temperature – the higher the
temperature, the lower the solubility and vice versa.
The solubility of a gas in a liquid depends on
temperature and the partial pressure of the gas over the
liquid. It also is governed by the nature of the solvent and
the nature of the gas. Nitrogen, for instance, is about five
times more soluble in fatty tissue than in watery tissue.
This difference in solubility is something that must be
considered in decompression theory.
Solubility of Gases
1. Equilibrium =
1 ATM
2. Non-equilibrium
with pressure
increased
3. Equilibrium at
Increased Pressure
4. Non-equilibrium
with pressure
decreased
The amount of a gas which dissolves in a liquid is proportional
to the pressure of the gas in contact with the liquid
Figure 2-3 Henry’s Law: the solubility of a gas is
directly related to the pressure of the gas.
30m / 99 ft
3 ata
4 ata
20m / 66 ft
1 ata
Surface
Figure 2-2 Boyle’s Law: pressure and volume are
inversely related.