Cited Passage
CHAPTER 2 — Underwater Physics 2-29
actually passing through the membrane at all times in both directions due to kinetic
activity, but more will move from the side of higher concentration to the side of
lower concentration.
Body tissues are permeable membranes. The rate of gas diffusion, which is related
to the difference in partial pressures, is an important consideration in determining
the uptake and elimination of gases in calculating decompression tables.
2-12.3 Humidity. Humidity is the amount of water vapor in gaseous atmospheres. Like
other gases, water vapor behaves in accordance with the gas laws. However,
unlike other gases encountered in diving, water vapor condenses to its liquid state
at temperatures normally encountered by man.
Humidity is related to the vapor pressure of water, and the maximum partial pres -
sure of water vapor in the gas is governed entirely by the temperature of the gas.
As the gas temperature increases, more molecules of water can be maintained in
the gas until a new equilibrium condition and higher maximum partial pressure are
established. As a gas cools, water vapor in the gas condenses until a lower partial
pressure condition exists regardless of the total pressure of the gas. The tempera -
ture at which a gas is saturated with water vapor is called the dewpoint .
In proper concentrations, water vapor in a diver’s breathing gas can be beneficial to
the diver. Water vapor moistens body tissues, thus keeping the diver comfort able.
As a condensing liquid, however, water vapor can freeze and block air passageways
in hoses and equipment, fog a diver’s faceplate, and corrode his equipment.
2-12.4 Gases in Liquids. When a gas comes in contact with a liquid, a portion of the gas
molecules enters into solution with the liquid. The gas is said to be dissolved in
the liquid. Solubility is vitally important because significant amounts of gases are
dissolved in body tissues at the pressures encountered in diving.
2-12.5 Solubility. Some gases are more soluble (capable of being dissolved) than others,
and some liquids and substances are better solvents (capable of dissolving another
substance) than others. For example, nitrogen is five times more soluble in fat than
it is in water.
Apart from the individual characteristics of the various gases and liquids, tempera -
ture and pressure greatly affect the quantity of gas that will be absorbed. Because a
diver is always operating under unusual conditions of pressure, understanding this
factor is particularly important.
2-12.6 Henry’s Law. Henry’s law states: “The amount of any given gas that will dissolve
in a liquid at a given temperature is directly proportional to the partial pressure of
that gas.” Because a large percentage of the human body is water, the law simply
states that as one dives deeper and deeper, more gas will dissolve in the body
tissues and that upon ascent, the dissolved gas must be released.
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