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Chapter 2 Gases and Gas Mixtures 29
Henry’s Law addresses the quantity of a gas that will
dissolve, but it does not describe the rate at which the gas
will dissolve. Whenever the pressure of a gas on a liquid
is increased, molecules of gas begin to diffuse into the
liquid, and we say ingassing occurs. In the beginning, the
gas moves rapidly into solution, driven by the high partial
pressure of the gas on the liquid compared to the gas
tension of the dissolved gas – the high pressure gradient.
As the gas tension increases, the pressure gradient
becomes less, and the gas dissolves less rapidly until
equilibrium is reached and ingassing stops (Figure 2-4).
This is known as saturation. When the pressure of the gas
on the liquid is reduced, offgassing occurs, again more
rapidly at first, then slowing until equilibrium is reached.
This can be seen by looking at the surface interval credit
table on a set of standard dive tables. The diver offgasses
rapidly for the first two hours or so after surfacing, then
progressively more slowly until offgassing is complete.
Dalton’s Law: Partial Pressure in
Gas Mixtures
An understanding of partial pressure and its
consequences is probably the most important concept
to grasp for safe diving with enriched air nitrox. The
partial pressure of a gas in a mix is the portion of the
total pressure exerted by that gas. Whether we are at the
surface or diving, our body responds to each gas in a gas
mixture according to its partial pressure. With oxygenenriched
air as well as with the more exotic mixtures of
technical diving, we are manipulating the gas percentages,
and therefore the partial pressures, of the gas mixtures
that we choose to breathe. We must know what we are
doing and be able to plan safe limits to our diving.
In the balance of this chapter, you will learn about
partial pressure and how to determine the partial pressure
of any gas in your breathing mixture at any depth. In the
next chapter, we will explore how nitrogen and oxygen
affect your body at different pressures.
For any single, pure gas, the pressure of the gas
is the total pressure. Its effect upon us or in chemical
reactions, its solubility, and so on are directly related to
its pressure. Oxygen, for instance, supports our life and
also combustion. If there is too little oxygen, even pure
oxygen, we will lose consciousness and die, and materials
will burn poorly if at all. If the oxygen pressure is high, it
can be toxic to us and objects will burn furiously.
In any mixture of gases, such as air, the total pressure
of the mixture is equal to the sum of the individual
pressures exerted by each individual gas. Physically
and chemically, each individual gas acts according to
its partial pressure. This was first observed in the early
nineteenth century by the English chemist John Dalton.
Dalton’s Law states: “The total pressure exerted by
a mixture of gases is equal to the sum of the pressures
that would be exerted by each of the gases if it alone were
present and occupied the volume.” In other words, the
whole is equal to the sum of the parts (Figure 2-5). The
pressure exerted by each component gas is termed the
partial pressure of that gas. Expressed mathematically:
Ptotal = P1 + P2 + P3 + ... + Pn
where Ptotal is the total pressure of the gas mixture,
and P1, P2, etc. are the partial pressures of each
component gas.
Ingassing
Offgassing
Gas Tension
Time
Figure 2-4 Rate of ingassing and offgassing is
related to the pressure gradient.
Figure 2-5 Dalton’s Law: as the total pressure
increases or decreases, the partial
pressure of each component gas
increases or decreases proportionately.