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. 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
Chapter 2-Gases & Gas Mixtures
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. 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.
Dalton’s Law can be expressed another way: “The
partial pressure of any component gas in a mixture is
the fraction of that gas in the mixture times the total
gas pressure.” Expressed as a formula:
Pg = Fg x Ptotal
Gases & Gas Mixtures
21
FIGURE 2-5: DALTON'S LAW: AS THE TOTAL PRESSURE INCREASES
OR DECREASES, THE PARTIAL PRESSURE OF EACH
COMPONENT GAS INCREASES OR DECREASES
PROPORTIONATELY