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Chapter 2 — 4
Gauge Pressure
As described above, hydrostatic pressure in diving is generally measured by a
pressure
or
depth gauge
. Such a gauge is normally set to register a pressure of zero at sea level and so it
ignores the pressure due to the atmosphere (1ATA).
The pressure registered by a gauge at 10 metres sea water depth would thus be one
atmosphere gauge (1ATG) or equivalent units. Gauge pressure is converted to absolute
pressure by adding 1 atmosphere pressure.
Partial Pressure
With a mixture of gases, the proportion of the total pressure contributed by each of the gases
is termed its partial pressure (its part of the pressure). The partial pressure contributed by each
gas is proportional to its percentage of the mixture. Each gas contributes the same proportion
to the total pressure of the mixture, as is its proportion in the composition of the mixture.
e.g. air at 1 ATA contains 21% oxygen, hence the partial pressure of oxygen is 0.21 ATA and
air at 1 ATA contains 78% nitrogen, hence the partial pressure of nitrogen is 0.78 ATA.
GAS LAWS
Gases behave in nature and in diving according to several laws. Knowledge of these laws is
important to the diver because they influence the duration of the air supply and affect the gas
containing spaces in the body such as the ears, sinuses and lungs. They also cause other diving
illnesses.
Boyle's Law
This defines the relationship between pressure and volume. It states that the
volume of a
given mass of gas varies inversely with the absolute pressure (if the temperature remains
constant).
Stated simply, for a given amount of gas, if the pressure is increased, the volume is
proportionally decreased and vice versa. This means that if the pressure is doubled, the
volume is halved and vice versa.
Stated mathematically: V varies as 1
(where V = volume and P = pressure)
P
It follows that for a given amount of gas, the volume multiplied by the pressure always has a
constant value.
i.e. P
!
V is constant.
So if a sample of gas has an original volume of V
1
and an original pressure of P
1
, and either
the pressure or volume are changed, the new volume V
2
and the new pressure P
2
will
multiply out to the same value.
i.e
. P
1
!
V
1
= P
2
!
V
2