Cited Passage
2.0 GENERAL
In all diving operations, safety is the primary considera-
tion. One key to safety is a clear understanding of the physics
of diving. Physics is the field of science dealing with matter
and energy and their interactions. This chapter explores
physical laws and principles that pertain to the diving envi-
ronment and its influence on the diver. Gravity is passive,
vision and hearing may be misleading, color perception
changes at varying depth, and breathing dynamics are ever
changing. The principles of physics provide the keystone for
understanding the reasons for employing various diving pro-
cedures and the operation of associated equipment. Many of
these principles receive further elaboration in other sections
of the NOAA Diving Manual.
2.1 PRESSURE
Pressure is force acting on a unit area. Stated mathemati-
cally,
Pressure = force/area P = F/A
In the United States, pressure is typically measured in
pounds per square inch (psi). Under water, two kinds of
pressure affect a person, the weight of the surrounding
water and the weight of the atmosphere over that water.
One concept that must be remembered at all times is: a
diver, at any depth, must be in pressure balance with the
forces at that depth.
At all depths, the diver must compensate for the pres-
sure exerted by the atmosphere, by the water, and by the
gases being used for breathing under water. This compen-
sation must always be thought of in terms of attaining and
maintaining a balance between the pressure inside the body
and the external pressure.
2.1.1 Atmospheric Pressure
Atmospheric pressure is the pressure exerted by the
earth's atmosphere; it decreases with altitude above sea
level. At sea level, atmospheric pressure is equal to 14.7
pounds per square inch (psi) or one atmosphere (atm). The
higher the altitude above sea level, the lower the atmospher-
ic pressure. For example, at 18,000 ft. (5,486 m), atmos-
pheric pressure is 7.35 psi, or half that at sea level (see
Figure 2.1). At sea level, atmospheric pressure is considered
constant and universal; that is, anywhere on the earth at sea
level, the pressure is 14.7 psi. The pressure inside a person’s
lungs is the same as the pressure outside.
2.1.2 Hydrostatic Pressure
Pressure due to the weight of water is called “hydro-
static pressure.” The weight of water is cumulative; the
deeper the dive, the more water there is above the diver
and the greater the weight of that water. This weight
affects a diver from all sides equally and increases at a
rate of 0.445 psi per foot of seawater. Thus, at a depth of
33 ft. (10.1 m) of seawater (fsw), the hydrostatic pressure
is 14.7 psi, or one atmosphere, the same pressure as
atmospheric pressure at sea level. In freshwater, 34 ft.
(10.4 m) equals 14.7 psi or 0.432 psi per foot of freshwa-
ter (ffw). Thereafter, for every 34 ft. of additional depth in
freshwater, the hydrostatic pressure increases by one
atmosphere (see Figure 2.1).
2.1.3 Absolute Pressure
The sum of atmospheric pressure plus hydrostatic
pressure is called the “absolute pressure.” Absolute pres-
sure can be expressed in many ways, including "pounds
per square inch absolute" (psia) , "atmospheres absolute"
(ata) , feet of seawater absolute (fswa) , feet of freshwater
absolute (ffwa) , or millimeters of mercury absolute
(mmHga) .
To understand the effects of absolute pressure on a
diver, consider this: the feet of a 6-foot tall man standing
under water will be exposed to pressure that is almost
three pounds per square inch greater than that exerted at
his head.
2-1
2
Physics of Diving
The PDF viewer opens on the cited page and searches this passage for highlight.