First of all, we should point
out that the pressure on a diver under water is the result of two separate
forces which act simultaneously upon him or her. These are:
1. The weight of the water
2. The weight of the atmosphere over the surface of the water.
on the left provides mathematical equivalents necessary for converting
barometric pressure units. The various types of pressure exerted upon divers
are summarized further below.
As atmospheric pressure
increases, the height of the mercury in the tube also increases and vice
versa. That is, the weight of the mercury in the tube always corresponds
to the atmospheric pressure.
In the middle of the 17th
century Italian scientist Evangelista Toricelli determined the value of
normal atmospheric pressure with the following experiment. A mercury-filled
glass tube with a section area of 1cm2
and a closed end was vertically immersed in a vessel full of mercury (Hg),
the open end pointing downwards. The level of the mercury inside the tube
decreased to a certain extent. Further decrease was impeded by the atmospheric
pressure that acted upon the surface of the mercury in the vessel. It turned
out that the mercury level in the tube measured 760mm and weighed 1033g.
If water had been used instead, a different tube would be necessary. It
would have to be longer as many times as water is lighter than air. The
level of the water in the tube would correspond to the atmospheric pressure
and would equal 10.33m. Therefore, at sea level air exerts a pressure of
in mind that the total area of the human body is 17,000–18,000cm2,
it can be calculated that atmospheric air exerts upon us a pressure of
17 to 18 tonnes!
Scientists have proven that
the critical point of the mechanical effect of hydrostatic pressure depends
on the evolution level of the organisms. Lower unicellular organisms such
as spores, bacteria, and viruses can withstand pressures of thousands of
atmospheres. A further increase of pressure causes physical and chemical
changes in the cellular structures, thus altering the characteristics of
do not feel the great pressure because the tissues of the human organism
contain 65% of liquids that practically do not shrink. In inner cavities,
the pressure of the inhaled air counteracts the external pressure. During
descent, divers usually do not feel the increasing pressure. They only
feel a slight difficulty while breathing because they inhale gases that
are under a pressure equal to that of the surrounding water. All underwater
diving suits ensure the intake of air held under a pressure that corresponds
to the depth at which the diver is. Otherwise, the absence of this condition
would cause quick death.
Although divers do not feel
the pressure itself, its rapid change may lead to different sicknesses.
A quick decrease of pressure during ascent is particularly dangerous and
may result in a serious disease called decompression sickness. Read more
on that in the Medicine Section.
While under water, a diver
feels unequal pressure on the different parts of his or her body. Low parts,
if in greater depths than the upper body, endure pressure that is greater
by .15–.20x105 Pa
than the one on the upper body.
Atmospheric Pressure is
produced by the weight of the gases in the atmosphere, acts on every body
and in all directions. Its effects are therefore neutralized. At sea level,
it equals 14.7 psi or 1.03 kg/cm2;
larger values are often expressed in atmospheres. Atmospheric pressure
decreases with the increase of height.
is produced by the weight of a fluid, acts upon every body in the fluid,
and is one and the same in all directions at a particular depth. Its increase
rate is .445 psi/foot (1 kg/cm2
per 9.75 meters) when descending in seawter, whereas in fresh water it
increases at .432 psi/foot (1 kg/cm2
per 10 meters).
Absolute Pressure = Atmospheric
Pressure + Hydrostatic Pressure
Its units of measurement
are pounds per square inch absolute (psia) or kilograms per square centimeter
absolute (kg/cm2 absolute).
is the difference between the absolute pressure and a specific pressure.
It is measured with gauges that read zero at sea level. To convert to absolute
pressure add 14.7 to the value in psi or 1.03 to the value in kg/cm2.
is the fraction of the total pressure contributed by a gas in a mixture.
It is in direct proportion to the volume percentage of the gas in the mixture.
The Buoyant Force
is a very characteristic force that acts upon all submerged bodies. This
is how Archimedes' Principle explains buoyancy:
body immersed in a liquid, either wholly or partially, is buoyed up by
a force equal to the weight of the liquid displaced by the body.
The following mathematical
equation can be derived from Archimedes' Principle:
the buoyancy of a submerged
body = weight of displaced liquid – weight of the body. Therefore, we may
1 The body will float if the buoyancy is positive
(body weight < weight of displaced liquid).
2 The body will be suspended if the buoyancy is neutral
(body weight = weight of displaced liquid).
3 The body will sink if the buoyancy is negative
(body weight > weight of displaced liquid).
The buoyant force of a liquid
depends on its density, which equals its weight per unit volume.
A body immersed in seawater
will, therefore, be buoyed up by a greater force than a body immersed in
fresh water, so it is easier to float in seawater than in fresh water.
The density of fresh water is
62.4 pounds per cubic foot (28.3 kg/ 0.03 m3).
Seawater, however, is denser:
64 pounds per cubic foot (29 kg/0.03 m3).
Lung capacity affects
the buoyancy of a person. A diver with full lungs displaces a greater volume
of water and, according to Archimedes' Principle, is more buoyant than
a diver with deflated lungs. With full lungs, a diver’s relative weight
is .96-.99; with deflated lungs: 1.021-1.097. Bone structure, bone weight
and body fat are other factors that have an effect on buoyancy and
vary from person to person. That is why some people float more easily than
Divers who wear wet suits
often add diving weights to their weight belts to create the negative
buoyancy needed for descent. At the desired depth, they adjust their buoyancy
to an appropriate level so that work can be accomplished without extra
physical efforts to oppose positive or negative buoyancy.
Usually, a person’s weight
is slightly less than the weight of the displaced amount of water. For
example, a person who weighs 80kg displaces 79dm2 of water, which weighs
79kg, that is, he has about 1kg of negative buoyancy. Balance under water
depends on the location of the center of weight and that of buoyancy. If
they are situated on a vertical line (the symmetry axis of the human body)
and the center of buoyancy is higher than that of weight, equilibrium will
The relative weight of seawater
is considered 1. That is why the loss of weight of submerged bodies in
Newtons corresponds to the displaced volume in liters. The human body has
a relative weight of about 1, which is why it weighs little under water
To ensure normal descent,
it is not enough to regulate the weight of the diver. It is also necessary
that the additional weights attached to the weight belt be situated so
as to provide stable equilibrium, on condition that the equipment is intact.
All weight forces can be
added and presented as a single force applied to the center of weight.
Similarly, buoyant forces correspond to the center of buoyancy, located
just above the center of weight. The distance between the two centers must
be about 20cm. This fact allows the diver to maintain the erect position
of his or her body.
If the belt weights are situated
too high and the center of weight turns out to be above that of buoyancy,
the diver will be rotated upside-down.
If the belt weights are situated
too low, the center of weight would be much lower than that of buoyancy.
As a result, the diver will be unable to bend or accomplish underwater