Pressure is defined as a force acting upon a particular area of matter. It is typically
measured in pounds per square inch (psi) in the English system and Newton per
square centimeter (N/cm2) in the System International (SI). Underwater pressure
is a result of the weight of the water above the diver and the weight of the atmosphere
over the water. There is one concept that must be remembered at all
times—any diver, at any depth, must be in pressure balance with the forces at that
depth. The body can only function normally when the pressure difference between
the forces acting inside of the diver’s body and forces acting outside is very small.
Pressure, whether of the atmosphere, seawater, or the diver’s breathing gases,
must always be thought of in terms of maintaining pressure balance.
Given that one atmosphere is equal to 33 feet of sea water
or 14.7 psi, 14.7 psi divided by 33 feet equals 0.445 psi per foot. Thus, for every
foot of sea water, the total pressure is increased by 0.445 psi. Atmospheric pressure
is constant at sea level; minor fluctuations caused by the weather are usually
ignored. Atmospheric pressure acts on all things in all directions.
Most pressure gauges measure differential pressure between the inside and outside
of the gauge. Thus, the atmospheric pressure does not register on the pressure
gauge of a cylinder of compressed air. The initial air in the cylinder and the gauge
are already under a base pressure of one atmosphere (14.7 psi or 10N/cm2). The
gauge measures the pressure difference between the atmosphere and the increased
air pressure in the tank. This reading is called gauge pressure and for most
purposes it is sufficient.
In diving, however, it is important to include atmospheric pressure in computations.
This total pressure is called absolute pressure and is normally expressed in
units of atmospheres. The distinction is important and pressure must be identified
as either gauge (psig) or absolute (psia). When the type of pressure is identified
only as psi, it refers to gauge pressure. Table 2-10 contains conversion factors for
pressure measurement units.
Four terms are used to describe gas
pressure:
-
Atmospheric.Standard atmosphere, usually expressed as 10N/cm2, 14.7 psi,
or one atmosphere absolute (1 ata).
-
Barometric.
Essentially the same as atmospheric but varying with the weather
and expressed in terms of the height of a column of mercury. Standard
pressure is equal to 29.92 inches of mercury, 760 millimeters of mercury, or
1013 millibars.
- Gauge. Indicates the difference between atmospheric pressure and the
pressure being measured.
- Absolute. The total pressure being exerted, i.e., gauge pressure plus
atmospheric pressure.
The water on the surface pushes down on the water below
and so on down to the bottom where, at the greatest depths of the ocean (approximately
36,000 fsw), the pressure is more than 8 tons per square inch (1,100 ata).
The pressure due to the weight of a water column is referred to as hydrostatic
pressure.
The pressure of seawater at a depth of 33 feet equals one atmosphere. The absolute
pressure, which is a combination of atmospheric and water pressure for that depth,
is two atmospheres. For every additional 33 feet of depth, another atmosphere of
pressure (14.7 psi) is encountered. Thus, at 99 feet, the absolute pressure is equal
to four atmospheres. Table 2-1 shows how pressure increases with depth.
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Table 2 -1. Pressure Chart.
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The change in pressure with depth is so pronounced that the feet of a 6-foot tall
person standing underwater is exposed to pressure that is almost 3 pounds per
square inch greater than that exerted at his head.
Buoyancy is the force that makes objects float. It was first defined by
the Greek mathematician Archimedes, who established that “Any object wholly or
partly immersed in a fluid is buoyed up by a force equal to the weight of the fluid
displaced by the object.” This is known as Archimedes’ Principle and applies to all
objects and all fluids.
According to Archimedes’ Principle, the buoyancy of a
submerged body can be established by subtracting the weight of the submerged
body from the weight of the displaced liquid. If the total displacement (the weight
of the displaced liquid) is greater than the weight of the submerged body, the
buoyancy is positive and the body will float or be buoyed upward. If the weight of
the body is equal to that of the displaced liquid, the buoyancy is neutral and the
body will remain suspended in the liquid. If the weight of the submerged body is
greater than that of the displaced liquid, the buoyancy is negative and the body
will sink.
The buoyant force on an object is dependent upon the density of the substance it is
immersed in (weight per unit volume). Fresh water has a density of 62.4 pounds per cubic foot. Sea water is heavier, having a density of 64.0 pounds per cubic
foot. Thus an object is buoyed up by a greater force in seawater than in fresh
water, making it easier to float in the ocean than in a fresh water lake.
Lung capacity has a significant effect on buoyancy of a diver. A
diver with full lungs displaces a greater volume of water and, therefore, is more
buoyant than with deflated lungs. Individual differences that may affect the buoyancy
of a diver include bone structure, bone weight, and body fat. These
differences explain why some individuals float easily while others do not.
A diver can vary his buoyancy in several ways. By adding weight to his gear, he
can cause himself to sink. When wearing a variable volume dry suit, he can
increase or decrease the amount of air in his suit, thus changing his displacement
and thereby his buoyancy. Divers usually seek a condition of neutral to slightly
negative buoyancy. Negative buoyancy gives a diver in a helmet and dress a better
foothold on the bottom. Neutral buoyancy enhances a scuba diver’s ability to
swim easily, change depth, and hover.
