Reference 8-47
8.15 Concepts Applicable to Testing
Pressure
In physics, pressure is a force measured in terms of its distribution over an area of
an opposing force. This is expressed as force (F) divided by unit area (A) of the
surface area to which the force is applied. Air pressure most commonly refers to a
force exerted uniformly in all directions. Force X Area = pressure.
Absolute pressure is pressure measured with respect to zero pressure (a very high
vacuum)
Gauge pressure is pressure measured with respect to surrounding air pressure (the
pressure exerted by the weight of the atmosphere).
Barometric pressure is the surrounding pressure caused by the atmosphere. At
average sea level, barometric pressure is about 14.7 pounds per square inch, or
29.9 inches of mercury. This is equivalent to 101.3 Kilopascals.
Negative Pressure (Vacuum)
A vacuum can be defined as a volume of space that contains no matter. For
practical purposes, this means a volume where as much matter as possible has been
removed. A perfect vacuum does not exist even in the depths of space. Any given
volume will probably contain one or more particles of matter or one or more units
of energy, which is the equivalent of matter (relativity). Even a vacuum with no
measurable energy level is only a “virtual” vacuum.
Air Composition
The atmosphere is composed almost entirely of oxygen and nitrogen in their
diatomic forms (two atoms bound together by chemical forces). Diatomic nitrogen
makes up about 78% of the total molecules in the atmosphere. Diatomic oxygen
represents nearly 21%. The inert noble gas, argon, accounts for about 0.9%, and
the remaining 0.1% is composed of many trace gases, the most significant being
carbon dioxide and water vapor. Carbon dioxide makes up only 325 parts per
million of the atmosphere by volume. Water vapor is present in highly variable
quantities ranging from 0 to 4% by volume.
Air Density
If the atmosphere were like water and incompressible, pressure would decrease
uniformly as you went up. In reality, the atmosphere is compressible and density
(mass per unit volume) is proportional to pressure. This relationship, called
Boyle’s Law, implies that density decreases with height in the atmosphere: As
height increases, less mass remains above a given point; therefore less pressure is
exerted. At sea level, the density of air is about 1 kg per cubic meter (8 oz. per
cubic foot). Both pressure and density decrease by about a factor of 10 for every 16
km (10 miles) increase in altitude.
Density does not depend solely on pressure. For a given pressure, density is
inversely proportional to temperature. This relationship, known as Charles’ Law,
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