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6.3
Physical Laws.
Presented here is an abbreviated history and overview of the fundamental laws of phys-
ics dealing with pressure and ow measurement.
Pressure — In physics, pressure is a force measured in terms of its distribution over a
given area. This is expressed as force (F) divided by a unit area (A) of the surface area
to which the force is applied. Air pressure most commonly refers to a force exerted uni-
formly in all directions. Force x Area = Pressure.
Absolute Pressure — Pressure measured with respect to zero pressure (a very high
vacuum).
Gauge Pressure — Pressure measured with respect to surrounding air pressure (the
pressure exerted by the weight of the atmosphere).
Barometric Pressure — the surrounding pressure caused by the atmosphere. At
average sea level, barometric pressure is approximately 14.7 pounds per square inch, or
29.9 inches of mercury. This is equivalent to 101.3 Kilopascals.
Negative Pressure (Vacuum) — Vacuum is dened as a volume void of matter. For
practical purposes, this means a volume where as much matter as possible has been re-
moved. A perfect vacuum does not exist even in the depths of space, where any given
volume will probably contain one or more particles of matter or one or more units of en-
ergy, which is the equivalent of matter (Relativity). Even a vacuum with no measurable
energy level is only a “virtual” vacuum.
Air Composition — Our atmosphere is composed almost entirely of oxygen and nitro-
gen in their diatomic forms (two atoms bound together by chemical forces). Diatomic
nitrogen makes up approximately 78% of the total molecules in the atmosphere. Di-
atomic 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 signicant
being carbon dioxide and water vapor. Water vapor is present in highly variable quanti-
ties ranging from 0 to 4% by volume.
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