Physical Laws
We present here an abbreviated history and overview of fundamental laws dealing with pressure and
flow 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 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 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 defined as a volume void of 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, where 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
Our atmosphere is composed almost entirely of oxygen and nitrogen in their
diatomic forms (two atoms bound together by chemical forces). Diatomic nitrogen makes up
approximately 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. Water vapor is present in highly
variable quantities ranging from 0 to 4% by volume.
Air Density If the atmosphere was 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, call Boyle’s Law, implies that density decreases with height
in 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’s Law, implies that the depth of an air column
bounded by two constant-pressure surfaces will increase as the temperature in the column increases.
Density varies mostly with pressure over large vertical distances; at constant height, pressure variation
with temperature becomes important. In the low atmosphere, air is heavy, with a stable mass of roughly
one kilogram per cubic meter (1 oz/cubic foot). A room of 500 cubic meters (650 cubic yards) thus
contains 0.05 metric ton of air. At an altitude of 3 km (2 miles), however, density is 30% less than at
sea level.
This difference in air density can cause variations in flow readings from one location to another when
elevations are quite different and no corrections are made.
To Next Page
Loading...
