Equus 3660 - Hand Vacuum Pump & Brake Bleeding Kit - Manual (Page 5)

3660 3
B. HOW IS VACUUM MEASURED?
The earth’s atmosphere exerts a force on all surfaces it contacts. This
force is called “atmospheric pressure”. There is a direct relationship
between atmospheric pressure and vacuum. In order to understand
vacuum, we need to understand what atmospheric pressure is and how
it's measured.
Air is composed of tiny particles called molecules. Air molecules have
weight. Atmospheric pressure is defined as the force exerted by the
total weight of the air above a specific area at any elevation. At sea
level, the total weight of a column of air (air molecules) above an area
of one square inch equals 14.7 pounds (1030g/cm2). At higher
elevations there are fewer air molecules, and the weight of the air
above a comparable surface area is lower than at sea level. This is why
the air pressure at higher elevations is lower than at sea level.
Atmospheric pressure is measured using an instrument called a
Manometer/Barometer. A Manometer/Barometer is a hollow glass tube
with one end sealed and the other end immersed in a container of
mercury.
At sea level, the force of the surrounding air pressure (atmospheric
pressure) of 14.7 psi (1030g/cm2) acting on the mercury in the
container forces the mercury up the glass tube to a measured height of
30 inches (76 cm).
Vacuum can be defined as a pressure that is less than the surrounding
air pressure (atmospheric pressure). Vacuum gauges base their
readings on atmospheric pressure as indicated on a Manometer/
Barometer. The “0” indication on a vacuum gauge is calibrated to
represent "atmospheric pressure"; the “30” indication represents "zero
pressure" or a "perfect vacuum". Any reading between these two
indications (“0” and “30”) represents a pressure that is less than the
surrounding air pressure (atmospheric pressure).
C. HOW IS VACUUM CREATED/GENERATED?
Vacuum (a low pressure area) can be created in a sealed container in
one of two ways:
1. If part of the air is removed from a sealed container, there will be
fewer air molecules per square inch inside the sealed container
than in the surrounding outside air. Fewer air molecules within a
constant area equates to a lower air pressure.
2. If the area inside a sealed container is increased but the amount of
air inside the container stays the same. The area inside the
container will have fewer air molecules per square inch than the
surrounding outside air. Increasing area while maintaining a
constant volume of air equates to a lower air pressure.