UNDERSTANDING PRESSURE IN VACUUM: DEFINING TERMS AND EXAMPLES
Definition
Example
Ambient
Pressure
Pressure
surrounding
an
object at the time of
measurement.
Pressure of air pushing ag
ainst the outside of a
closed
tank
.
Air
Pressure
A
n indication of
th
e force per unit area
that
molecules
exert on an object. Vacuum is used when that
pressure is less than ambient pressure.
Air pressure is a measure of the weight of the air pushing down on
any particular location. Since there is less air above us at the top of
the mountain, the air pressure at the top of a mountain is less than
the air pressure at sea level
Absolute
measure
Absolute measure is a real measure of how much of
something is there. Zero means we have nothing, a
non-zero number means we have something.
If you have zero blocks, you don’t have any blocks. If you have 10
blocks, you have 10 blocks. If you have a perfect vacuum with zero
molecules, you have zero pressure. If you have 760 Torr of
absolute pressure/vacuum, then you have standard atmospheric
pressure.
Differential
measure
Indicates the difference between two known
quantities.
If
in
there are 10 blocks in one stack
and 15 blocks in
another
stack, then there is a differential block count of 5 blocks. If there is
760 Torr of absolute pressure inside an airplane cabin, and there is
700 Torr of absolute pressure outside the airplane at altitude, then
there is a differential pressure of (760-700) or 60 Torr between the
two pressures. Differential pressure = Pressure A – Pressure B
Gauge
measure
Indicates the difference in pressure between a
measured and a known or reference standard.
If the standard block count is 10 blocks
, and I have 15 blocks, then
the gauge measure of blocks is (15-10) or 5 blocks. If gauge tire
pressure is 30 PSI, and the surrounding absolute air pressure is 15
PSI, then the absolute pressure of the tire is (30+15) or 45 PSI.
Gauge pressure can be thought of as differential pressure where
one of the pressures is the ambient air pressure.
Gauge Pressure = Measured Absolute Pressure – Standard
Absolute Pressure;
Measured Absolute Pressure=Gauge Pressure + Standard Absolute
Pressure
