What Is A Vacuum?
Vacuum [Latin word "empty"] is well defined in several documents.
According to the definition of the American Vacuum Society (1958)
the term "vacuum" refers to a given space filled with gas at
pressures below atmospheric, i.e. having a density of molecules
less than about 2.5x1019 molecules/cm3. In other word, a vacuum is
the state of a gas where the density of the particles is lower than
atmospheric pressure at the earth's surface. In general the term
"vacuum" includes nowadays about 19 orders of magnitude of
pressures (or densities) below that corresponding to the standard
atmosphere.
The vacuum can be divided into two catagories, artificial and
natural vacua.
1. Artificial Vacuum: On the earth vacuum is normally achieved by
pumping on a vessel, the degree of vacuum increasing as the
pressure exerted by the residual gas decreases below atmosphere.
The degree of vacuum is classified by measuring a system's absolute
pressure. Traditional limits of vacuum ranges are rather arbitrary.
Nonetheless, low, medium, high, and ultrahigh vacuum can be
specified corresponding to regions oflower and lower pressures. The
concept of vacuum is normally undersood in terms of molecular
density, mean free path, and the time constant to form a
monolayer
Pressure of Air at 25 degree
Torr
Molecular Density
molec/cm3
Molecular Incident Rate
molec/cm2-sec
Mean Free Path
cm
Time to Form a Monolayer
second (8x1014molec/cm2)
760
2.46x1019
2.88x1023
6.7x10-6
2.9x10-9
1
3.25x1016
3.78x1020
5.1x10-3
2.2x10-6
10-3
3.25x1013
3.78x1017
5.1
2.2x10-3
10-6
3.25x1010
3.78x1014
5.1x103
2.2
10-9
3.25x107
3.78x1011
5.1x106
2.2x103
10-12
3.25x104
3.78x108
5.1x109
2.2x106
10-15
3.25x10
3.78x105
5.1x1012
2.2x109
Degree of Vacuum
Pressure Range (Pa) : 1 Pa = 7.5x10-3 Torr
Low Vacuum (LV)
3.3x103 < P < 105
Medium Vacuum
10-1 < P < 3.3x103
High Vacuum (HV)
10-4 < P < 10-1
Very High Vacuum
10-7 < P < 10-4
Ultrahigh Vacuum (UHV)
10-10 < P < 10-7
Extremely High Vacuum (XHV)
P < 10-10
2. Natural Vacuum: No natural high vacuum is known on earth. Some
of these applications are very vital. Human beings are pumping to
about 740 Torr during respiration, and may achieve pressures as low
as 300 Torr by suction. The octopus is able to achieve pressures of
about 100 Torr. In space the pressure decreases with the altitude
from the pressure of 760 Torr at sea level. Up to 100 km altitude
(troposphere and stratosphere) the pressure decreases quite
regularly by a factor of 10 per 15 km altitude, which results in a
pressure of 10-3 Torr at about 90 km altitude. The ionosphere
(100-400 km) contains a large number of ionized atoms, and its
pressure decreases only by a factor of 10 every 100-200 km. This
decrease results in a pressure of about 10-10 Torr at an altitude
of 1,000 km. About 400 km ultrahigh vacuum conditions exist. At
10,000 km a pressure of about 10-13 Torr exists. The pressure
measured on space crafts are determined by the spacecraft velocity
and gas particle concentration.
