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Absolute Vacuum: vacuum, high vacuum and ultra high vacuum resource pages:
What is 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.
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