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| 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 |
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