| 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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