1.2.1 Definition of vacuum

Pressure is defined as the ratio of force acting perpendicular and uniformly distributed per unit area.

In an enclosed vessel the gas particles perform thermal movements. In their interaction with the vessel wall, the atoms and molecules are subjected to a large number of collisions. Each collision exerts a force on the vessel wall. Where an enclosed gas is not exposed to outside influences, the numerous collisions that take place result in the same pressure occurring at any point within the vessel, no matter where and in what direction the measurement is carried out.

Figure 1.2: Definition of total pressure

In practice, it is very rare that only one gas is available. Mixtures of different gases are much more common. Each single component of these gases will exert a specific pressure that can be measured independently of the other components. This pressure exerted by the various components is called partial pressure. In ideal gases, the partial pressures of the various components add up to the total pressure and do not interfere with each other. The sum of all partial pressures equals the total pressure.

Figure 1.3: Definition of partial pressure

An example of a gas mixture is our ambient air. Its partial pressure composition is shown in Table 1.1 [3].

In space, depending on the proximity to galaxies, pressures of under 10^-18 hPa prevail. On Earth, technically generated pressures of less than 10^-16 hPa have been reported. The range of atmospheric pressure down to 10^-16 hPa covers 19 decimal powers. Specifically adapted types of vacuum generation and measurement for the pressure range result in subdivisions of the various pressure ranges as shown in Table 1.2.

The unit for measuring pressure is the pascal. This unit was named after the French mathematician, physicist, writer and philosopher Blaise Pascal (1623 – 1662). According to Formula 1-3, the SI unit pascal is composed of Pa = N m^-2. The units mbar, torr and the units shown in Table 1.3 are common in practical use.