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1.1 General

1.1.1 Vacuum – Definition

A vacuum is defined colloquially as the state encountered in a room at pressures below atmospheric pressure. These pressures can be generated by gases or vapors that are evenly distributed over the room.
The standard definition of vacuum is “the state of a gas at which its pressure in a vessel and therefore its particle density is lower than that of the ambient surrounding atmosphere or in which the pressure of the gas is lower than 300 mbar, i. e. lower than the pressure of the atmosphere on the Earth’s surface.“ [2]

1.1.2 Overview of vacuum

The significance of the 300 mbar specified in the standard becomes apparent when the barometric formula is considered. Atmospheric pressure sinks with increasing altitude due to the decreasing weight of the column of air over a certain area.

    p_{h}=p_0\cdot\exp\left(-\frac{\rho_0\cdot g\cdot h}{p_0}\right)

    Formula 1-1: Barometric formula

  • p_h Atmospheric pressure at height h
  • p_0 Atmospheric pressure at sea level = 1,013.25 mbar oder 101,325 Pa
  • g Acceleration of gravity = 9.81 m s-2
  • ρ0 Density of air at sea level at 0 °C = 1.293 kg m-3
If for the purposes of simplification we assume that the density of the air, the acceleration of gravity and atmospheric pressure at sea level are constant, we obtain by summarizing:
    p_{h}=p_0\cdot\exp\left(-\frac{h}{8,005\mbox{ m}}\right)

    Formula 1-2: Numerical barometric formula

  • Chart titled 'Vacuum at a Glance' showing pressure ranges, vacuum levels, vacuum pump and gauge ranges, applications, and altitudes with pressure values.

    Figure 1.1: Overview of vacuum

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