1.1.2 Overview of vacuum
Consequently, in order to achieve a vacuum it is necessary to generate a pressure in a vessel that is lower than the ambient pressure. Due to the Earth's gravity, atmospheric pressure varies with altitude in accordance with the barometric altitude formula:
Formula 1-1:
Barometer formula
Where:
Atmospheric pressure at sea level p0 = 1,013 mbar
Earth acceleration g = 9.81 m/s2
Density of air at sea level at 0 °C ρ0 = 1.293
kg/m3.
Combining the constants yields:
Formula 1-2
Barometer formula number
If p = p0 / 2 and the formula is solved for h, the result is the half altitude value h½ = 5,548 m. In other words: Atmospheric pressure declines by one half every 5.548 km.
At the cruising altitude of a passenger jet, i.e. at approximately 10 km above the surface of the Earth, atmospheric pressure has already decreased to 290 mbar. Weather balloon data are measured at an altitude of approximately 30 km at a pressure of 24 mbar. The pressure levels prevailing at these altitudes can still be categorized as being in the low vacuum range, which will be discussed in greater detail below. An even greater distance from the Earth's surface, finally, brings us to satellite orbits at an altitude of 250 km, where the pressure has now decreased to 10-5 – 10-6. What prevails farther out in space is an ultra high vacuum of less than 10-14.
Figure 1.1:
Overview of vacuum
Source: Paul Scherer Institut, CH