Prior to evacuation, every vacuum system on earth contains air and it will always be surrounded by air during operation. This makes it necessary to be familiar with the physical and chemical properties of atmospheric air.
The atmosphere is made up of a number of gases and, near the earth’s surface, water vapor as well. The pressure exerted by atmospheric air is referenced to sea level. Average atmospheric pressure is 1013 mbar (equivalent to the “atmosphere”, a unit of measure used earlier).
In terms of vacuum technology the following points should be noted in regard to the composition of the air:
a) The water vapor contained in the air, varying according to the humidity level, plays an important part when evacuating a vacuum plant.
b) The considerable amount of the inert gas argon should be taken into account in evacuation procedures using sorption pumps.
c) In spite of the very low content of helium in the atmosphere, only about 5 ppm (parts per million), this inert gas makes itself particularly obvious in ultrahigh vacuum systems which are sealed with Viton or which incorporate glass or quartz components. Helium is able to permeate these substances to a measurable extent.
The pressure of atmospheric air falls with rising altitude above the earth’s surface:
High vacuum prevails at an altitude of about 100 km and ultrahigh vacuum above 400 km. The composition of the air also changes with the distance to the surface of the earth:
