Since vacuum is described as a “reduced pressure” we must have some understanding of what pressure means. There are two ways of presenting it:
- every-day experience with atmospheric pressure
- what is really happening at the molecular level
The layer of atmosphere surrounding the earth is not thick (very roughly 100 km compared to the earth's diameter of 12,800 km). However, a column of air 1" square (6.45 sq cm) at sea level projected to the top of the atmosphere weighs about 14.7 lbs. (6.7 kg) on the average day. Expressed another way, this air column creates a pressure at sea level of 14.7 lbs. per square inch (psi) (1.035 kg/ cm2).
But air is a fluid and 14.7 psi applies to all surfaces no matter what their orientation. If we evacuate a 1" cubic box at sea level, then the top (horizontal) surface will experience 14.7 psi pushing down and the bottom (horizontal) surface will experience 14.7 psi pushing up. Equally, opposite sides of the cube experience forces of 14.7 psi pushing left and right. So the cube experiences no net force pushing it in any direction (other than gravity, of course).
Air is a mixture of molecules (nitrogen, oxygen, carbon dioxide, etc.) and atoms (argon, helium, etc.), which at normal temperatures are all moving at high speed, making a huge number of elastic collisions with each other in a gas phase and non-elastic collisions with surfaces.
At room temperature, the average nitrogen molecule is traveling at ~900 mph (400 m/sec). At any moment ~90 percent of the N2 molecules have velocities between 100 mph and 1,800 mph. But N2 has a mass of only 4.8 x 10-23 gm, so despite its high velocity its kinetic energy is unnoticeably small. However, as noted in Number Density (below), 1 cubic centimeter (cc) of air contains a gigantic number of atoms/molecules. It is the force generated by the high speed surface bombardment of those myriad tiny particles that we experience as pressure.
All pressure measurement units are of the form-force per unit area. However, for many units like this relationship is hard to identify. A few of the more commonly used pressure units in vacuum applications are noted here with approximate conversion factors to 1 atmosphere pressure (1 atm) to show their relative magnitude.
- millimeter of mercury: 760 mmHg = 1 atm
- Torr*: 760 Torr = 1 atm
- millitorr: 760000 mTorr = 1 atm
- micron of mercury: 760000 µHg = 1 atm
- bar: 1.013 bar = 1 atm
- millibar: 1013 mbar = 1 atm
- pascal * *: 101325 Pa = 1 atm
* Preferred unit in the USA and used throughout these notes
* Sl units (1 Pa = 1 newton/ m 2)