What do ‘high vacuum’ and ‘low vacuum’ mean? (FAQ - Pressure)
The terms high- and low- (and also medium-) vacuum are not very intuitive; they are used to describe the various pressure ranges known under the general banner of vacuum but are neither defined nor used consistently. The most important point, though, is that high-vacuum is a lower pressure than low-vacuum.
The adjectives essentially are a mirror-image of those used at higher pressures; the farther away (upwards/increasing) from atmospheric pressure the higher the pressure is said to be and similarly the farther away (downwards/decreasing) from atmospheric pressure the higher the degree of vacuum is said to be. Historically, early pumping terminology also used words such as rough vacuum (ie not very good) and fine-vacuum (ie good, pure, refined, harder to achieve).
The use of atmospheric pressure as a 'mental datum’ pressure does lead to confusions, however. Although, as said above, the pressure ranges corresponding to high- and low-vacuum are not particularly well defined, they are all considered to refer to absolute pressures (their magnitude is expressed with respect to a theoretically perfect reference vacuum, that is no molecules at all - see pressure modes). They more-or-less correspond to the pressure ranges shown in the table below:
Pressure ranges corresponding to 'degree of vacuum'
|Pressure range (See note below)||'Degree of vacuum'|
|1×105 to 3×103||100 000 to 3 000||low vacuum|
|3×103 to 1×10-1||3 000 to 0.1||medium vacuum|
|1×10-1 to 1×10-4||0.1 to 0.000 1||high vacuum|
|1×10-4 to 1×10-7||0.000 1 to 0.000 000 1||very high vacuum|
|1×10-7 to 1×10-10||0.000 000 1 to 0.000 000 000 1||ultra-high vacuum (UHV)|
|<1×10-10||<0.000 000 000 1||extreme-ultrahigh vacuum (EHV or XHV)|
Note: The two lists of pressure ranges shown in the table above are numerically identical, the left column style being known as scientific notation. Atmospheric pressure is nominally 1×105 Pa (100 000 Pa) so, for example, high vacuum covers a range that is somewhere between one millionth and one thousand millionth the value of nominal atmospheric pressure.