4.1 What is Ultra High Vacuum (UHV) ?

Vacuum technology has advanced considerably over the last 25 years and very low pressures are now routinely obtainable.

Firstly, we need to remind ourselves of the units of pressure :

• The SI unit of pressure is the Pascal ( 1 Pa = 1 N m^-2 )
• Normal atmospheric pressure ( 1 atm.) is 101325 Pa or 1013 mbar ( 1 bar = 10⁵ Pa ).
• An older unit of pressure is the Torr ( 1 Torr = 1 mmHg ). One atmosphere is ca. 760 Torr ( i.e. 1 Torr = 133.3 Pa ).

Whilst the mbar is often used as a unit of pressure for describing the level of vacuum, the most commonly employed unit is still the Torr. (The SI unit, the Pa , is almost never used ! )

Classification of the degree of vacuum is hardly an exact science - it very much depends upon who you are talking to - but as a rough guideline :

Virtually all surface studies are carried out under UHV conditions - the question is why ? This is the question that we will address in Section 4.2 .