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 :

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 :

Rough (low) vacuum :

1 - 10-3 Torr

Medium vacuum :

10-3 - 10-5 Torr

High vacuum (HV) :

10-6 - 10-8 Torr

Ultrahigh vacuum (UHV) :

< 10-9 Torr

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 .

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