High Quality Content by WIKIPEDIA articles! Ultra high vacuum (UHV) is the vacuum regime characterised by pressures lower than about 10?7 pascal or 100 nanopascals (10?9 mbar, ~10?9 torr). UHV requires the use of special materials in creating the vacuum system, extreme cleanliness to maintain the vacuum system, and baking of the entire system to remove water and other trace gases that adsorb on the surfaces of the chamber. At these low pressures the mean free path of a gas molecule is approximately 40 km, so gas molecules will collide with the chamber walls many times before colliding with each other. Almost all interactions therefore take place on various surfaces in the chamber.

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