Ultra high vacuum (UHV) is the vacuum regime characterise by pressures lower than about 10−7pascal or 100 nanopascals (~10−9torr). UHV requires the use of special materials in creating the vacuum system, extreme cleanliness to maintain the vacuum system, and baking the entire system to remove water and other trace gases that are accumulated due to the difficulties of maintaining a UHV. At these low pressures the mean free path of a gas molecule at 10−7 pascal or 100 nanopascals (~10−9 torr) 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.