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<p><b>Ultra-high vacuum</b> (often spelled ultrahigh in American English, <b>UHV</b>) is the <a href="page.php?w=vacuum">vacuum</a> regime characterised by  <a href="page.php?w=pressure">pressure</a> lower than about 1e-9 Torr. UHV conditions are created by pumping the gas out of a UHV chamber, and in some cases by cooling the chamber to cryogenic temperatures. In general, there are two regimes of gas flow, <a href="page.php?w=Laminar_flow">laminar</a> and <a href="page.php?w=Free_molecular_flow">free molecular</a>, defined by whether the dominant</p><p>
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