I need a vacuum to provide 21" Hg but looking online many list an ultimate vacuum of 10Pa but 1"Hg = 3300Pa? I know I am missing something and just want to make sure I get it right. Thanks for the help!


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    $\begingroup$ You're right 1"Hg = 3386 Pa. The pump that you've linked to indicates that it goes to 75 microns of Hg (or 10 Pa) those units match. Are you skeptical that the pumps can go so low? $\endgroup$
    – Dan
    Jul 12, 2016 at 5:11

1 Answer 1


I think your confusion is in absolute vs gauge pressure. Atmospheric pressure is about 101 kPa, or 101,000 Pa, or about 30 inches of Hg. Vacuum is commonly measured relative to atmospheric pressure, and represents a negative gauge pressure but still a positive absolute pressure. You want 21 inches Hg vacuum - i.e. -21 inches gauge, or about 9 inches absolute. The 10 Pa "ultimate" vacuum is in absolute units, not gauge, and is about 0.003 inches Hg.


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