[Jewelry making - Article 00197] - Re: [Orchid] Venturi Vacuum thoughts

[John Burgess]

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>         Hi All, I thought I might chip in with the fact that running
>     water will pull a vacuum using the Venturi prin.  I first saw this
>     working in a industrial chem. lab, we had a simple metal tube that
>     screwed on to a standard spigot..... 

    G'day; I have never seen a chemistry lab without several!  Nearly 60
    years ago when I took a lab technician's exam we had to do scientific
    apparatus glassblowing  and a "Buchner Pump" was one of the items we
    had to make - in glass.  A good glassblower could turn out 6 in an
    hour and guarantee they all worked.  The lab pumps were commonly used
    to increase the rate of filtration; hence they were also known as
    filter pumps. 

    We measured vacuum differently though.  What we measured was the
    actual pressure of gas left in a system at maximum vacuum.  Our
    little glass water pumps if properly made, would leave a pressure of
    only three millimetres of mercury.  What that means in practical
    terms is if one took a long glass tube closed at one end and filled
    it with mercury, inverted it and attached it to a vessel being pumped
    down, the mercury level would fall to a height of 3 mm instead of the
    atmospheric pressure of 760mm; a much more accurate way to measure
    vacuum. Putting it another way, normally the atmosphere will exert
    enough pressure to hold up a column of mercury about 760 mm high (it
    varies with the weather).  The pumps exhausted the air from a closed
    vessel, to the extent that was was left would only hold up a column
    3mm long. -- Cheers for now, 

John Burgess;   johnb AT ts.co.nz of Mapua Nelson NZ

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