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Vacuum Nomenclature

        I've been looking at some vacuum pumps recently and noticed
some of the specifications for vacuum are in microns instead of
inches of Hg. How do they relate? Thanks, John 

I am cross-posting this to several places where vacuum pumps are
talked about.

To measure inches of mercury: If you stick a long glass tube into a
dish of mercury, and slowly suck on the open end, the mercury will
slowly rise up to around 29-30 inches (depending on barometric
preassure) and then stop at a full vacuum. Not very good for measuring
levels of full vacuum but quite fine for de-bubbling, clamping,
casting, investing, etc.

To measure in microns of mercury Close one end of the tube and fill it
with mercury and invert (don’t let any air in) it and seal it into a
flask of mercury and the column will be about 29 inches (710 mm) high
with a full vacuum above the column. Now, if you slowly suck on the
outlet of the flask, the column will slowly fall towards zero (at full
vacuum) and the actuall indicated residual preasure is not affected by
barometric preasure.-- So 20 microns means that there is enough
residual preasure to support a column 0.02mm high. At such low levels
, vacuum is not actually measured this way. Rather, such things as;
thermal conductivity (hot wire gauge {piani, thermocouple}), ionic
conductivity (ion gauge, penning gauge), compressability of a sample
(McLeoid gauge), thermal momentum transfer (a gismo like a spring
loaded radiometer), viscous drag (a spinning something slows down),
etc (I wonder how well sound transfer would work) are measured, then,
the mercury highth is calculated/ calibrated. Each of these have
their own favorite range to work in. These ranges would be used for
things like sputtering, vacuum coating, electron tubes, gas disharge
tubes, etc. - You would not measure your vacuum bagging rig with these

Any pump rated in microns will be way more than good enough for
de-bubbling, investing etc as long as it is fast enough (liters per
minute) for the task at hand. – Looking forward: Alan Shinn

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