John Smith said:
Claims a perfect vacuum is 30 inches of mercury, and is unobtainable on
earth ... he further goes on to state he has used pumps able to pull 28
inches--28 inches and then the use of a "getter" in the tube should do
it ...
Snort. Beware of the web, for it full of people that know part of
something, pontificating as if they know it all. While 28 inches may be
a limit for a particular class of mechanical pump (probably dry vane)
pulling on wood (a porous substance) through a system that is not too
well sealed (typical lathe vacuum setup), it's hardly a limit "on earth".
1 atmosphere (the pressure at the surface of the earth, on average) is
29.92 inches of mercury, or 760 mm of mercury (a unit also known as
torr). Most serious vacuum measurements are in the direction of "0 is a
perfect vacuum", so rather than refer to 760 torr being a perfect
vacuum, we say that the room is at 760 torr, and the chamber is being
pumped down to some figure approaching 0 - 1 torr, 1 millitorr (1x10-3),
1 microtorr (1x10-6) etc.
A half-decent mechanical "roughing" (wet-vane, oil bath) pump can pull a
well-sealed chamber to less than 1 torr, no problem. A turbomolecular
pump, oil diffusion pump, or cryo pump can pull a rather casually
prepared chamber to 10-6 torr, and one that's better prepared can be
pulled to 10-13 or better. If you leave a crapload of stuff in the tube
envelope by stopping at 49 torr (28 inches of mercury) the getter will
"get" full and leave far too much pressure in the tube. You'd want at
least 10-6 (one millionth of a) torr before sealing for any hope of
having the getter work properly (which will further lower the pressure
in the tube). Lower is typically better, except for certain abnormal
types that are using plasma in the tube.
