A "vacuum pump" is so named for its function. It is really an air pump that is
specifically designed to reduce the air pressure of a connected chamber.

 You may have seen the use of such a pump in class as part of a classical physics demonstration.

 The problem with the use of such a pump is that its process is hidden deeply within an iron
casing, no knowledge imparted to the class during physics demonstrations as to its specific operation.
 
 Because it is an expensive and delicate piece of scientific equipment, instructors often do not
allow students to touch or operate the machine.

 It is very important to introduce newcomers to both the concept and the feel as what it
takes to create this kind of reduced pressure in a connected vessel.

     What the vacuum service air pump isn't.

  It is a common misconception that a lab vacuum pump is nothing more than an air compressor  that is used to "suck" air out of things.

Such reciprocating piston machines are quite unsuitable for the low pressures necessary for
modern physics demonstrations like electrical discharges in rarefied air and other gases.
 
 
 

There are spaces in a  typical piston-type air  compressor that are not  part of the volume that is  displaced or swept by  the piston.   These include the valve  passages, pockets and the annular spaces  around the piston above  the sliding ring seals.   This leftover parcel of air  cannot be expelled, stays  in the fixed volume of the pump and is uselessly  reworked over repeatedly.

Some amount of air  pressure is necessary  to open the check  valves in a typical air  compressor.    There is plenty  of pressure to open the  valves in  pressure  service.   There is not enough  force by the lowered  pressure air of vacuum  service to open  the spring loaded  valves.

 

   The same goes for diaphragm pumps.         Next page---->