DIGIVAC 201 series Instruction Manual Page 8
Microns are not really a measure of vacuum at all, but rather of absolute pressure. It will be recalled that
the pressure of the atmosphere is 14.696 or approximately 14.7 pounds per square inch at sea level.
This pressure is due to the weight of all of the air in the earth's atmosphere above any particular square
inch. This 14.696 psi is equivalent to the pressure produced by a mercury column of approximately
29.92 inches high or .76 meters (about 3/4 of a yard) or 760 millimeters of mercury. Atmospheric
pressure varies greatly with altitude. It decreases approximately 1 inch of mercury per thousand feet of
altitude. It also varies widely with local weather conditions. (Variations of one half inch in a single day are
common.) The word vacuum means pressure lower than atmospheric or "suction," but, in describing
negative pressure, the atmosphere is only a satisfactory reference if we are dealing with values of
vacuum down to about 27 inches of mercury. Below that, it is much more useful to talk in terms of
absolute pressure, starting from absolute zero. The DIGIVAC and all similar instruments do just this.
One TORR, a commonly used unit, is an absolute pressure of one millimeter of mercury. A millitorr is
equal to one thousandth of a TORR. A MICRON is the same as a millitorr. The full scale reading of a
DIGIVAC is 1999 microns and is equivalent to 1.999 TORR of approximately 2/760 of atmospheric
pressure. This is less than .1 inches of mercury, and less than .05 PSI.
