DigiVac SNAP Руководство по эксплуатации - Страница 16
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Section 8. Understanding Torr
This instrument and many similar instruments are calibrated in Torr.
The pressure of the atmosphere is 14.696 or approximately 14.7 pounds per square
inch at
sea level. One TORR 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.
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 (~ 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 atmosphere or "suction." However, 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
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