Vacuum
Vacuum is defined as air pressure below atmospheric pressure.
The vacuum level is the difference in pressure between atmospheric pressure and pressure in the evacuated system:
 0% vacuum = 760 torr = 14.7 psia = 29.92 inc mercury abs = 101.4 kPa abs
 50% vacuum = 380 torr = 7.3 psia = 15 inc mercury abs = 50.8 kPa abs
 99.9% vacuum = 1 torr = 0.01934 psia = 0.03937 inc mercury abs = 1.3 kPa abs
For perfect vacuum (100%)  the pressure is 0 torr, 0 psia or 0 Pa abs.
Vacuum versus Absolute Air Pressure
Example  Vacuum
If the absolute pressure is 60 kPa the vacuum according the diagram above is approximately 40%  or approximately 450 Torr.
Vacuum Ranges
 atmospheric pressure  760 torr
 low vacuum  760 to 25 torr
 medium vacuum  25 to 10^{3} torr
 high vacuum  10^{3} to 10^{9} torr
 ultra high vacuum  10^{9} to 10^{12} torr
 extremely high vacuum  < 10^{12} torr
 perfect vacuum  0 torr
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