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Vacuum is the state of a gas whose particle density is lower than that of the earth’s atmosphere at sea level. In general, pressure in pneumatics is specified as positive or gauge pressure (relative to the ambient pressure). This in turn means that vacuum is always quoted as a negative value (measured down from the ambient pressure level). The units generally used are bars or millibars (mbar) (1 bar = 1000 mbar). This unit is derived from the SI unit for pressure, pascal (Pa). Previously common units of pressure such as Torr, Kp/cm2, at, atm, mWS and mmHg should no longer be used.


Article in Wikipedia on vacuum