Compressed Air is air that has been reduced in volume and held under pressure. An Air Compressor is a machine that mechanically reduces the volume of space and increasing the air pressure. 

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There are 3 basic types of Air Compressors: Reciprocating Compressors, Rotary Screw Compressors (Positive Displacement) and Centrifugal Compressors (Dynamic). 

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Pounds Per Square Inch (PSI) is a common method of how air pressure is measured.