Displacement compressors A bicycle pump is the simplest form of a displacement compressor, where air is drawn into a cylinder and is compressed by a moving piston. The piston compressor has the same operation principle, with a piston whose forward and backward movement is accomplished by a connecting rod and a rotating crankshaft. If only one side of the piston is used for compression this is called single acting. If both the piston's top and undersides are used the compressor is called double acting. The difference between the pressure on the inlet side and the pressure on the outlet side is a measurement of the compressor's work. The pressure ratio is the relation between absolute pressure on the inlet and outlet sides. Accordingly, a machine that draws in air at atmospheric pressure and compresses it to 7 bar overpressure works with a pressure ratio of (7 + 1)/1 = 8.