Pressure switches consist of a switch mechanism that is connected to a bellow or diaphragm that reacts to system pressure. They are selected by operating range and differential. Operating range is the pressure limits in which the switch will work. Differential is the difference between the cut-in and cut-out pressures. Pressure switch settings, common to many air compressors, are set for a range of 40 to 250 PSI and an adjustable differential of 35 to 60 PSI. This means that the lowest pressure that the switch can be set to close at is 40 PSI, and the highest pressure it can be set to open at is 250 PSI. The differential that is actually available to use varies with the pressure range you�??re actually working in.