The switches listed are universal replacement pressure switches, which can replace Furnas, Square-D and other brands. The switch is equipped with an unloader, which is used to remove the air pressure in the compressor head when the compressor is stopped, so that there is no pressure on the piston when starting, and the compressor can be started without load. The valve usually has a 1/4" plastic or copper pipe connected to the head of the check valve or compressor. If your compressor does not have an unloading line, there is no need to connect a valve, and the pressure switch will still work normally. The preset pressure of this switch is most commonly used in two-stage compressors, as long as the amperage at 120 volts is less than 20 amperes or the amperage at 240 volts is less than 12 amperes. The switch is preset to turn on the compressor at 95 psi and turn off at 125 psi. This can be adjusted to any pressure within the factory minimum and maximum set points listed below.