The differential value of the pressure switch is also called dead zone of pressure switch. After the pressure switch acts, only the pressure must be slightly lower than the action value of the pressure switch, can the micro switch be reset, the difference between the action value and the reset value is called the differential value of the pressure switch. Install the pressure switch on the calibration device, firstly rotate the differential knob to zero in order to make the pressure reache the action value, adjust the control rod, monitor the output contact state using the resistance of the multimeter, and stop adjusting when the contact is just closed. Then make pressure drop to the specified dead zone, adjust the differential knob, to make the contact just disconnect. Repeatly check twice, until the action is accurate.