Procedure of Pressure switch calibration .
Materials required to calibrate a pressure switch are :
Source of compressed air (Adjustable )
Circuit for pressure switch calibration.
If you need to calibrate the pressure switch for 21PSI you need to adjust limit screws.
Apply pressure to the inlet of pressure switch .
When the pressure goes above 21 PSI, the switch opens. You can see it from Multimeter.
You can adjust the setpoint by adjusting the screws. It varies by manufacturer .Refer datasheet and manual of pressure switch for more information.