Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove tubes from APS.
Attach multimeter to APS using crocodile clips.
Remove tubes from APS.
Attach multimeter to APS using crocodile clips.
Measurement:
Note the operating pressure indicated on the pressure switch.
Connect the air pressure switch charge kit to +ve connection on the air pressure switch.
Increase the pressure via the syringe to that indicated on the pressure switch.
Check the multimeter to see if you have continuity / zero resistance.
Note :
If continuity is not obtained when the specified pressure is applied it is permissible to increase the pressure by 50%.
100 pascals = 1mbar / 50 pascals = 0.5 mbar.
Connect the air pressure switch charge kit to +ve connection on the air pressure switch.
Increase the pressure via the syringe to that indicated on the pressure switch.
Check the multimeter to see if you have continuity / zero resistance.
Note :
If continuity is not obtained when the specified pressure is applied it is permissible to increase the pressure by 50%.
100 pascals = 1mbar / 50 pascals = 0.5 mbar.
Isolate as per TOPs P4
and refit as necessary