If you want to detect a both positive differential(HP > LP) and a negative differential (HP < LP) then you choose a switch with the appropriate range and calibrate it accordingly. For example if you require to measure say +50 PSI and -50 PSI then choose a switch with a range of say -75 PSI to +75 PSI. The unit would also require to be equipt with 2 switches, one to detect the -ve dp and one to detect the +ve dp. You cannot achieve both on one switch.