Pressure Switch (reverse differential)

If you have a Differential Pressure Switch and the "High" port goes lower pressure than the "Low" port... will it activate the switch ever? (reverse differential)

No, not the switches I have ever dealt with. But you should test it and make sure
it preforms as you want it to.

Ok thanks! I've been trying to find a diagram or drawing that shows how the insides of the thing works but have been unsuccessful thus far.

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.

Or if you only want to check for a negative differential simply pipe the switch the other way round.

If you want to check for both + and - d.p. use 2 switches piped the opposite way to each other.

If your differential is going + & - don't forget to check the manufacturers data on maximum reverse pressure differentials.

sounds like some good ideas. thanks guys!