Abstract: A very low differential pressure switch for detecting pressure differential across two points of a pressurized system. A flexible diaphragm divides two cavities which are exposed to fluid pressures across two points of the pressurized system, and moves a sensor actuating target axially against a compression spring relative to the sensed differential pressures. The switch has a proximity sensor responsive to the axial location of the target to produce an output indicative of a pressure differential condition between the two cavities. The switch has a selected pressure sensitivity and volumetric displacement threshold dependent in part on the cross-sectional area of the flexible diaphragm, size of the compression spring and on the distance between the diaphragm and the proximity sensor.