given dechuck operation the membrane 22 is vented the perforated plate 40 is extended until contacting the substrate extending of the perforated plate 40 also causes encapsulated region 28 to be pressurized due to the spring action of spring 42 acting upon plunger 46 and causing the captured resilient sealing ring 44 to seal off sensor venting ports 53 the pressurized region 28 is sensed by pressure sensor 32 vacuum is pulled on membrane 22 wherein vacuum is drawn behind membrane 22 and within the openings of perforated plate 40 causing a suctioning effect between the membrane 22 and the substrate in response to suctioning of the by membrane 22 the substrate acts upon plunger 46 in an upward fashion causing plunger 46 to be displaced displacement of plunger 46 moves the captured resilient sealing ring 44 in a controlled manner to break the seal thereby allowing region 28 to vent the venting of region 28 causes a change in pressure of the encapsulated region accordingly pressure sensor 32 senses the change in pressure thus indicating the presence of the substrate according to one embodiment a system for polishing a substrate includes a controller a platen and a carrier head the carrier head is coupled to the controller the carrier head is.