Patent ID: 7195032

Claim:
A flap valve for controlling the air pressure within a protected space defined by walls, said flap valve comprising: a valve frame attachable to an opening made in said walls, a valve flap, articulated at least indirectly to said frame and being subjectable to both a sealing force which forces said flap against said valve frame and to an opening force which lifts a portion of the flap off said valve frame; said flap valve having a first position in which said protected space is sealed off from a contaminated environment, and a second position in which air from said protected space is allowed to escape into said environment via said opening; at least one spring at least indirectly attached to said valve flap and said valve frame and located in a plane substantially parallel to the plane of said valve frame, and bent leaf spring extending along one surface of said flap and affixed at one end to said flap while freely bearing against the flap at its other end, and a slider coupled to a slot in the frame and movable along said bent leaf to vary the opening force applied to the surface of said flap.