Pressure-responsive apparatus having hysteresis to prevent hunting

Claim 1

A pressure responsive apparatus, comprising: a case defining an internal volume and having a first fluid port and a second fluid port, said ports each being made of electrically conductive materials but electrically insulated from each other; a snap-action diaphragm which has a first stable state and a second stable state, with a predetermined force required to cause it to go from each one of said states to the other, mounted nonrigidly in said case and electrically insulated therefrom by flexible peripherally disposed seals on both sides for partitioning said internal volume into a first chamber accessible to fluid via said first fluid port and a second chamber accessible to fluid via said second fluid port, said diaphragm having an electrically conducting portion; an electrically conducting spring of known stiffness contacting at a first end said electrically conducting first fluid port and contacting at a second end said electrically conducting portion of said diaphragm, said spring then being deflected from its unstressed form by maximum predetermined biasing force on said diaphragm directed towards said second port such that when a predetermined pressure difference between said first and said second chambers is exceeded the combined fluid pressure force and spring bias force causes said diaphragm to move from said first stable state into said second state in which said electrically conducting portion of said diaphragm establishes contact with said electrically conducting second port, such that said first port, said spring, said electrically conducting portion and said second port cooperate to establish an electrically conductive path when said diaphragm is in contact with said second port.