Patent ID: 8710385

Claim:
A fire pressure switch comprising: a case having a first down-hole end and a second up-hole end and an axial passage therethrough the axial passage having at least two sections having different diameters; an electrically conductive piston having a down-hole end and an up-hole end, the second end having a recessed portion; a portion of the up-hole portion of the piston surrounded by an injection molded sheath; the piston having one or more circumferential grooves which are filled with insulating material comprising portions of the sheath; the sheath frictionally engaging a portion of the axial passage of the case; a space between the up-hole end of the piston and the down-hole end of an insert; a lubricant in the space comprising approximately 0.0065 ounces of lubricant; a conductive rigid dart having a conical down-hole head and a tapered up-hole tail; a portion of the tapered tail surrounded by a spring; the spring maintaining the dart in electrical connection with the piston and permitting movement of the dart between a position which is electrically insulated from an insert and a position which is electrically connected to the insert; the dart having an insulating sleeve surrounding a portion of the tail; an insert having an electrically conductive inner portion and an electrically insulated sheath and having an axial passage having a diameter larger than the diameter of the dart; the electrically insulating sheath having a uniform outer diameter which is larger than the interior diameter of a corresponding portion of the case by between 0.001 and 0.01 inches.