Capacitance bar sensor

Claim 1

A capacitance sensor for measuring thickness variations in film materials passed over and in contact with a relatively smooth housing surface, comprising (a) a housing plate having an upper surface comprising said relatively smooth surface, and having an elongate slot therethrough opening into an enlarged recess extending through the bottom of said housing plate, said housing plate being made from a material having a coefficient of linear temperature expansion of less than 1×10 -6 per degree Centigrade; (b) a sensor support plate affixed to said housing plate in said enlarged recess, said support plate being constructed of an insulator material having a coefficient of linear temperature expansion of less than 1×10 -6 per degree Centigrade, said material also having a dielectric temperature characteristic so as to produce a capacitance change with temperature no greater than 1×10 -5 picofarads per degree centigrade; (c) a sensor affixed to said sensor support plate, said sensor having an upstanding elongate portion sized smaller than said elongate slot, said upstanding portion being positioned centrally in said slot and having an upper surface projecting at least to said housing plate upper surface, said sensor being made from a material having a coefficient of linear temperature expansion of less than 1×10 -6 per degree Centigrade; (d) a bottom plate affixed against the lower side of said housing plate, said bottom plate being made from a material having a coefficient of linear temperature expansion of less than 1×10 -6 per degree Centigrade, said bottom plate further having at least one enlarged opening therethrough aligned beneath said sensor support plate and said sensor; and (e) electrical conductor means for electrically contacting said sensor, said conductor means passing through said at least one enlarged opening.