Electric proximity sensor

An electric proximity sensor of the type which in use is exposed to pressure fluid and which comprises a casing having an interior chamber space open at one axial end and bounded circumferentially by a sidewall and at the opposite axial end by a transverse wall, a pick-up coil and pot core unit disposed within said interior space, a closure closing said one axial end and an O-ring seal circumferentially sealing between said closure and said sidewall, characterized in that a separate insert member is provided between said unit and said walls bounding said interior space, said insert member being dimensioned to fit closely with respect to said walls and comprising a cavity which is open toward said one axial end, said cavity being dimensioned to receive said unit with a close fit, said insert and said unit substantially filling said interior space, said sensor further comprising means defining a shoulder which extends circumferentially around the inside perimeter of said sidewall at said one axial end of said casing, said closure having a circumferential margin which is disposed to radially overlap said shoulder, said O-ring seal being disposed on said shoulder, and means for retaining said closure on said casing such that said O-ring seal is axially compressed against said shoulder by the circumferential margin of said closure and forms a circumferential seal between said closure and said casing.