Valve unit

In a valve unit having a switch comprising an electrically conductive valve which slidably moves within a guide hole defined in an electrically conductive body and a layer for maintaining an electrically insulated state between said valve and said body for forming a switch wherein an electrically conductive state is established between said valve and said body when said valve is seated on a valve seat associated therewith and an electrically insulated state is established between said valve and said body when said valve is lifted from said valve seat,

• the improvement wherein said layer is made of three different portions which contain different compositions of a specific metal and a specific reaction gas, said specific metal being selected as having a property as metal of strong adherence to said valve and a property as a compound of the metal and the gas of being electrically insulating, wherein a first portion is disposed as an inner surface of said layer closer to said valve which is composed substantially of said metal, a second portion is disposed as an outer surface of said layer which is composed of an insulating compound of said metal and said gas, and a third portion is disposed as the region in between said inner and outer surfaces which is composed of a non-stoichiometric compound of said metal and said gas which varies progressively in the direction of thickness of said third portion from substantially the metal composition at said inner surface to the insulating compound composition at said outer surface so as to strongly bind said first portion of said layer to said second portion thereof, and said layer is coated on the peripheral surface of said valve such that said metal composition of said inner surface of said layer forms a strongly adhering bond to the surface of said valve and said outer surface of said layer forms an electrically insulating surface destined to make insulating contact with the wall of said guide hole.