Abstract: An apparatus for the continuous measurement of the oxygen content of molten copper and alloys thereof is disclosed. The apparatus comprises an austenitic stainless steel tube possessing a large heat capacity and capable of withstanding the operative conditions of molten copper and alloys thereof, a closed end tube of solid electrolyte inserted inside the stainless steel tube and projecting from the end of the stainless steel tube a distance at least equal to the diameter of the solid electrolyte tube, a relatively thick layer of fine grained alumina cement inserted between the stainless steel tube and the solid electrolyte tube for cementing the stainless steel tube to the solid electrolyte tube, and threads on the inside of the stainless steel tube for permitting the alumina cement to stick to the stainless steel tube.