PROTECTED THERMOCOUPLE AND PROTECTION TUBE

1,184,306. Thermocouples. CARBORUNDUM CO. 24 June, 1968 [26 June, 1967], No. 29916/68. Heading H1K. [Also in Division G1] A composite protection tube 10, e.g. for housing a thermocouple for insertion in a molten metal bath comprises an inner tube 12 of a heat resistant refractory oxide disposed inside an expandable metal tube 14, the space between the two tubes being filled with a granular refractory material 18 bonded together by a carbonized binder material. The tube 12 is preferably of alumina, zirconia, thoria or beryllia, and the tube 14 of the metal contained in the melt whose temperature is to be measured, e.g. cast iron, steel, stainless steel, nickel, copper or aluminium. The material 18 is preferably granular magnesia, though zirconia, alumina, mullite or dolomite may be used and in addition carbides, borides and nitrides may be used in place of oxides, particularly zirconium carbide, diboride or carbonitride or aluminium nitride. In order to obtain the optimum density the material 18 is preferably a mixture of different grain sizes and is vibrated as it is poured between the tubes 12, 14 during manufacture. A carbonizable binder, e.g. binding pitch is then added and carbonized, and this is followed by further addition of furfuryl alcohol copolymer or phenolformaldehyde which is also carbonized. The protection tube 10 is formed with a closed end 30, but the inner tube 12 may extend to this end or project from the end of the tube 14, if desired. In use the outer tube 14 melts preventing thermal shock to the thermocouple on insertion into a melt and the refractory material 18 then protects the thermocouple from the melt.