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Image Number 2 for United States Patent #5071258.
A thermocouple assembly for sensing temperatures of molten metals in a metallurgical vessel, such as a tundish used in continuous steel casting, includes a double bore insulator containing the two noble metal alloy lead wires of the thermocouple. Platinum-rhodium alloys are preferred. The weld bead joining the ends of the leads or hot junction of the thermocouple is positioned within one of the bores of the insulator such that a lower purity alloy lead is exposed in the loop. The loop resides in a blind bore formed in the end of the double bore insulator and is enclosed in a containmemt zone by a refractory plug or foil disc closing off the end of the blind bore. The double bore insulator is received in an axial bore of a refractory sheath having a closed end adjacent the loop area of the thermocouple. The bores of the double bore insulator and refractory sheath communicate with the atmosphere to permit oxygen to diffuse therealong to the loop area to protect the platinum alloy from reaction with carbon and silica which may be present in trace amounts. A closed end molybdenum sheath having an axial bore receives the refractory sheath therein. High purity alumina powder is preferably packed around an upper annular space between the bore of the molybdenum sheath and the refractory sheath to prevent diffusion of atmospheric oxygen within the molybdenum bore and thereby minimize the formation of gaseous molybdenum oxide at molten steel temperatures.
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