Thermocouple and method of making the thermocouple and of mounting the thermocouple on a heat exchanger tube

The method of mounting a thermocouple assembly on the fired surface of a heat exchanger tube, wherein the tube is externally subjected to high temperature gases and carries a fluid under pressure to be heated, and wherein the thermocouple assembly includes a cold junction end having a temperature readout instrument and a hot junction end having a temperature sensing junction member, the ends being connected by sheathed thermocouple conductors, said sheathed thermocouple conductors including an elongated tubular metal sheath, a pair of spaced thermocouple conductors within said sheath, electrical insulating material spacing and supporting said conductors within the sheath in insulating relation from each other and from the sheath, said hot junction end being defined by a thermojunction tip, said tip including a face [in] of which the conductor s end[s], [insulation and sheath are substantially coplanar], a junction member in contact with face and welded to said conductors, said junction member extending from said sheath and spaced from said sheath to thermally electrically isolate the junction member, which method comprises, the steps of positioning the thermojunction tip on the tube surface so that the outer face of the junction member mates with said tube surface and the sheath is in spaced relation thereto, and applying a plurality of weld passes between the periphery of the sheath and the tube to define a void-free and leakproof weldment to prevent contact between the gases and the conductors, the number of weld passes being such that the cross section of the weldment is greater than the cross section of the sheath wall to define an unimpeded heat transfer path between the sheath and the tube, whereby the natural shrinkage of the weldment pressurizes the junction member against the tube surface, the face is in spaced relation to the tube surface and the heat energy of the sheath goes through the weldment to the tube and bypasses the junction member.

The method of mounting a thermocouple assembly on the fired surface of a heat exchanger tube, wherein the tube is externally subjected to high temperature gases and carries a fluid under pressure to be heated, and wherein the thermocouple assembly includes a bulk length of mineral oxide insulated metal sheathed thermocouple wires having an elongated tubular metal sheath, a pair of spaced thermocouple wires within said sheath, and said mineral oxide insulating material spacing and supporting said conductors within the sheath in electrical and thermal insulating relation from each other and from the sheath, and a thermojunction tip formed on an end of said length defining a face [where the sheath, insulation and wire ends are substantially coplanar], and a junction member in contact with said wires but in spaced relation to the sheath so that the junction member protrudes from said face, which method comprises, the steps of positioning the thermojunction tip on the tube surface so that the outer face of the junction member mates with said tube surface and the face is in spaced relation thereto, and applying a plurality of weld passes between the periphery of the sheath and the tube to define a void-free and leakproof weldment to prevent contact between the gases and the conductors, the number of weld passes being such that the cross section of the weldment is greater than the cross section of the sheath wall, whereby the natural shrinkage of the weldment pressurizes the junction member against the tube surface, said face being in spaced relation to the tube surface whereby the heat energy of the sheath goes through the weldment to the tube and bypasses the junction member.

In a method of measuring the surface temperature of a fired heat exchanger tube, the steps of

• supporting a pair of thermocouple wires within a length of elongated tubular metal sheath with a mineral oxide material, said mineral oxide material spacing said conductors within the sheath in electrical and thermal insulating relation from each other and from the sheath,
• terminating the thermocouple wires and the length of metal sheath to form a hot junction end with the terminated ends of the wires and sheath defining a face,
• securing a thermojunction member across the ends of the wires in spaced relation to the sheath so that the member protrudes from said face,
• positioning the thermojunction member against the heat exchanger tube, and
• applying a void-free continuous weldment around the periphery of the sheath to secure the sheath to the heat exchanger tube, whereby the natural shrinkage of the weldment pressurizes the junction member against the tube surface.