Sheathed thermocouple lance for measuring temperature in molten metal bath

A thermocouple lance for measuring temperatures in molten metals has a protective sheath(1,21) of multilayered structure that makes it possible to improve the life expectancy of repetitive measuring services. The thermocouple lance comprises a protective sheath(1,21) with one closed end and one open end wherein porous heat-resisting ceramic filler(8,25) is compacted. Embedded in the filler(8,25) is a pair of W-Re alloy wires(6,7,26,27) dissimilar in their composition from each other. The protective sheath(1) has the layered structure made from the cermet layers(2) each having a molybdenum matrix, and other layers(3) of at least one selected from the class consisting of C, MgO, CaO, Al2O3 and ZrO2, both of which layers are overlapped each other in alternate layer. As an alternative, the protective sheath(21) may be composed of layers in which are embedded filaments(24) oriented predominantly in the perimetral direction of the sheath(21), and boundary layers(23) which are provided between confronting surfaces of any adjoining filament-embedded layers, the boundary layer(23) being weaker in bonding power than the filament-embedded layers. The filaments (24) are of carbon filaments or ceramic filaments whereby the protective sheath(21) may be enhanced in strength against the tensile stress that will be occur due to the temperature difference in the protective sheath(21).