A device for measuring a temperature of a molten metal bath , comprising (a) a cored wire (1, 1'-1′′′ʺ) having an immersion end and an opposite end , the cored wire (1, 1'-1′′′ʺ) comprising (a1) an optical fiber (2, 2'-2′′′ʺ), (a2) a first metal tube (3, 3'-3′′′ʺ) accommodating the optical fiber (2, 2'-2′′′ʺ), (a3) a filler layer (4, 4'-4′′′ʺ) surrounding the first metal tube (3, 3'-3′′′ʺ) and (a4) a second metal tube (5, 5'-5′′′ʺ) accommodating the filler layer (4, 4'-4′′′ʺ), and (b) a detector to receive a signal transmitted by the optical fiber (2, 2'-2′′′ʺ), the detector being coupled to the opposite end of the cored wire (1, 1'-1′′′ʺ), wherein the first metal tube (3, 3'-3′′′ʺ) has a yield force of at least 140 N, wherein the yield force is defined as the product of the yield stress of the first metal tube (3, 3'-3""') and the cross-sectional area of the first metal tube (3, 3'-3′′′ʺ) and wherein the first metal tube (3, 3'-3′′′ʺ) has a cross sectional area defined by the thickness of the walls of the first metal tube (CW1) and a total cross sectional area defined by the outer diameter (TC1), wherein the cross sectional area defined by the thickness of the walls of the first metal tube (CW1) is less than 45 % the total cross sectional area of the first metal tube (TC1) and wherein the filler layer (4, 4'-4′′′ʺ) has a density in the range of 0,3 - 1,1 g/cm 3 and comprises a filler layer material which is an organic material having an ash content of not more than 10 mass-%. (a) a cored wire (1, 1'-1′′′ʺ) having an immersion end and an opposite end , the cored wire (1, 1'-1′′′ʺ) comprising (a1) an optical fiber (2, 2'-2′′′ʺ), (a2) a first metal tube (3, 3'-3′′′ʺ) accommodating the optical fiber (2, 2'-2′′′ʺ), (a3) a filler layer (4, 4'-4′′′ʺ) surrounding the first metal tube (3, 3'-3′′′ʺ) and (a4) a second metal tube (5, 5'-5′′′ʺ) accommodating the filler layer (4, 4'-4′′′ʺ), and (a1) an optical fiber (2, 2'-2′′′ʺ), (a2) a first metal tube (3, 3'-3′′′ʺ) accommodating the optical fiber (2, 2'-2′′′ʺ), (a3) a filler layer (4, 4'-4′′′ʺ) surrounding the first metal tube (3, 3'-3′′′ʺ) and (a4) a second metal tube (5, 5'-5′′′ʺ) accommodating the filler layer (4, 4'-4′′′ʺ), and (b) a detector to receive a signal transmitted by the optical fiber (2, 2'-2′′′ʺ), the detector being coupled to the opposite end of the cored wire (1, 1'-1′′′ʺ), wherein the first metal tube (3, 3'-3′′′ʺ) has a yield force of at least 140 N, wherein the yield force is defined as the product of the yield stress of the first metal tube (3, 3'-3""') and the cross-sectional area of the first metal tube (3, 3'-3′′′ʺ) and wherein the first metal tube (3, 3'-3′′′ʺ) has a cross sectional area defined by the thickness of the walls of the first metal tube (CW1) and a total cross sectional area defined by the outer diameter (TC1), wherein the cross sectional area defined by the thickness of the walls of the first metal tube (CW1) is less than 45 % the total cross sectional area of the first metal tube (TC1) and wherein the filler layer (4, 4'-4′′′ʺ) has a density in the range of 0,3 - 1,1 g/cm 3 and comprises a filler layer material which is an organic material having an ash content of not more than 10 mass-%. <EOS>