Abstract: A thermocouple protection tube is designed such that it is capable of adjusting the length of thermocouple inside reactor after assembly. The design includes an outer casing, an inner casing and a means for fixing the relative position of the outer casing and the inner casing. The inner casing is positioned relative to, and within the outer casing. The fixing means temporarily fixes the relative position of inner casing and outer casing. The disclosed design permits the adjustment of the thermocouple length inside thermocouple protection tube according to the relative thickness the refractory lining inside reactor.