An immersion device for measuring the temperature of a melt with an optical fiber, preferably a metal coated optical fiber, in a metallurgical vessel includes a feeding channel, feeding means for feeding an optical fiber into a disposable guiding tube and for feeding the disposable guiding tube together with the optical fiber into the melt, and control means for monitoring the position of an end of the optical fiber relative to an end of the disposable guiding tube. The relative position determines the quality of a temperature measurement.