Immersion device for an optical fiber for measuring the temperature of a melt

The invention refers to an immersion device for measuring the temperature with an optical fiber, preferably a metal coated optical fiber in a metallurgical vessel. An immersion device according to the present invention comprises a feeding channel and 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 a melt. The immersion device comprises control means for monitoring the position of an end of the fiber relative to an end of the guiding tube. It was found that said relative position determines the quality of a temperature measurement. Consequently, to monitor said relative position makes it possible to determine the quality of a temperature measurement and thus to improve the temperature measurements.