A method is provided for determining temperature changes of an optical fibre having Fiber Bragg Grating (FBG) patterns provided in at least one portion (Portion 1) of the optical fibre. The optical fibre is connected between a first detector arrangement and a second detector arrangement. Light is emitted into the optical fibre in a first direction from the first detector arrangement, which receives reflections from the FBG patterns of the emitted light. The reflections are processed for determining a current temperature change related to the optical fibre. On the basis of predetermined criteria, light is emitted into the optical fibre in an opposite second, direction from the second detector arrangement. The second detector arrangement receives reflections from the FBG patterns of the light emitted in the second direction and the reflections are processed for determining a current temperature change related to the optical fibre.