A device for detecting characteristics of a fluid, with a light source for emitting several light beams, of which one of the light beams is a measurement beam, which is provided for a passage through the fluid, and another light beam is a reference beam, which is provided for bypassing the fluid, with a movable cover device arranged downstream of the light source, which cover device is provided for covering the light beams and which is arranged so as to be transferable between a first position releasing the measurement beam and covering reference beam, and a second position covering the measurement beam and releasing the reference beam, and with a light detector arranged downstream of the cover device.