Abstract : This paper addresses the metrological stability of spectral measurements performed with a smart FBG sensing instrument. With a sophisticated tracking system based on a real-time drift compensation, we obtained, during 16 days, an unrivalled measurement stability, as low as: 0.0074 fm/h with a standard deviation: 0.032 pm. This level of performance opens the way, not only to high quality metrology, but also to long-term structure monitoring.