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With the high luminosity upgrade of the Large Hadron Collider, some zones in the Long Straight Section will expect annual radiation doses of 100 kGy. Hydrostatic levelling systems will be deployed for vertical and tilt monitoring. To resist to this high radiation doses, the electronics have to be placed up to 200 m apart from the sensors. A candidate for this task is a hydrostatic levelling sensor based on extrinsic fibre Fabry-Perot interferometer technology. Any variation in the level of the reference surface is detected as a variation in the optical path length between the fibre probe situated in the sensor head and the mirror that is submerged in the liquid. In laboratory conditions the prototype operated at a range of 10 mm with a resolution of 5 nm. Thermal and atmospheric influences as well as the internal drift of the interferometer are compensated for by the differential measurement carried out with a single laser source. The sensing fibre can be several kilometres long and the systems allows signal processing up to 10 kHz. These design parameters have been tested in two field tests, one at the Laboratoire Souterrain à Bas Bruit for a sensor validation test. The other one at CERN in comparison with the currently used capacitive hydrostatic levelling sensor type. The results show that the technology can be applied for future hydrostatic levelling sensors. The approach to an absolute measurement solution is still under investigation.
