This paper describes the development of a sensor designed to monitor the closed loop recirculation gas system of the Resistive Plate Counter (RPC) muon detectors which are used for the Compact Muon Solenoid (CMS) experiment installed at the Large Hadron Collider (LHC) accel-
erator of the European Center for Nuclear Research (CERN). The sensor is designed to detect low concentrations of hydrogen fluoride (HF) in the gas mixtures and employs a plastic optic fiber (POF) which is covered with a thin layer of a glass-like material. The fluoride ions attacks the
glass and alters the fiber transmission capability so that the detection simply requires a LED and a photodiode. The coating is obtained by means of a low pressure plasma enhanced chemical vapor deposition (PECVD) that allows the glass-like film to be deposited at low temperature without damaging the fiber core.