An inerting system is disclosed that is adaptable to inert the fuel tank of a vehicle, most typically an aircraft, that includes an oxygen detector to monitor the oxygen partial pressure of the vapors in the ullage (i.e., the overfuel) volume of the tank, a source of an inert gas (e.g., nitrogen) in valved communication with the ullage of the tank, and a detector for sensing the oxygen content in the ullage of the tank and controlling the flow of inert gas to the tillage to maintain that volume with a proportion oxygen that will not support combustion in the event of an ignition source or intrusion of another potentially explosive occurrence within said tank. A specific fiberoptic probe which enables monitoring oxygen content within the tank without introducing a source of electrical current within the tank is also disclosed.