Abstract: A system and method for measuring pressure inside a pipeline or pressure vessel is disclosed. The system includes one or more units locatable around an external surface of the pipeline or vessel, each unit containing: a light source for generating a light beam; a single fiber Mach Zehnder interferometer (SF-MZI) sensor bendable against an outer surface of the pipeline or vessel, for receiving the light beam; a light receiver for receiving the light bean from an output of the SF-MZI sensor, said light receiver including an optical spectral analyzer; and a local processor for signal conversion; a data acquisition system associated with each of the one or more units to provide power to the elements of the unit; and a central processor in communication with each of the local processors of each of the units, said central processor containing stored strain data, and the stored bend induced birefringence data.