A system for determining a process variable of a medium arranged in a container is disclosed, including an optical fiber Bragg sensor with an optical waveguide with a fiber Bragg grating, a signal generating unit designed to generate an optical input signal and couple it into the waveguide, a receiving unit designed to receive an optical output signal from the waveguide and converts it into an electrical output signal, and an evaluating unit which determines the process variable using the electrical output signal, wherein a subsection of the optical waveguide is arranged inside or in a wall of the container, the subsection designed such that the fiber Bragg grating is affected by the process variable to be determined of the medium.