Abstract: A concentration measuring apparatus is provided having a fluid sensing means, an optic transmission means, a signal processing means, and an interface means. The fluid sensing means comprises a reflective sensing media in a self-contained cassette. The cassette is operationally associated with a sample chamber. The cassette advances the reflective sensing media by a motor which is driven by the signal processing means. The optic means comprises a fiber optic line which is partially associated with a lamp and partially associated with a photocell. Light having a wave length in a specified range originates from the lamp, passes through a portion of the fiber optic cable and is cast upon the reflective sensing media. The altnerate portion of the fiber optic cable receives the reflected light from the reflective sensing media for transfer to the photocell. A signal is generated by the photocell representative of the change of reflectivity of the light from the reflective sensing media.