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An optical fiber liquid level sensor based on differential optical absorption is described. The sensor uses a two-wavelength ratiometric approach to cancel out errors arising from variations in fuel characteristics, tank vibrations, etc. The sensor is mechanically simple and uses inexpensive LED sources and multimode optical fiber. The sensor displays ±2-mm resolution over an 18-cm range with immunity to 5° surface tilt variations and immunity to optical fiber connector and bending loss variations up to 1 dB.