US Patent 7116236 Oil tank sight glass monitor

An oil tank sight glass monitor comprising a housing having an opening therein sized to receive the sight glass of an oil tank; an emitter for emitting a beam of light, the emitter being mounted to the housing on one side of the opening so that the beam of light is directed across the opening; a detector for detecting the beam of light generated by the emitter, the detector being mounted to the housing on the other side of the opening, in optical alignment with the emitter, so as to detect the beam of light emitted by the emitter; and an alarm for producing an audible sound when the detector fails to detect the beam of light generated by the emitter.