Abstract: The apparatus includes at least one light-reflecting element (11) disposed on a portion of at least one of the conventional rotary elements (10) of the mechanism of a mechanical fluid meter. The apparatus further includes a light-emitting circuit (130) and a light-detecting and converting circuit (140) with first and second concentric fibers (21, 22) providing an optical connection between the circuits and the meter. The electrical signal delivered by the conversion circuit (140) is amplified and the amplified electrical signal is compared with first and second thresholds (S1, S2). An alarm and indicator circuit is responsive to the comparison made with the first threshold (S1), and a circuit (190) counts and stores the pulses delivered after comparison with the second threshold (S2).