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An optical fiber moisture sensor is prepared by coating a 1 mm polymer optical fiber with a film 30 micron thick of Polyvinyl Acetate PVA. We have experimentally studied how the light transmission characteristics of such coated optical fiber changes as a function of moisture conditions within an oil-paper insulation, to which the coating is exposed. An optical fiber moisture sensor that can be used to sense moisture present in liquid transformer oil and composite paper insulation in a wide range of concentrations is characterized. The resulting optical fiber sensor can be used for on-line measurements in electrical apparatus that use oil-paper insulation under large electrical field gradients. A light guide such as an optical polymer fiber operating in the 400 nm to 970 nm wavelength range is embedded in a body of paper insulation material immersed in oil, with the free ends of the fiber extending to or beyond the edges of the structure so that radiation such as light can be directed through one end of the carrier, and measured at the other end thereof, to monitor and detect the presence of moisture in the interior of the structure. Under normal operating conditions of power transformers the moisture concentration in the oil is in the range of 10 to 100 ppm and 0.5 to 5% in paper for a temperature range of 0 to 100degC. In order to compensate for the oil optical properties, an additional optical polymer fiber in contact with the transformer oil is used as a cero reference outside of the paper structure, since amount of moisture in oil is at least 200 times smaller than the moisture in the paper.