Problems concerning the adjustment of fiber-optic refractometers with a U-shaped fiber for concentration measurement of sugar solutions are discussed. Proposed methods are based on the application of computer models. A regression analysis has been used for the setting-up of laboratory fiber-optic refractometer. A precision better than 0.3% dry substance has been obtained. Due to the hard working conditions industrial refractometers have been adjusted with mathematical models by self-organization methods. The maximum deviation of the industrial refractometer for measurement of sugar in apple concentrate does not exceed 0.6% dry substance.