Abstract: In a method and apparatus for measuring the concentration of a fluid depending on the dielectric constant of the fluid, electric oscillations are supplied from a source through an impedance matching link to an antenna placed in the liquid within the confines of a Faraday cage, and the electric power reflected from the antenna towards the source is measured to determine deviations from ideal matching resulting from changes of the dielectric constant of the fluid. In an important embodiment, the measurement is used for the automatic programmed control of the supply of syrup in a sugar crystallization process in accordance with a predetermined program.