The use of compacted landfills for filtered tailings has grown to replace dams. Considering that this is a structure that must remain in the unsaturated condition, it is necessary to monitor the flow pattern and water retention in these landfills. The use of sensors for the automatic monitoring of volumetric water content is a fundamental tool for use in the field. Sensors that use the technique called FDR (Frequency Domain Refractometry) emit an electromagnetic signal that, associated with the ability of materials to store electrical charge, allows the determination of the so-called dielectric constant. The dielectric constant relates the capacitance of the material to the capacitance of the air. As the dielectric constant in air is, by definition, equal to 1 and of the water, at 20 degrees Celsius, equal to 80, materials with a range proportion of air and water have variable dielectric constant values, considering the dielectric constant of minerals being constant. Many commercial sensors already come with a calibration curve that is based on typical soils (quartz and feldspar). However, in the case of iron mining tailings, in addition of having a very different mineral composition from the soils normally founded, there is the possibility that the presence of very fine particles could alter the dielectric constant of the liquid. In this way, it is essential to obtain a specific calibration curve, taking into account not only the material, but also the density of it. The sensor used in the present study is the TEROS 12. The results obtained for the sensor calibration in the case of iron ore tailings, demands a specific calibration to be performed for the material and each density to be used. The process used for the calibration and the equations obtained are presented in this paper.