The second method is a two-step procedure as employed by Bogena et al. (2010) [13], [24]. In this method, the output signal is first related to apparent relative permittivity (e.g., based on the standardized sensor characterization method using standard solutions with known relative permittivity [25]), and is then related to volumetric water content . The – relationship in the second step is relatively well understood [24], [26], [27]. An advantage of the two-step calibration method is that, assuming that the – relationship in the second step is valid for the soils of interest, recalibration of all sensors is not required when the sensors are installed in different soils.