The Competence Centre for Material Moisture (CMM) at the Karlsruhe Institute of Technology (KIT) is specialized on the development and testing of EM moisture sensors for various materials with a special focus on dielectric methods such as Time Domain Reflectometry (TDR) and Frequency Domain (FD) Methods. This paper gives a short review on the recent developments of new dielectric sensors and techniques for soil and snow moisture measurements of this institution. A short introduction to the basic measurement principles of dielectric measurement techniques will be given in the next paragraph.2.?Electromagnetic Moisture (EM) Measurement MethodsEM moisture measurement methods are so-called indirect methods that determine an electrical property which is closely related to the water content of the material, such as the electrical conductivity (EC) or the dielectric permittivity.
Since EC is also considerably affected by the salinity of the material, mainly the dielectric methods promise good opportunities for accurate measurements.There exists a broad variety of different dielectric moisture measurement methods . Depending on the type of the material, the geometry of the sample, frequency range or desired accuracy, a suitable measurement method can be chosen. All methods are based on the interaction of an EM field and the material to be measured. A classification of the dielectric methods is possible by the way the EM field is generated and observed.So-called FD methods on the one hand show a sinusoidal time dependence of the field.
The steady state signals can be analysed by methods of the complex EM field theory.TD methods, on the other hand, use signals with a transient character, confering a pulse-like time shape on the EM field. Both devices and analyzing techniques of these two methods differ significantly and are explained in more detail in the following.2.1. Batimastat Frequency Domain (FD) TechniqueFD or capacitive techniques use capacitance to measure the dielectric permittivity of a surrounding medium and operate at one single measurement frequency. When the amount of water changes in the soil, a probe will measure a change in capacitance due to the change in dielectric permittivity that can be directly correlated with a change in soil water content . During the last two decades, FD sensors have been gaining more and more acceptance in a broad range of agricultural, environmental, and engineering disciplines. Their widespread use has resulted partly from the simpler and thus less expensive electronics compared to TD methods. Currently, FD sensors are commercialized as single and multi-sensor capacitance probes with different installation and monitoring techniques .2.2.