Capacitive frost depth indicator
¹Czech Technical University in Prague, Faculty of Electrical Engineering, Department of Electrotechnology, Technicka 2, CZ166 27 Pague 6, Czech Republic
²Czech University of Life Sciences in Prague, Faculty of Engineering, Department of Electrical Engineering and Automation, Kamycka 129, CZ165 21 Prague 6 ˗ Suchdol, Czech Republic
The depth of soil freezing, i.e., the depth at which water is frozen, is a significant factor in meteorology, as it affects many processes in agriculture, building, etc. Soil frost penetration is an important factor for overwintering organisms, but also for physical and chemical processes in soil, particularly for its mechanical properties. The depth of freezing is normally determined directly, i.e. mechanically, using a special soil freezing meter, i.e., frost-depth indicator, according to the process of water freezing in the probe. Another method lies in determining the soil temperature by the interpolation of the curves from the graph, as measured by soil thermometers according to the change in the resistivity of soil or water, when frozen. The principle of the frost-depth indicator function is to evaluate the temperature dependence of water permittivity, which decreases, when water is frozen, from εr ~ 87 at 1 °C to εr ~ 3.2 at -1 °C, typical for ice. The water permittivity is evaluated by a measuring capacitor, which is adapted into the shape of the frost depth indicator probe, whose dielectric is deionized water. During freezing, the capacity is reduced in this area. The capacity of the partially frozen probe is directly proportional to the length of its non-frozen section. The measuring capacitor is a part of the resonant circuit of the oscillator. The frequency of the oscillator varies with the capacity of the probe and is further evaluated. The achievable measurement accuracy is approximately 2% for the probe of a 1 m length and in direct evaluation by an autonomous counter. For the computer evaluation, with the possibility to compensate the probe non-linearity, the measurement accuracy is approximately 0.5%.