what is dielectric permittivity and dielectric constant?
Dielectric permittivity (ε) is the ability of a substance to hold an electrical charge. The ε is grounded in complex physics but in simple terms it can be described as the ability of a substance to hold an electrical charge.
The dielectric constant (Ka) is the ratio of the permittivity of a substance to free space. The value of Ka in air is 1 and in water Ka is approximately 80.
Many materials have an ε or Ka. For example, the Ka of glass is between 5 and 10, the Ka of paper is between 2 and 4, and the Ka of body tissue is approximately 8. An example list of materials, with their respective Ka, can be found here.
Soil water content sensors, such as capacitance sensors and time domain reflectometry (TDR) sensors, measure the dielectric permittivity of soil and water to estimate soil water content. As the Ka of air is 1, pure water is 80, mineral soil is around 3, then there is a gradient between pure air and water, with a soil mixture, from very dry to very wet. Therefore, there is a gradient in Ka from dry to wet soil and this gradient can be measured via electromagnetic methods. A measured value of Ka can then be converted to % volumetric water content via a calibration equation.
The Ka of water varies slightly with temperature and pressure. A Ka value of 80 assumes water is at room temperature. Owen et al (1961) cite Ka values for water across a range of temperature and pressure. There is an inverse relationship between temperature and the Ka of water, where Ka decreases with increasing temperature.
The temperature dependency of water’s Ka has significant implications for the calibration of soil water content sensors. Typically, calibration of sensors is conducted at room temperature. However, soil temperature in the field can vary from extremely low to very high. Most researchers, and certainly most growers, ignore the effects of temperature on Ka when reporting soil water content values. Other variables, particularly soil electrical conductivity, can compound the effects of temperature on the accuracy of soil water content sensors.
dielectric permittivity explained
measuring the dielectric permittivity (ε) of soils
The TEROS-12 Soil Water Content, Temperature and Electrical Conductivity Sensor can measure the dielectric permittivity of soils. The TEROS-12 is a digital sensor that has multiple outputs, including dielectric permittivity.
Therefore, the sensor can be installed in any substrate, including mineral soils or soilless substrates, and the dielectric permittivity of that substrate can be directly measured.
The TEROS-12 is easily supported with the Procheck portable handheld meter. Or continuous measurements can be recorded with an Em50 Data Logger. Alternatively, the TEROS-12 can be supported by a system designed and installed by Edaphic Scientific that can measure related parameters such as drainage, hydraulic conductivity, weather parameters, or vegetation/plant physiology parameters.