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Dielectric-based moisture measurements (PDF Download)
Abstract
Nondestructive measurements of soil moisture have been practiced in agriculture for over fifty years. With few exceptions, these are performed by measuring the dielectric permittivity of soil. The two most popular techniques adopted in low-cost moisture sensors are Capacitance measurements and Time Domain Reflectometry (TDR). Both methods, although for different reasons, are limited in their ability to measure permittivity in electrically conductive materials. This is usually not a concern with agricultural soils, due to the modest conductivity values typically met in the field. In greenhouse cultures, on the other hand, the growing substrate typically retains water and nutrients in high concentration, and is therefore a lot more conductive than soil. Complex Dielectric Sensors (CDS) do not suffer from these limitations. Once considered too expensive for the industry, this technique is now available at low-cost. In this brief note we will describe the working principles of the three technologies and compare their performance.