Gypsum blocks use two electrodes placed into a small block of gypsum to measure soil water tension. Wires connected to the electrodes are connected to either a portable hand-held reader or a data logger. The amount of water in the soil is determined by the electrical resistance between the two electrodes within the gypsum block. …
Neutron probes are another way to measure soil moisture content. A probe inserted in the ground emits low-level radiation in the form of neutrons. These collide with the hydrogen atoms contained in water, which is detected by the probe. The more water content in the soil, the more neutrons are scattered back at the device. …
The Coaxial Impedance Dielectric Reflectometry (also referred to as “Ratiometric Coaxial Impedance Dielectric Reflectometry”) method of soil moisture measurement employs an oscillator to generate an electromagnetic signal that is propagated through the sensor by metal tines and into the soil. It characterizes the ratio of the amplitudes of reflected radio waves at 50 MHz with a …
Time Domain Transmissometry (TDT) is similar to TDR, however it measures the transmission, rather than reflection, of a pulse along a looped, or closed circuit, rod. TDT measures the time taken for an electromagnetic wave to propagate (travel) along a given length of a transmission line in the soil. With TDT, a step pulse with …
Capacitance determines the dielectric constant (Ka) by measuring the charge time of a capacitor, which uses soil as a dielectric medium. The charge time of the capacitor is a linear function of the dielectric permittivity of the soil. Capacitance sensors measure the dielectric constant of the soil in order to find its water content. Since the dielectric constant of water …
Sensors that use the Time Domain Reflectometry (TDR) are somewhat similar way to FDR probes, but the mechanics behind the measurement system are different. TDR sensors use parallel rods, acting as transmission lines. A voltage is applied to the rods and reflected back to the sensor for analysis. The speed or velocity of the voltage …
Capacitance can be measured from the change in frequency of a reflected radio wave or resonance frequency (Kelleners 2004). This method of measurement uses an oscillator to propagate an electromagnetic signal through a metal tine or other wave guide. The difference between the output wave and the return wave frequency is measured to determine soil …
Frequency Domain Reflectometry (FDR) / Capacitance (Frequency) Read More »
