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There are two types of sensors:
* ''[[Capacitance probe]]'', or fringe capacitance sensor. Capacitance probes use capacitance to measure the dielectric permittivity of the soil. The volume of water in the total volume of soil most heavily influences the dielectric permittivity of the soil because the dielectric constant of water (80) is much greater than the other constituents of the soil (mineral soil: 4, organic matter: 4, air: 1). Thus, when the amount of water changes in the soil, the probe will measure a change in capacitance (from the change in dielectric permittivity) that can be directly correlated with a change in water content. Circuitry inside some commercial probes change the capacitance measurement into a proportional millivolt output. Other configuration are like the [[neutron probe]] where an access tube made of [[PVC]] is installed in the soil. The probe consists of sensing head at fixed depth. The sensing head consists of an [[oscillator]] circuit, the frequency is determined by an annular electrode, fringe-effect [[capacitor]], and the [[dielectric constant]] of the soil.
* ''Electrical [[Electrical impedance|impedance]] sensor'', which consists of soil probes and using electrical impedance measurement. The most common configuration is based on the [[standing wave]] principle (Gaskin & Miller, 1996). The device comprises a 100 MHz sinusoidal [[oscillator]], a fixed impedance [[coaxial]] [[transmission line]], and probe wires which is buried in the soil. The oscillator signal is propagated along the transmission line into the soil probe, and if the probe's impedance differs from that of the transmission line, a proportion of the incident signal is reflected back along the line towards the signal source.
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