Frequency ___domain sensor: Difference between revisions

Revision as of 04:41, 3 August 2006

Revision as of 17:05, 11 August 2006

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There are two types of sensors:

* ''Capacitance probe'', or fringe capacitance sensor. The configuration is like the [[neutron probe]] where an access tube made of [[PVC]] is intalledinstalled 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 [[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 burriedburied 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.

Compared with [[time ___domain reflectometer]] (TDR), FD sensors are cheaper to built and a has a quicker response time. However becuasebecause of the complex electrical field around the probe, the sensor needs to be calibrated for different soil type.

==References==