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Instrument: TDR : Time Domain Reflectometry Probe
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Description
In the past 20 years use of the soil?s dielectric properties to
determine moisture content have been met with increasing
technological success through use Time Domain Reflectometry
(TDR).

The TDR method is based on determining the propagation time of
an electromagnetic pulse traveling along a probe segment in the
soil. The TDR device first measures the round-trip time for the
pulse to travel to the beginning of the segment, which is
indicated by the presence of a diode. Then the round-trip
propagation time to the end of the segment is measured. The
resulting value is used to determine the dielectric constant of
the material. This value, when compared to the dielectric
constant of pure water and mineral soil, will indicate the
amount of water found in the medium.

[Summary provided by North Dakota State University]