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The AQUAFLEX soil moisture probe uses a process called Time Domain Transmission (TDT).
An
electrical pulse is sent along a transmission line, which is embedded
within the AQUAFLEX sensor, and the electrical field around the
transmission line interacts with the surrounding medium.
The
speed and shape of this pulse is affected by the dielectric properties
of the medium. Water has a much higher dielectric constant than most
materials that constitute soil, so the presence of water has a profound
impact on the electrical signal. This factor enables a soil moisture
probe to measure moisture content.
TDT
can be considered similar to TDR (Time Domain Reflectrometry), however,
an AQUAFLEX soil moisture probe differs from traditional TDR in several
ways.
- The sensor of an AQUAFLEX
soil moisture probe consists of a transmission line that extends to the
end of the sensor ribbon and then returns back to the sensor
electronics. This means that AQUAFLEX measures the original pulse, not
a reflection from an open metal rod.
- The
AQUAFLEX pulse circulates many times through the sensor cable before the pulse delay is measured, resulting in
extremely high precision in the time delay measurement.
- The
AQUAFLEX soil moisture probe has a large measurement area. The sensor
is 3 meters in length (10 feet). Whereas TDR measures a very small
sample, AQUAFLEX spatially averages over a 50mmx3000mm cylinder of
soil. This represents a sample size of approximately 6 litres. This
sample size is significantly larger than other techniques and
facilitates reliable and repeatable results.
- The
shape of the pulse used by the AQUAFLEX electronics differs from that
used by traditional TDR instrumentation, with the consequence that
losses and noise in the transmission are reduced. As a result, an
extremely good signal level can be achieved in a 3 metre long sensor.
- AQUAFLEX
extracts information from the shape of the pulse, which gives a good
indication of soil conductivity - information that is used to
compensate the moisture measurement reading to maintain accuracy in
conductive soils.
- AQUAFLEX includes automatic temperature compensation and also provides a soil temperature output.
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