Field Scout TDR 100, Руководство по эксплуатации продукта

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6
Volumetric Water
Content Mode
The volumetric water content (VWC) is the ratio of the
volume of water in a given volume of soil to the total soil
volume. At saturation, the volumetric water content
(expressed as a percentage) will equal the percent pore
space of the soil.
The underlying principal of TDR involves measuring the
travel time of an electromagnetic wave along a wave-
guide. The speed of the wave in soil is dependent on the
bulk dielectric permittivity (
ε
) of the soil matrix. The fact
that water (
ε
= 80) has a much greater dielectric constant
than air (
ε
= 1) or soil solids (
ε
= 3-7) is exploited to de-
termine the VWC of the soil. The VWC measured by
TDR is an average over the length of the waveguide.
Electronics in the TDR 300 generate and sense the return
of a high energy signal that travels down and back,
through the soil, along the waveguide composed of the
two replaceable, stainless steel rods. The sampling vol-
ume is an elliptical cylinder that extends approximately 3
cm out from the rods. The high frequency signal infor-
mation is then converted to volumetric water content.
However, high amounts of clay and high electrical con-
ductivity (EC>2 dS/m) will attenuate the high-frequency
signal and affect the reading displayed by the meter. Very
high organic matter content will similarly affect the VWC
reading.
When taking a measurement, it is important that the rods
be fully inserted into the soil. If not, part of the sampling
volume will be composed of air and the reading will be