Campbell HydroSense II HS2, Руководство по эксплуатации продукта

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HS2 and HS2P
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constituents making possible the correlation of water content to measured
dielectric permittivity. Additionally, water and air are the only soil constituents
that change appreciably over biological time scales.
The electronics contained in the water content sensor generate the high
frequency electromagnetic energy necessary to polarize water molecules such
that their permittivity can be determined. The energy passes along a waveguide
formed by the two rods and reflects from the end of the rods and back into the
sensor head where the reflected signal is detected and time of travel is
measured. The time of travel along the waveguide is predominantly dependent
on the dielectric permittivity. Since the measured time is the net result of
passing down the length of the rods and back again, it reflects an average of the
water content over the volume of the waveguide. The sensor electronics also
detect electrical conductivity (EC) between the rods and use this to correct the
permittivity measurement. This allows the sensor to operate in a wider range of
soil EC. The calibration coefficients to convert measured time of travel to
dielectric constant and water content are contained within the sensor head and
are the intellectual property of Campbell Scientific.
6.4.4.2 Rod Insertion
For accurate, repeatable measurements, the rods of the sensor must be fully
inserted into the soil. Since the water content is averaged over the length of the
rods, the reading from a 20 cm sensor inserted vertically will be the average of
the soil moisture over the top 20 cm; however, the same rods inserted at 45°
will yield an average of the top 14 cm. This is often used for shallow rooted
crops, such as turf, to measure the average water content in the root zone of the
plant. Inserting the rods completely at an angle often requires more attention
during the insertion action.
The measurement volume of a sensor varies somewhat with soil type. As a
guide, volume extends along the full length of the rods and outward radially
from each rod a distance of approximately 3 cm.
Soil is not homogeneous. Cracks, rocks, pore size, plant roots, and texture
layers are not usually distributed uniformly throughout a measured profile. If
the water content over a large area such as a cropped field is to be determined,
several measurements may be required to establish a representative
measurement.
6.4.4.3 Soil Factors Affecting Measurement
The HydroSense II is predominantly sensitive to dielectric permittivity, and
therefore soil water content (see Section 6.4.4.1 , Measurement Principle
(p. 12)
).
Other physical properties of the soil can affect the measurement. If the soil
contains a large clay fraction or has high electrical conductivity (EC), the
applied signal can be attenuated sufficiently to affect detection of the reflected
signal in the sensor electronics. A very high organic matter fraction has a
similar effect. The HydroSense II will still respond to changes in water content
in these atypical soils, but its response will deviate from that of soils wherein
the attenuation factors are present in small non-interfering amounts. The
calibration coefficients fixed in the HydroSense II sensor were determined in
laboratory studies on typical soils. When measuring atypical soils, user
determined coefficients can often be applied to the measured period value.
Rocky soils can make rod insertion difficult and introduce variability in water
content measurements taken in the same general area. Rocks occupy space