Campbell Scientific HS2 and HS2P (HydroSense II) - Manual (Page 21)

HS2 and HS2P
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
otherwise occupied by the fine soil fraction, but they do not hold water in the
same manner as soil. If two proximal measurements are made in rocky soil,
the measured water content can differ significantly if large quantities of rock
occupy part of the sensitive volume of one measurement but not the other.
6.4.4.4 Measurements in Special Materials
The HydroSense II was designed for use in agricultural soils, but the
measurement technique underlying the instrument supports other potential
applications. Other porous media can be monitored using the period value
shown on the display. The period is strongly related to dielectric permittivity
of the material surrounding the sensor rods and can be used as a relative value
to measure changes in the material of interest. Period generally increases
proportionally with water content. For actual water content values, a soil
specific calibration can be performed using an independent measure of water
content such as gravimetric analysis. A calibration equation can then be
derived to relate period to water content.
7. User Interface
The following section contains a detailed description of the HydroSense II user
interface.
Screenshots included in this section were captured using factory default
settings (except where noted); however, they may not reflect the exact image
seen on your screen because of configuration settings chosen or operating
system updates
Red highlights on the images mark areas of interest. The highlights are added
for illustrative purposes and are not present on the screen of an actual display.
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