Campbell Scientific HS2 and HS2P (HydroSense II) - Manual (Page 20)

HS2 and HS2P
Selecting NEW PROBE will overwrite the soil profile, clearing the other
reference value, whereas selecting OLD PROBE will discard the measurement
and leave the soil profile unchanged. This feature forces both reference values
to use the same rod length. This step cannot be reversed. Using ,
select the desired option and press to accept or to discard the new
measurement and return to the soil profile screen.
Once the soil profile changes are complete, use to choose SELECT
THIS SOIL and press . This will return to the main screen (p. 14) and use
the new soil profile.
6.4.3 Storage
The HydroSense II does not store relative water content or deficit
measurements to flash. Only water content measurements are stored.
6.4.4 Proper Measurement Technique and Limitations
6.4.4.1 Measurement Principle
The HydroSense II uses soil dielectric permittivity to estimate volumetric water
content. Dielectric permittivity of water is much greater than that of other soil
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.
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