HydroSense II Soil Moisture Measurement System
20
When a new zone is created, the screen above is displayed. Use the Up and
Down keys to select the radius to use for this zone and press
OK
to save the new
zone details. Press
Back
to return to the
main screen
without creating a new zone
and without storing any data.
Once a zone is created, the water content data will be stored. By default the zone
name will be in the form “Zone xxxxx” where xxxxx is a number that
automatically increases each time a new zone is created, however the zones can
be renamed and updated at a later time using the HydroSense II software.
If the
Store
button is pressed when the GPS does not have a valid GPS sync the
screen above is displayed. This warning is intended to serve as a reminder that
these measurements will not be stored with an accurate date, time or position. To
continue and store data without GPS information, press
OK
, to return to the
main
screen
without storing, press
Back
.
The screens shown above are only displayed when a measurement is stored in a
new location. If the current position is within an existing zone, the
Store
button
simply causes data to be stored without any queries or warnings.
9
Proper measurement Technique & Limitations
9.1
Measurement Principle
The HydroSense II uses the soil dielectric permittivity to estimate the volumetric
water content. The dielectric permittivity of water is much greater than the other
soil constituents making it possible to relate water content to measured dielectric
permittivity. Additionally, water and air are the only soil constituents that change
over a reasonable amount of time, without human intervention.
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
probe 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 and 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 probe electronics also detect
electrical conductivity (EC) between the rods and use this to correct the
permittivity measurement, allowing the probe to operate in a wider range of soil
EC. The calibration coefficients to convert this measured time of travel to
dielectric constant and ultimately water content are contained within the probe
itself and are the intellectual property of Campbell Scientific.
