Campbell Scientific HydroSense® Soil Water Content System (CS620, CD620) - Manual (Page 14)

HydroSense Soil Water Measurement System (CD620, CS620)
6.1 Probe Rod Insertion
Making a measurement with HydroSense is a simple matter of inserting the
probe rods and pressing the READ button. However, the accuracy of the
measurement can be degraded if the rods are inserted incorrectly.
The rods must be completely inserted. If the probe rods are not fully inserted
and air instead of soil occupies the space around the rods, a portion of the
sensitive volume is not occupied by the medium intended to be measured and
error will result. Additionally, if the rods are inserted closer than 3 cm from a
material other than the soil being measured, e.g. the wall of a container, error
can result.
The most sensitive region for the measurement is that region immediately
adjacent to the rod surface. Sensitivity decreases with distance away from the
rod surface. It is important to push the probe so rods insert in a straight path
with minimum sideways motion to prevent establishing an air void around the
rod.
During insertion, contact of probe rods with consolidated material such as rock
can cause deflection and possibly bend the rods. Bent rods should be replaced
as soon as possible because continued bending can lead to breakage at the
point where the rod threads into the probe body making repair difficult. See
Section 8.2 for replacement instructions. The 12 cm rods are less likely to
bend than 20 cm rods.
If rods become loose in probe body threading, bending of
the rods near the thread is more likely than if the rods are
properly tightened. Please see maintenance Section 8.2. CAUTION
6.2 Soil Factors Which Can Affect the Measurement—Clay, Soil
Electrical Conductivity, Organic Matter and Rocks
HydroSense is very sensitive to changes in dielectric permittivity and the probe
has water content measurement resolution better than 0.1%. While the
HydroSense is predominantly sensitive to dielectric permittivity (and
consequently volumetric water content), other physical properties of the soil
can affect the measurement. If the soil contains a large amount of clay or has
high electrical conductivity (EC), the applied signal can be attenuated and this
affects detection of the reflected signal in the probe electronics. Very high
organic matter content has a similar effect. HydroSense will still respond to
changes in water content but not the same as if the attenuation factors were
present in small, noninterfering amounts. The calibration coefficients used to
transform the probe output signal to water content or deficit reside in the
HydroSense operating system and were determined in laboratory studies on
typical soils. User determined coefficients can be applied to the probe output
period value that is displayed in the Water Content Measurement Mode.
Rocky soils can make rod insertion difficult (see Section 6.1) and introduce
variability in water content measurements taken in the same general area. The
rocks occupy space otherwise occupied by the fine soil fraction but do not hold
water in the same manner as soil. If two proximal measurements are made in a
rocky soil, the measured water content can differ by several percent if a large
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