TDR Probes CS605, CS610, CS630, CS635, CS640, CS645
value is related to the geometrical configuration of the probe (size and spacing
of rods) and also is inversely related to the dielectric constant of the
surrounding material. A change in volumetric water content of the medium
surrounding the probe causes a change in the dielectric constant. This is seen
as a change in probe impedance which affects the shape of the reflection. The
shape of the reflection contains information used to determine water content
and soil bulk electrical conductivity.
4.2 Installation
TDR probes can be installed in any orientation, horizontally, vertically or at an
angle to the surface. The measured water content is the integral or average of
the water content over the length of the probe rods. The probe rods should be
completely surrounded by the soil or other media being measured. If portions
of the probe rods are exposed to air, the algorithm for analyzing the waveform
reflection may not be able to correctly locate the beginning and end of the
probe rods.
Care must be exercised when inserting probe rods into the soil to minimize soil
compaction around the rods. Compaction can leave air voids along the length
of the rods. The region adjacent to the rod is the most sensitive so voids near
the rods can be a significant source of error.
After the soil is disturbed for probe installation, most soils will experience
rejuvenation of the soil structure with wetting/drying cycle and freeze/thaw
cycles.
TDR probes can be buried or inserted into the soil. The CS605G is a guide for
inserting the CS605 and CS610 into the material being measured. A guide is
generally not needed for the smaller diameter probes.
4.3 Probe Offset for Water Content Measurement
4.3.1 General
A portion of the TDR probe rods is surrounded by the probe head material and
so is not exposed to the material being measured. Probe offset is used to
correct for this. Table 3-2 lists offset values for probes manufactured by
Campbell Scientific. These values are used in the datalogger instruction or in
PCTDR.
4.3.2 Calculating Probe Offset
Probe offset can be calculated using information from PCTDR. The probe rods
are immersed in water of known temperature, algorithm values are collected in
the terminal emulator mode of PCTDR and simple calculations provide custom
offset values. See Appendix A for calculation method.
The values listed in Table 3-2 were determined using TDR probes with short
cables. The shape of the waveform reflection is affected as cable length
increases, and this can introduce error into the water content measurement.
Using probe offsets determined by the method described in Appendix A with
all cabling from TDR100 to probe in place will compensate for the cable
losses. Probe offset values obtained this way will be greater than those listed
in Table 3-2.
3
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