Campbell Scientific TDR100 Time Domain Reflectometry Systems - Manual (Page 30)

TDR100
beginning of the probe can be read from the x-axis. Subtract 0.5 meters from
the beginning distance and use that value in parameter 7 of datalogger
instruction 119.
It is important that the V
p
used during determination of value for cable length
be the same V
p
used during subsequent measurements with PCTDR and
datalogger instruction 119. A Vp value of 1.0 is recommended for soil water
measurements.
6.3.8 Parameter 8: Window Length (meters)
Window length specifies the length of waveform used in water content
determination and the distance for a saved waveform. The waveform analysis
algorithm begins at the distance specified by cable length and uses data
contained for the value specified by Window Length. It is important that
Window Length be large enough to contain the entire probe waveform at the
highest expected water contents. See Table 4-1.
6.3.9 Parameter 9: Probe Length (meters)
Physical length of the exposed portion of the TDR probe rods.
6.3.10 Parameter 10: Probe Offset (meters)
Most TDR probes have a block of epoxy or other material which holds the rods
rigidly spaced and houses an impedance matching transformer (balun) if used.
The algorithm in the TDR100 uses changes in the reflection coefficient to
identify end points of the probe. The transition from the coaxial cable to the
TDR probe occurs inside the probe block and causes an increase in the
reflection coefficient if the probe impedance is greater than the cable
impedance. However, this is not the same physical position as the beginning
of the probe’s exposure to the soil being measured. Therefore, a correction
must be made for the distance which the signal travels inside the probe block.
This offset is determined empirically. For the CS600 probe, the value is 69
mm (enter 0.069 in parameter 10), and for the CS605 and the CS610, the value
is 85 mm (enter 0.085 in parameter 10). The offset for probes not obtained
from Campbell Scientific must determined by the user. Contact Campbell
Scientific for guidance.
6.3.11 Parameter 11: Input Location
Input storage location for measurement result. If waveforms are chosen for
output, this location is occupied by the first data point.
6.3.11.1 Reflection Waveform Values to Input Storage
Saving reflection waveforms or reflection waveform plus first derivative using
Instruction 119 requires manual allocation of datalogger input storage. The
Insert Block function of the Edlog input location editor is used to allocate input
locations for the waveform array header elements (see Table 6-1) and
waveform values. The array header contains nine elements. The minimum
number of locations that must be allocated is the Points value defined in
Instruction 119 parameter 6 plus 9. An array size allocation of 9 more than the
number of waveform points the minimum.
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