Campbell Scientific TDR100 Time Domain Reflectometry Systems - Manual (Page 15)

TDR100
4.4.2 Waveform
Average - sets the number of measurements averaged at a given distance from
the TDR100. A value of 4 is recommended. Higher values can be used when
noise is present. Averaging is useful when noise from power sources or when
noise of random nature is superimposed on the reflection waveform.
Averaging is accomplished by collecting n values at a given distance before
collecting values at the next distance increment where n is the value entered in
Average.
Points - the number of points in the displayed or collected waveform (using
File/Save ASCII Waveform). For water content measurements, a value of 251
is recommended and will provide 250 waveform increments. A higher value
can provide better resolution when collecting waveforms.
Start - the apparent distance from the TDR100 to where the displayed
waveform will begin (using File/Save ASCII Waveform). For water content
measurements this value should be the apparent distance from the TDR100 to
the beginning of the probe minus approximately 0.5 meter. Figure 3-2 is an
example of the display when the correct start is chosen.
The apparent distance is the (actual distance) x (selected
Vp/actual Vp). For example, if the actual length of a cable
having a Vp of 0.78 is 5 meters and the selected Vp is 1.0, the
apparent distance to the end of the cable is 5 x (1.0/0.78) = 6.41
meters. NOTE
Length - Beginning at distance Start, the length of the display window and
apparent length depicted by the number of data Points selected. For water
content measurements with the CS605 or CS610 TDR probes (30 cm rods) a
length of 4 meters is recommended. See Table 1.
4.4.2.1 A Discussion of Start and Length Parameters
Only the waveform reflection near the probe is used for water content
determination. The reflections for most of the cable between the TDR100 and
the TDR probe are not used for TDR100 measurements. The apparent probe
length algorithm begins analysis of the probe waveform at the distance set by
Waveform Start. The Waveform Start value must include a short section of
cable near the probe head to establish reference values. Subtracting 0.5 m
from the PCTDR x-axis value for the actual probe beginning is recommended.
The actual beginning of the probe displayed in Figure 4-1 is approximately 6.2
m. A Waveform Start value of 5.7 m will provide the complete data needed by
the algorithm to determine apparent probe length.
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