Campbell Scientific TDR100 Time Domain Reflectometry Systems - Manual (Page 39)

TDR100
9.3 Algorithm for Calculation of Bulk Electrical Conductivity
9.3.1 Algorithm Description
Section 7, TDR Principles, presents the equation and method used by the
TDR100 to calculate bulk electrical conductivity. The electrical conductivity
calculation uses waveform values for the region immediately before the TDR
probe and values from about 200 meters from the TDR100. The value at the
location before the probe is used to calculate the applied signal and the 200
meter values are used for the reflected signal. Each of these values are
normalized by a value collected from inside the TDR100.
A brief description of the algorithm is presented here.
1. Begin at waveform data point defined by parameter 1 (default = 40) and
search for the point of the maximum first derivative.
2. Calculate the location of the point halfway between the value of parameter
1 and the point of maximum first derivative.
3. Calculate the mean and standard deviation of waveform values between
parameter 1 and the halfway point.
4. Calculate a threshold value using:
threshold = a*(maximum derivative) + b*(mean from step 3) +
c*(standard deviation from step 3)
The default values for a, b and c are 0, 1 and 2 respectively.
5. With the point of the maximum first derivative as the right hand point,
define a window of 10 waveform values. Incrementally move this
window toward the TDR100 until all values are less than the threshold
calculated in step 4.
6. Calculate the average of this window of values and use for the applied
signal.
7. Calculate the average of the last six waveform values and use for reflected
signal.
8. Apply equation [5] of Section 7 to calculate electrical conductivity.
9.3.2 Algorithm Parameter Adjustment for Special Conditions
Under conditions of very long TDR probe cables, signal loss can lead to
erroneous electrical conductivity measurements. Parameters can be adjusted
using the terminal emulator mode of PCTDR to optimize measurements under
these conditions.
Figure 9-2 lists the parameter adjustment commands and the required
numerical format. The commands are discussed in detail here. These
commands are active only in the terminal emulator mode of PCTDR which is
reached using menu Options/Terminal Emulator.
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