TDR200-Based Time-Domain Reflectometry System
19
The combined effect of long cable runs and high soil electrical conductivity
must be considered when TDR measurements are taken.
8.4 Filtering and Averaging
The following parameters for the
TDR200
instruction are available for
improving measurements in noisy environments.
Noise from AC power lines can be reduced by using the
Noise Rejection
Frequency
setting. Option
50
takes two contiguous measurements that are
exactly 10 ms out of phase to cancel 50 Hz noise. If the
WaveAvg
parameter is
an odd number, the actual number of averages will be the next highest even
integer. Option
60
takes two contiguous measurements that are exactly 8.33 ms
out of phase in order to cancel 60 Hz noise. If the
WaveAvg
parameter is an odd
number, the actual number of averages will be the next highest even integer.
The
TDR Filter Level
setting reduces noise from the TDR trace by applying a
weighted moving average (arithmetic convolution) to the data points. The filter
level determines the number of neighboring values to include in the weighted
average. Acceptable values are 0 (no filtering) to 10 (maximum filtering). As
an example, choosing a
Filter Level
of 2 will use five measured values in
calculating the final output value: two values prior to the point of interest, the
actual measured point of interest, and the two values after that point. Each of
those points is weighted with the closer points having more weight than those
further away.
The
WaveAvg
setting defines the number of waveform reflections averaged by
the TDR200 to give a single result. A waveform averaging value of 4 provides
good signal-to-noise ratio under typical applications. Under high noise
conditions, averaging can be increased. The maximum averaging possible is
128.
8.5 Algorithms for Calculation of TDR Probe Rod Apparent
Length
Three algorithms are available for calculating the TDR probe apparent length.
8.5.1 Legacy Algorithm
The legacy algorithm was used with the TDR100, and is consistent over
varying soils and water contents. However, the tangent-line and second-order
bounded mean oscillation algorithms are better for noisy environments and
short probe lengths. When using the legacy algorithm for calculating the probe
apparent length, the measurements can be improved by increasing the points
(10112 maximum) or using the methods provided in Section
(p. 19)
. Details of the legacy algorithm are provided in the TDR100
manual.
8.5.2 Tangent Line Algorithm
This algorithm is the flat line method used in the
WinTDR
software package
(Or et al. (2004)). It works well without any predefined waveform attributes
and is more resistant to noisy environments than the legacy algorithm.