Campbell Scientific TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 - Manual (Page 16)

Appendix A. Discussion of TDR Probe Offset and a Simple Laboratory Method for Calculation
A.2 The Compounding Effect of Signal Attenuation in
Connecting Cables
The probe offset values provided in the operating manual were calculated from
measurements in the Campbell Scientific soils laboratory. The method is
described below. The length of cable for the laboratory calculations was 3
meters or less. As cable length increases, signal loss occurs in both amplitude
and bandwidth. As a result of bandwidth loss, the slope of the waveform at the
beginning of the probe decreases with increasing cable length. The probe start
is determined from the intersection of a line tangent to the waveform at the
steepest point and of a line that is essentially horizontal. See FIGURE A-1.
The probe offset correction identifies the location where the rods exit the probe
head.
15 20 25 30 35 40
0.4
0.2
0.2
0.4
horizontal line
waveform
tangent line
calculated and corrected start
data point
reflection coefficient
indexstartindexstartcorr
FIGURE A-1. Example of start of TDR probe determination
The slope of the tangent line decreases as cable length increases, and the
intersection of the two lines will shift in the direction of greater apparent probe
rod length.
Calculating the probe offset using the method described below and with all
cables and multiplexers in place will optimize the accuracy of water content
measurements.
A-2