TDR Probes CS605, CS610, CS630, CS635, CS640, CS645
4.4 Probe Constant for Electrical Conductivity Measurement
The electrical conductivity measurement requires a probe constant to account
for probe geometry. The probe constant is commonly referred as K
p
. The
probe constant is entered as a multiplier in the datalogger instruction for
TDR100 EC measurement. K
p
is set in PCTDR using Settings/Calibration
Functions/Bulk Electrical Conductivity. Using the K
p
values in Table 3-2 will
give electrical conductivity in the units siemens/meter. For the more common
units of decisiemens/meter, multiply the Table 3-2 K
p
values by 10.
Probe constant can be calculated using PCTDR. Selecting Settings/Calibration
Functions/Bulk Electrical Conductivity will present a button to Measure Cell
Constant. The method requires submersion of the TDR probe rods in de-
ionized water of known temperature. See PCTDR HELP for simple
instructions. It is recommended to make several K
p
determinations and use the
average value.
Probe constant can also be calculated using the method presented in Appendix
B. This method accounts for signal losses in system cabling and multiplexers.
4.4.1 Electrical Conductivity Error from Attenuation
Attenuation of the applied and reflected signal in the cable and multiplexers
will affect the accuracy of the electrical conductivity measurement. For
accurate electrical conductivity measurements this attenuation must be
accounted for.
A paper published by Castiglione and Shouse (2003) describes the error and a
method to account for the error. The method requires electrical conductivity
measurement with the probes in air and with the rods shorted with all system
components in place (cable and multiplexers).
Appendix B presents a summary of the Castiglione and Shouse (2003) method
and an adaptation of the method for the TDR100 system.
5. TDR Measurement Error from Cable Attenuation
and Soil Electrical Conductivity
5.1 Water Content Measurement Error from Cable
The determination of water content using the TDR system relies on the
evaluation of a pulse reflection from the TDR probe. The pulse generated by
the TDR100 and its reflections are subject to distortion during travel between
the TDR100 and the TDR probe. The cable connecting the probe to the
reflectometer has a characteristic impedance resulting in both resistive and
reactive losses. Distortion of the waveform caused by cable impedance can
introduce error into the water content determination.
Figure 5-1 presents waveforms collected from a 3-rod probe (CS610) for
various cable lengths. As cable length increases, the rise time and the
amplitude of the reflection are affected. The slopes and extrema used by the
datalogger algorithm to analyze the waveform are shifted by the cable losses
resulting in error. For the data shown in Figure 5-1, the water content
measurement using the 66 meter cable was in error by about 1.5% volumetric
water content when electrical conductivity is low. However, in saline soils the
4
(75 pages)
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