TDR200-Based Time-Domain Reflectometry System
17
presents waveforms collected from a 3-rod probe (CS610) with
varying cable lengths. Increasing the cable length affects the rise time and
amplitude of the reflection. The slopes and extrema used by the data logger
algorithm to analyze the waveform are shifted by the cable losses resulting in
an error. For the data shown in FIGURE
, the water content measurement
error using the 66-met
re
cable is about 1.5% volumetric water content when
electrical conductivity is low. However, in saline soils, the error is higher. See
Bilskie (1997) for complete results of the study.
FIGURE 8-1. Waveforms collected in a sandy loam using CS610 probe
with RG8 connecting cable. Volumetric water content is 24% and
bulk electrical conductivity is 0.3 dS m
–1
.
In general, water content is overestimated with increasing cable length. A
calibration of volumetric water content with apparent dielectric constant for a
given cable length can improve accuracy. Measurement precision at longer
cable lengths will be maintained if the soil electrical conductivity does not
prevent a reflection from the end of the probe rods. This is discussed later in
this section.
Minimizing cable lengths should always be considered in the design of a
measurement system using TDR. If long cable lengths are necessary, the
adverse effects can be minimized by using low attenuation cable such as RG8.
Careful probe design ensures correct probe impedance giving robust
reflections. All TDR probes offered by Campbell Scientific are designed to
optimize accuracy when longer cable lengths are used.
8.3 Soil Electrical Conductivity Effect on Water Content
Measurement
The signal at the probe is attenuated when ionic conduction occurs in the soil
solution. This inherent attenuation is used in TDR measurements to determine
soil electrical conductivity as described by equation [5]. The presence of ions
in the soil solution provides a path for electrical conduction between TDR
16 met
re
cable
26 met
re
cable
45 met
re
cable
66 met
re
cable
