Campbell Scientific TDR100 Time Domain Reflectometry Systems - Manual (Page 35)

TDR100
cable lengths will be maintained as long as 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.
8.2 Soil Electrical Conductivity Effect on Water Content
Measurement
The signal at the probe will be 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 probe rods. The attenuation of the signal can affect the accuracy
and resolution of water content measurements. Figure 8-2 presents a series of
waveforms when a solution with an electrical conductivity of 1.0 dS m
-1
is
added to a soil which has essentially no salt present. Figure 8-3 shows data for
solution with high electrical conductivity.
water content = 9.5%
water content = 25%
FIGURE 8-2. Waveforms collected in a sandy loam using CS610 probe
with RG8 connecting cable. Volumetric water content values are 10,
16, 18, 21 and 25%. Solution electrical conductivity is 1.0 dS m
-1
.
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