Campbell Scientific TDR100 Time Domain Reflectometry Systems - Manual (Page 32)

TDR100
The probe constant (K
p
) is easily determined using PCTDR or can be
determined through calibration. The probe rods are immersed in solutions of
known electrical conductivity and values of
1 1
1 Z
c
−
+ ρ
ρ
collected by the
datalogger. A linear regression of known electrical conductivity to the result
from the datalogger will have a slope equal to the probe constant. When
performing a calibration with independently measured electrical conductivity,
take caution to account for the strong temperature dependence of electrical
conductivity. This calibration is generally recommended if accurate absolute
electrical conductivity values are required.
Laboratory calibrations were performed to determine a K
p
value for the
CS600, CS605 and CS610 probes with the following results.
TABLE 6-2. Probe Constant Values for Campbell Scientific Probes
Probe model probe constant (K
p
)
CS600 3.16
CS605 & CS610 1.74
Entering these constants as the multiplier when using Instruction 119 for
electrical conductivity measurement will give units of Siemens/meter.
For example, when using a CS600 the value 3.16 can be entered into parameter
12 of Instruction 119 and the electrical conductivity value in Siemens per
meter will be written to the input location specified in parameter 11.
7. TDR Principles
The travel time for a pulsed electromagnetic signal along a waveguide is
dependent on the velocity of the signal and the length of the waveguide. The
velocity is dependent on the dielectric constant of the material surrounding the
waveguide. This relationship can be expressed by
Δt
L K
c
a =
2
[1]
where K
a
is the apparent dielectric constant, c is the velocity of
electromagnetic signals in free space,
Δt
is the travel time, and L is the
waveguide length. The dielectric constant of water relative to other soil
constituents is high. Consequently, changes in volumetric water content can be
directly related to the change in the dielectric constant of bulk soil material.
Equation [1] can be simplified to express the apparent dielectric constant as the
ratio of the apparent probe length (
L
a
= c Δ t/2 ) to the real probe length.
L
K
L a
a =
[2]
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