Campbell Scientific TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 - Manual (Page 21)

Appendix B. Correcting Electrical
Conductivity Measurements for System
Losses
TDR system cabling and multiplexers introduce losses of the applied and
reflected signals which can lead to error in measurement of electrical
conductivity. The following information is based on a method presented in
paper published by Castiglione and Shouse (2003). The method has been
tested by Campbell Scientific and found to provide excellent results.
Refinement of the method is provided to allow implementation using Campbell
Scientific dataloggers and TDR100 system.
B.1 Description of Method
The method is essentially a calibration and involves collecting system
characterization measurements with all system components in place; TDR100,
multiplexers, all cabling, and probes. The steps in the process are:
1. measure reflection coefficient with probe rods open and with probe rods
shorted
2. determine probe constant, K
p
, using one solution of known electrical
conductivity
3. use values collected in above steps to generate simple function to correct
EC measurements
4. incorporate calibration function in datalogger program.
The method defines corrected reflection coefficient, ρ
corrected
, using the equation
1 2
shorted open
open d uncorrecte
corrected
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
ρ − ρ
ρ − ρ
= ρ [B1]
ρ
corrected
is then used to determine the conductance, G, with a TDR probe rods
immersed in a solution of known electrical conductivity. ρ
uncorrected
is the
refection coefficient at distance 200 m (example given below). The equation
for conductance is:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
ρ +
ρ −
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
corrected
corrected
u
1
1
Z
1
G [B2]
with Z
u
the system impedance, 50 ohms.
K
p
is the slope of a graph of electrical conductivity versus electrical
conductance, . Since this function passes through the origin, only
one measurement of G is needed with a probe immersed in a solution of known
G K
p
= σ
B-1