A.3.1 procedure for calculating probe offset, A.3.1 – Campbell Scientific TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 User Manual
Page 17
Appendix A. Discussion of TDR Probe Offset and a Simple Laboratory Method for Calculation
A.3 Method for Calculating Probe Offset Using
Information from the Terminal Mode of PC-TDR
Letting V
p
= 1 and solving [1] for ProbeOffset gives:
La
start
end
ProbeOff
−
−
=
[A3]
The start and end distance values are determined by an algorithm in the
TDR100 operating system. The apparent length, La, excluding the offset, is
related to actual rod length and permittivity by:
( )
T
•
L
:
La
ε
=
[A4]
The rod length, L, is the physical length of the rods (m). For 3-rod TDR
probes with longer outer rods, the length of the outer rods is used.
The dielectric permittivity of water can be calculated from water temperature
using:
[A5]
( )
(
)
(
)
(
)
[
]
3
8
2
5
3
25
T
•
10
•
8
.
2
25
T
•
10
•
19
.
1
25
T
•
10
•
5791
.
4
1
•
54
.
78
:
T
ε
−
−
−
+
−
−
=
−
−
−
TABLE A-1 contains dielectric permittivities for a typical range of
temperatures and may be used in lieu of equation [A5]. Substituting the
calculation of La using equations [A4] and [A5] into equation [A3] leaves the
end and start distances as the only unknowns. These values can be acquired
directly from the TDR100 algorithm by using the terminal emulator mode of
PC-TDR.
A.3.1 Procedure for Calculating Probe Offset
Connect all cabling and multiplexers to be used for field or laboratory
measurements.
Immerse the TDR probe rods in DI or tap water. The container must be large
enough to ensure rods are at least 5 cm from container walls.
Use PC-TDR as follows:
1. Enter values for Waveform parameters. Suggested values are: Average =
4, Points = 251. Relative propagation velocity, Vp, must be 1. Choice of
start point and waveform length depends on the length of the cable and the
actual probe rod length. There should be about 0.5 meters before the
probe, enough distance for probe apparent length in water (approximately
9 times rod length), and enough distance for the waveform past the end of
the probe. The distance for the end can be approximated as 3 times rod
length.
2. Enter the value for Probe Rod Length in meters and set Probe Offset to 0
m.
3. Click the Water Content button to send the values to the TDR100 and to
have it calculate La/L and provide start and end values.
A-3