Campbell Scientific TDR User Manual
TDR Probes CS605, CS610,
CS630, CS635, CS640, CS645
Revision: 9/13
Copyright © 2006-2013
Campbell Scientific, Inc.
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Table of Contents
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1. Introduction.................................................................1
2. Cautionary Statements...............................................1
3. Initial Inspection .........................................................1
4. Overview......................................................................1
5. Specifications .............................................................2
5.1 Physical Description.............................................................................2
5.2 Measurement Parameters .....................................................................2
5.3 Electromagnetic Compatibility ............................................................3
6. Installation...................................................................3
7. Operation.....................................................................3
7.1 Probe Offset for Water Content Measurement.....................................3
7.1.1 Calculating Probe Offset...............................................................3
7.2 Probe Constant for Electrical Conductivity Measurement...................4
7.2.1 Electrical Conductivity Error from Attenuation............................4
7.3 Water Content Measurement Error from Cable ...................................4
7.4 Water Content Measurement Error from Soil Electrical
Conductivity .....................................................................................5
8. References ..................................................................7
Appendices
A.
Discussion of TDR Probe Offset and a Simple
Laboratory Method for Calculation..................... A-1
A.1 Discussion of Probe Offset.............................................................. A-1
A.2 The Compounding Effect of Signal Attenuation in Connecting
Cables .......................................................................................... A-2
A.3 Method for Calculating Probe Offset Using Information from the
Terminal Mode of PC-TDR......................................................... A-3
A.3.1 Procedure for Calculating Probe Offset ................................... A-3
A.3.2 An Example Using CS605 ....................................................... A-4
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Table of Contents
B. Correcting Electrical Conductivity
Measurements for System Losses...................... B-1
B.1 Description of Method..................................................................... B-1
B.2 Detailed Method Description........................................................... B-2
B.2.1 Collecting Reflection Coefficient with Probes Open and
Shorted.................................................................................. B-2
B.2.2 Determining Kp........................................................................ B-2
B.2.3 Deriving Calibration Function.................................................. B-3
B.2.4 CR1000 Program for Collecting ρ
open
Figures
7-1. 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
7-2. 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
7-3. Waveforms collected in a sandy loam using CS610 probe with RG8
connecting cable. Volumetric water content values are 10, 18, 26,
30 and 37%. Solution electrical conductivity is 10.2 dS m
A-1. Example of start of TDR probe determination ................................ A-2
B-1. Example of corrected and uncorrected electrical conductivity
values. .......................................................................................... B-3
and ρ
-1
......................................... 5
Values......... B-4
shorted
-1
. ................. 6
-1
................. 6
Tables
5-1. TDR Probe Physical Properties ........................................................... 2
5-2. TDR Probe Measurement Properties................................................... 2
A-1. Dielectric permittivity values for range of temperatures. From
equation [A5]. .............................................................................. A-5
B-1. Standard KCl Solutions ................................................................... B-2
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TDR Probes CS605, CS610, CS630,
CS635, CS640, CS645
1. Introduction
This document presents descriptions and instructions for Campbell Scientific
Time Domain Reflectometry (TDR) probes and includes some TDR principles.
Consult the TDR100 operating manual for comprehensive TDR instructions.
A single TDR probe can be connected directly to the TDR100 or multiple
probes connected via the SDMX50-series Coaxial Multiplexers.
Before using the TDR probes, please study:
• Section 2, Cautionary Statements
• Section 3, Initial Inspection
2. Cautionary Statements
• Care should be taken when opening the shipping package to not damage or
cut the cable jacket. If damage to the cable is suspected, consult with a
Campbell Scientific application engineer.
• The CS605 and CS610 are shipped with rubber caps covering the sharp
ends of the rods. Remove the three caps before use.
• The TDR100 is sensitive to electrostatic discharge damage. Avoid
touching the center conductor of the panel BNC connector or the center
rod of TDR probes connected to the TDR100.
3. Initial Inspection
• Upon receipt of a TDR probe, inspect the packaging and contents for
damage. File damage claims with the shipping company.
• The model number and cable length are printed on a label at the
connection end of the cable. Check this information against the shipping
documents to ensure the correct product and cable length are received.
4. Overview
TDR probes are the sensors of the TDR measurement system and are inserted
or buried in the medium to be measured. The probes are a wave guide
extension on the end of coaxial cable. Reflections of the applied signal along
the waveguide will occur where there are impedance changes. The impedance
value is related to the geometrical configuration of the probe (size and spacing
of rods) and also is inversely related to the dielectric constant of the
surrounding material. A change in volumetric water content of the medium
surrounding the probe causes a change in the dielectric constant. This is seen
1
TDR Probes CS605, CS610, CS630, CS635, CS640, CS645
as a change in probe impedance which affects the shape of the reflection. The
shape of the reflection contains information used to determine water content
and soil bulk electrical conductivity.
5. Specifications
5.1 Physical Description
TABLE 5-1. TDR Probe Physical Properties
Probe
Model
Rods Probe Head Cable Type
Maximum Soil
Bulk Electrical
Conductivity
Maximum
Cable Length
(measured from
the tips of the
probe’s rods to
the TDR100
Reflectometer)
CS605
CS610 length 30.0 cm
CS630 length 15.0 cm
CS635 length 15.0 cm
CS640 length 7.5 cm
CS645 length 7.5 cm
length 30.0 cm
diameter 0.475 cm
diameter 0.475 cm
diameter 0.318 cm
diameter 0.318 cm
diameter 0.159 cm
diameter 0.159 cm
5.2 Measurement Parameters
length 10.8 cm
width 7.0 cm
thickness 1.9 cm
length 10.8 cm
width 7.0 cm
thickness 1.9 cm
length 5.75 cm
width 4.0 cm
thickness 1.25 cm
length 5.75 cm
width 4.0 cm
thickness 1.25 cm
length 4.5 cm
width 2.2 cm
thickness 1.0 cm
length 4.5 cm
width 2.2 cm
thickness 1.0 cm
TABLE 5-2. TDR Probe Measurement Properties
LMR-200
LMR-200
RG58
RG8
low loss
RG58
low loss
RG58
low loss
1.4 dS/m 15 m
1.4 dS/m 25 m
3.5 dS/m 15 m
3.5 dS/m 25 m
5.0 dS/m 15 m
5.0 dS/m 25 m
2
Probe Model
CS605 and CS610 0.090 1.74
CS630 and CS635 0.052 3.36
CS640 and CS645 0.035 6.40
Probe Offset
(meters)
Probe Constant for Electrical
Conductivity (EC) Measurement, Kp
(using this constant will provide EC in
siemens/meter)
5.3 Electromagnetic Compatibility
6. Installation
TDR Probes CS605, CS610, CS630, CS635, CS640, CS645
All TDR probes are compliant with performance criteria available upon
request. RF emissions are below EN55022 limit.
TDR probes can be installed in any orientation: horizontally, vertically, or at an
angle to the surface. The measured water content is the integral or average of
the water content over the length of the probe rods. The probe rods should be
completely surrounded by the soil or other media being measured. If portions
of the probe rods are exposed to air, the algorithm for analyzing the waveform
reflection may not be able to correctly locate the beginning and end of the
probe rods.
Care must be exercised when inserting probe rods into the soil to minimize soil
compaction around the rods. Compaction can leave air voids along the length
of the rods. The region adjacent to the rod is the most sensitive so voids near
the rods can be a significant source of error.
After the soil is disturbed for probe installation, most soils will experience
rejuvenation of the soil structure with wetting/drying cycle and freeze/thaw
cycles.
7. Operation
7.1 Probe Offset for Water Content Measurement
7.1.1 Calculating Probe Offset
TDR probes can be buried or inserted into the soil. The CS605G Installation
Guide should be used when inserting the CS605 and CS610 into the material
being measured. A guide is generally not needed for the smaller diameter
probes.
A portion of the TDR probe rods is surrounded by the probe head material and
is not exposed to the material being measured. Probe offset is used to correct
for this. TABLE 5-2 lists offset values for probes manufactured by Campbell
Scientific. These values are entered in the datalogger instruction or in the PCTDR software.
Probe offset can be calculated using information from PC-TDR. The probe
rods are immersed in water of known temperature, algorithm values are
collected in the terminal emulator mode of PC-TDR, and simple calculations
provide custom offset values. See Appendix A, Discussion of TDR Probe
Offset and a Simple Laboratory Method for Calculation, for calculation
method.
The values listed in TABLE 5-2 were determined using TDR probes with short
cables. The shape of the waveform reflection is affected as cable length
increases, and this can introduce error into the water content measurement.
Using probe offsets determined by the method described in Appendix A,
Discussion of TDR Probe Offset and a Simple Laboratory Method for
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