TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 Revision: 2/09 C o p y r i g h t © 2 0 0 6 - 2 0 0 9 C a m p b e l l S c i e n t i f i c , I n c .
Warranty and Assistance The TDR PROBES CS605, CS610, CS630, CS635, CS640, AND CS645 are warranted by CAMPBELL SCIENTIFIC, INC. to be free from defects in materials and workmanship under normal use and service for twelve (12) months from date of shipment unless specified otherwise. Batteries have no warranty. CAMPBELL SCIENTIFIC, INC.'s obligation under this warranty is limited to repairing or replacing (at CAMPBELL SCIENTIFIC, INC.'s option) defective products.
TDR Probes Table of Contents PDF viewers note: These page numbers refer to the printed version of this document. Use the Adobe Acrobat® bookmarks tab for links to specific sections. 1. Introduction..................................................................1 2. Electromagnetic Compatibility ...................................1 3. Specifications ..............................................................2 3.1 Physical Description .....................................................................
TDR Probes 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 ..............................................................................
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 coaxial multiplexer units (SDMX50). Warning The CS605 and CS610 are shipped with rubber caps covering the sharp ends of the rods.
TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 3. Specifications 3.1 Physical Description TABLE 3-1. TDR Probe Physical Properties Probe Model Rods Probe Head Cable Type CS605 length 30.0 cm diameter 0.475 cm length width thickness 10.8 cm 7.0 cm 1.9 cm RG58 CS610 length 30.0 cm diameter 0.475 cm length width thickness 10.8 cm 7.0 cm 1.9 cm RG8 low loss CS630 length 15.0 cm diameter 0.318 cm length width thickness 5.75 cm 4.0 cm 1.25 cm RG58 CS635 length 15.0 cm diameter 0.
TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 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 as a change in probe impedance which affects the shape of the reflection.
TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 4.4 Probe Constant for Electrical Conductivity Measurement The electrical conductivity measurement requires a probe constant to account for probe geometry. The probe constant is commonly referred as Kp. The probe constant is entered as a multiplier in the datalogger instruction for TDR100 EC measurement. Kp is set in PCTDR using Settings/Calibration Functions/Bulk Electrical Conductivity.
TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 error can be several percent. See Bilskie (1997) for complete results of the study. 16 meter cable 26 meter cable 45 meter cable 66 meter cable FIGURE 5-1. 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-1. In general, water content is overestimated with increasing cable length.
TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 water content = 9.5% water content = 25% FIGURE 5-2. 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. water content = 18% water content = 37% FIGURE 5-3. Waveforms collected in a sandy loam using CS610 probe with RG8 connecting cable. Volumetric water content values are 10, 18, 26, 30 and 37%.
TDR Probes CS605, CS610, CS630, CS635, CS640, CS645 6. References Bilskie, Jim. 1997. “Reducing Measurement Errors of Selected Soil Water Sensors.” Proceedings of the International Workshop on Characterization and measurement of the hydraulic properties of unsaturated porous media. 387396. P. Castiglione and P.J. Shouse. 2003. The effect of ohmic cable losses on timedomain reflectometry measurements of electrical conductivity. Soil Sci Soc Am J 2003 67: 414-424.
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Appendix A. Discussion of TDR Probe Offset and a Simple Laboratory Method for Calculation A.1 Discussion of Probe Offset Probe offset accounts for the segment of the TDR probe rods that is part of the probe head and is not exposed to the media surrounding the probe rods. The location of the beginning of the probe that is calculated in the TDR100 operating system is the point along the cable where the transition from the 50 ohm cable to the TDR probe impedance occurs.
Appendix A. Discussion of TDR Probe Offset and a Simple Laboratory Method for Calculation A.2 The Compounding Effect of Signal Attenuation in Connecting Cables The probe offset values provided in the operating manual were calculated from measurements in the Campbell Scientific soils laboratory. The method is described below. The length of cable for the laboratory calculations was 3 meters or less. As cable length increases, signal loss occurs in both amplitude and bandwidth.
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 PCTDR Letting Vp = 1 and solving [1] for ProbeOffset gives ProbeOff = end − start − La [A3] The start and end distance values are determined by an algorithm in the TDR100 operating system.
Appendix A. Discussion of TDR Probe Offset and a Simple Laboratory Method for Calculation 5. Enter terminal mode using Options/Terminal Emulator. 6. Hit Enter until get > 7. Type GMO then Enter. This will return La/L. 8. Type GVAR then Enter. 9. It is recommended that steps 7 and 8 be repeated several times and that the average values of Start and End used for following calculations. (the line commands are not case sensitive) GVAR returns the uncorrected Start and End.
Appendix A. Discussion of TDR Probe Offset and a Simple Laboratory Method for Calculation • converting waveform index to apparent distance start distance := start index ⋅ WindowLength datapoints − 1 end distance := end index ⋅ WindowLength datapoints − 1 start distance = 0.65 Pr obeOffset := end distance = 3.4 − (La ⋅ Vp − end distance + start distance ) Vp ProbeOffset = 0.086 TABLE A-1. Dielectric permittivity values for range of temperatures. From equation [A5].
Appendix A. Discussion of TDR Probe Offset and a Simple Laboratory Method for Calculation A-6 Temperature (°C) Dielectric Permittivity 24 78.9 24.5 78.72 25 78.54 25.5 78.36 26 78.18 26.5 78 27 77.82 27.5 77.65 28 77.47 28.5 77.29 29 77.12 29.5 76.94 30 76.
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.
Appendix B. Correcting Electrical Conductivity Measurements for System Losses electrical conductivity. Kp is calculated as the ratio of electrical conductivity to electrical conductance and presented in equation [B3]. Kp = σ G [B3] With Kp determined, a calibration equation can be derived that corrects EC measurements for system losses. B.2 Detailed Method Description B.2.
Appendix B. Correcting Electrical Conductivity Measurements for System Losses The temperature effect is described by EC T = EC 25 ∗ (1 + 0.02 * (T − 25)) [B4] where EC 25 is the electrical conductivity at 25ºC and EC T is the electrical conductivity at other temperatures. B.2.3 Deriving Calibration Function Using the Kp, ρopen and ρshorted values for each probe, the uncorrected electrical conductivity as measured by the TDR100 can be corrected to give accurate EC values that account for system losses.
Appendix B. Correcting Electrical Conductivity Measurements for System Losses B.2.4 CR1000 Program for Collecting ρopen and ρshorted Values ‘This example program is written for 4 TDR probes connected to ‘a single multiplexer. It will be necessary to add instructions in ‘subroutine TDR if more probes are used.
Appendix B.
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