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TDR Principles

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TDR100 The probe constant (Kp) is easily determined using PCTDR or can be determined through calibration. The probe rods are immersed in solutions of 1 1− ρ known electrical conductivity and values of Zc 1 + 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 Kp value for the CS600, CS605 and CS610 probes with the following results. TABLE 6-2. Probe Constant Values for Campbell Scientific Probes Probe model CS600 CS605 & CS610 probe constant (Kp) 3.16 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 2 L Δt = Ka c [1] where Ka 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 (La = cΔt/2) to the real probe length. Ka = L a L [2] 26

Section	Page
Revision and Copyright Information	1
Warranty and Assistance	2
Table of Contents	3
TDR100	7
1. Introduction	7
1.1 TDR100 Packing List	7
1.2 ENCTDR100 Packing List	7
2. System Specifications	8
2.1 General	8
2.2 Power Consumption	8
2.2.1 TDR100	8
2.2.2 SDMX50	8
2.3 TDR100 Performance Specifications	8
2.4. Electromagnetic Compatibility	9
3. Getting Started with TDR100 using PCTDR	9
3.1 Discussion of Distances and Propagation Velocity (Vp) when using TDR100	10
3.2 PCTDR Help	12
4. PCTDR Software	12
4.1 General	13
4.2 PCTDR Help	13
4.3 Menu Selections	14
4.3.1 File Menu	14
4.3.2 Settings Menu	14
4.3.3 Options Menu	14
4.4 PCTDR Parameter Selection Boxes	14
4.4.1 Cable	14
4.4.2 Waveform	15
4.4.2.1 A Discussion of Start and Length Parameters	15
5. System Components: Datalogger Control	17
5.1 General	17
5.2 Datalogger	17
5.3 TDR100	18
5.4 SDMX50	18
5.5 Power Supply	19
5.5.1 Grounding	19
5.6 SDM Communication	20
5.6.1 SDM Addressing for TDR100 System	20
5.6.2 SDM Cable and Cable Length Considerations	22
5.7 ENCTDR100	22
5.7.1 Mounting Equipment in ENCTDR100	22
5.8 Soil Probes	23
Campbell Scientific TDR Probes	24
5.8.1 Determining Probe Constant, Kp, using PCTDR	24
6. Datalogger Instructions for TDR Measurements	24
6.1 CR800, CR850, CR1000, or CR3000 Datalogger Instruction “TDR100”	24
6.2 CR10X and CR23X Datalogger Instruction 119	26
6.3 Discussion of TDR Instruction Parameters (Instruction 119)	27
6.3.1 Parameter 1: SDM Address	27
6.3.2 Parameter 2: Output Option	27
6.3.2.1 Enter 0: Measure La/L	27
6.3.2.2 Enter 1: Collect Waveform	28
6.3.2.3 Enter 2: Collect Waveform and First Derivative	28
6.3.2.4 Enter 3: Measure Bulk Electrical Conductivity	28
6.3.3 Parameter 3: Multiplexer and Probe Selection	28
Example 2	29
6.3.4 Parameter 4: Waveform Averaging	29
6.3.5 Parameter 5: Relative Propagation Velocity	29
6.3.6 Parameter 6: Points	29
6.3.7 Parameter 7: Cable Length (meters)	29
6.3.8 Parameter 8: Window Length (meters)	30
6.3.9 Parameter 9: Probe Length (meters)	30
6.3.10 Parameter 10: Probe Offset (meters)	30
6.3.11 Parameter 11: Input Location	30
6.3.11.1 Reflection Waveform Values to Input Storage	30
6.3.12 Parameter 12: Multiplier	31
6.3.13 Parameter 13: Offset	31
6.3.13.1 Probe Constant for Electrical Conductivity Measurement	31
7. TDR Principles	32
8. Cable Length and Soil Electrical Conductivity Effect on Water Content Determination	34
8.1 Cable Length Effect on Water Content Measurement	34
8.2 Soil Electrical Conductivity Effect on Water Content Measurement	35
9. Algorithm Description and Parameter Adjustment	36
9.1 Introduction	36
9.2 Algorithm for Calculation of TDR Probe Rod Apparent Length	36
9.2.1 Algorithm Description	36
9.2.1.1 Waveform Evaluation	37
9.2.2 Algorithm Parameter Adjustment for Special Conditions	38
9.2.2.1 Terminal Emulator Commands for Apparent Length Algorithm	38
9.3 Algorithm for Calculation of Bulk Electrical Conductivity	39
9.3.1 Algorithm Description	39
9.3.2 Algorithm Parameter Adjustment for Special Conditions	39
10. Programming Examples	41
10.1 CR1000 Program Example	41
Example 1, Measure and Record; Analog Measurements, Volumetric Water Content, and Capture TDR Probe Waveforms	41
10.2 CR10X/CR23X Program Examples	44
Example 1, Measure and Record; Analog Measurements and Volumetric Water Content	44
Example 2, Measure and Record; Analog Measurements, Volumetric Water Content, Electrical Conductivity, and Capture a Waveform	46
Example 3, Measure and Record; Volumetric Water Content and Analog Measurements	50
Example 4, Measure and Record; Analog Measurements and Volumetric Water Content	52
Example 5, Trouble Shooting Program for SDMX50 Multiplexer, Datalogger Control Ports, and Analog Measurements	56
11. References	57

Match case Limit results 1 per page

TDR100

The probe constant (K

) 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

−

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

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

)

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.

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

[1]

where

is the apparent dielectric constant,

is the velocity of

electromagnetic signals in free space,

is the travel time, and

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 (

= c

t/2

) to the real probe length.

[2]