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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 V

= 1 and solving [1] for ProbeOffset gives

LastartendProbeOff −

−

[A3]

The start and end distance values are determined by an algorithm in the

TDR100 operating system. The apparent length, La, is related to actual rod

length and permittivity by

(

)

TL:La ε⋅=

[A4]

The rod length, L, is the physical length of the rods (m). For 3-rod TDR

probes having longer outer rods the length of the outer rods is used.

The dielectric permittivity of water can be calculated from water temperature

using

[A5]

() () () (

[]

25T108.225T1019.125T105791.4154.78:Tε −⋅⋅−−⋅⋅+−⋅⋅−⋅=

−−−

)

Table 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 PCTDR.

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 so rods are at least 5 cm from container walls.

Use PCTDR as follows.

1. Using settings/calibration functions/volumetric water content , select La/L

as the output by choosing the upper User Defined equation and setting

slope to 1 and offset to 0.

2. Enter values for Waveform parameters. Suggested values are: Average =

4, Points = 251. Relative propagation velocity, Vp, must be 1. Choice of

waveform length depends on the actual probe rod length. There should be

about .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.

3. Enter the value for probe rod length in meters and set probe offset to 0 m.

4. 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

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