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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. The distance from this

transition to the point where the rods come out of the probe head is constant

and can be accounted for.

The TDR100 operating system uses the following equation to calculate the

ratio of apparent rod length to actual rod length, La/L. This ratio is equal to the

square root of dielectric permittivity,

ProbeOff

startend

−

= [A1]

La apparent length (m)

L actual rod length (m)

relative propagation velocity (1.0)

ProbeOff probe offset (m)

start distance into window for beginning of rods (m)

end distance into window for end of rods (m)

To consider the sensitivity of La/L to probe offset, in equation [A1] cancel the

L's and take the 1st derivative of La with respect to probe offset.

1ProbeOff

startend

)d(ProbeOff

−=

⎟

⎠

⎞

⎜

⎝

⎛

−

[A2]

The sensitivity of the apparent length measurement, La, is directly related to

probe offset. A probe offset difference of 0.005 changes La by -0.005. The

water content error for saturated soil is 0.16% volumetric water content. In

very dry soil the error is 0.20%. There is a slight dependence of probe offset

on water content but the amount is within the resolution of the water content

measurement.

A-1

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Campbell TDR Specifiche Pagina 13

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