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TDR100

parameter only has an affect when Option 0, La/L, is used for the

measurement.

Mult, Offset: The Mult and Offset parameters are each a constant, variable,

array, or expression by which to scale the results of the measurement.

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

∆t

[1]

where

is the apparent dielectric constant, c is the velocity of

electromagnetic signals in free space,

∆t 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 (

= c

∆

t/2) to the real probe length.

= [2]

The relationship between dielectric constant and volumetric water content has

been described by, among others, Topp et al. (1980) and Ledieu et al. (1986) in

an empirical fashion using both polynomial and linear forms. These

expressions are presented here since it has been shown in numerous research

efforts that these equations are appropriate for nearly all applications. With

the volumetric water content, the equation presented by Topp et al. (1980) is

vaaa

KKK=− ∗ + ∗ − ∗ + ∗

−− − −

5310 29210 5510 4310

22 4263

.. . . [3]

and that presented by Ledieu et al. (1986) is

K=−01138 01758..

[4]

The TDR100 generates a very fast rise time pulse that is sent to the connecting

cable and probe. Reflections over a specified length of transmission line are

sampled and digitized. Discontinuities in cable impedance will cause changes

in the amplitude of the reflected signal. The travel time of the reflected signal

is used with a velocity value to give distance information. A probe consisting

of metal rods can be used as an extension of a coaxial cable. When the probe

is inserted in the soil, the travel time of the applied pulse along the probe

length will depend on the soil water content. In general, as the water content

increases, the travel time of the applied pulse increases. The reflected

waveform of the probe can be used to identify the impedance transitions

1 2 ... 27 28 29 30 31 32 33 34 35 36 37 ... 55 56

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