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ZUPANC, V. in sod.: Comparison of laboratory TDR soil water measurements

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Figure 1: Scheme of TDR 100 experiment set, connections of PC with TDR 100 software via

cable to TDR 100 pulse charger and battery to TDR rods via clasps.

2.2 Methods

The differentially driven probe rods form a transmission line with a wave propagation velocity

that is dependent on the dielectric permittivity of the medium surrounding the rods.

Nanosecond rise-times produce waveform reflections characteristic of an open-ended

transmission line (Campbell Scientific, 2004). The TDR method relies on graphical

interpretation of the waveform reflected from that part of the waveguide that is the probe

(Figure 2) (Evett, 2000, Noborio, 2001). The return of the reflection from the ends of the rods

triggers a logic state change, which initiates propagation of a new wave front.

Figure 2: Graphic output on the PC screen for the TDR 100 Time Domain Reflectometer

measurements. Lines define the measurement interval of the waveform as

recommended by the producer (Campbell Scientific, 2004).

Since water has a dielectric permittivity significantly larger (ε

O) ≈ 80) than other soil

constituents, air (ε

(Air) ≈ 0) and the solid phase (ε

(Solid) ≈ 4) (Curtis and Defandorf, 1929),

the resulting oscillation frequency is dependent upon the average water content of the

medium surrounding the rods. Rods can be inserted from the surface or the probe can be

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Campbell TDR Manual de usuario Pagina 3