Campbell TDR100 Instrukcja Obsługi Pobierz pdf (Strona 33)

TDR100

caused by the probe beginning and end. This information is then analyzed to

determine soil water content.

While the velocity of the applied pulse along a waveguide is dependent on the

dielectric constant of the material surrounding the waveguide, the amplitude of

the reflected voltage is dependent on electrical conduction of the applied signal

between probe rods. The presence of free ions in the soil solution will result in

attenuation of the applied signal. Theory presented by Giese and Tiemann

(1975) has been applied to the measurement of soil bulk electrical

conductivity. A commonly used expression is

−

[5]

where σ is the bulk electrical conductivity,

is a probe constant, Z

is the

cable impedance (50 ohm), and ρ is the reflection coefficient. The reflection

coefficient is the ratio of the reflected voltage to the applied voltage and ranges

between plus and minus one.

The components of the Campbell Scientific TDR system are designed to apply

time domain reflectometry for the determination of soil volumetric water

content and bulk electrical conductivity as both hardware for the collection of

data and software for communications and analysis of the data.

8. Cable Length and Soil Electrical Conductivity

Effect on Water Content Determination

8.1 Cable Length Effect on Water Content Measurement

The determination of water content using the TDR system relies on the

evaluation of a pulse reflection from the TDR probe. The pulse generated by

the TDR100 and its reflections are subject to distortion during travel between

the TDR100 and the TDR probe. The cable connecting the probe to the

reflectometer has a characteristic impedance resulting in both resistive and

reactive losses. Distortion of the waveform caused by cable impedance can

introduce error into the water content determination.

Figure 8-1 presents waveforms collected from a 3-rod probe (CS610) for

various cable lengths. As cable length increases, the rise time and the

amplitude of the reflection are affected. The slopes and extrema used by the

datalogger algorithm to analyze the waveform are shifted by the cable losses

resulting in error. For the data shown in Figure 8-1, the water content

measurement using the 66 meter cable was in error by about 1.5% volumetric

water content when electrical conductivity is low. However, in saline soils the

error can be several percent. See Bilskie (1997) for complete results of the

study.

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Campbell TDR100 Instrukcja Obsługi Strona 33