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The TDR probe consists of two waveguides connected together: a coaxial one,
called the feeder, and a parallel one, called the sensor, made of two parallel metal
rods inserted into the measured medium. The initial needle pulse travels from the
generator by the feeder towards the sensor. The recorder registers this pulse as it
passes T-connector. In the connector, between the feeder and the sensor, there is a
rapid change in geometry of the electromagnetic wave travel path. At this point
some energy of the pulse is reflected back to the generator, like in radar, and the
remaining is traveled along the parallel waveguide to be reflected completely
from the rods ending. The successive reflections are recorded for calculation the
time distance between the two reflections (a) and (b). Three reflectograms
(voltage as a function of time at the chosen point in the feeder) are presented in
Fig. 1. They represent cases when the sensor was placed in dry, wet and water
saturated soil. The time distance, ∆t, necessary for the pulse to cover the distance
equal to the double length of metal rods in the soil increases with the soil
dielectric constant, thus water content. The reason for that is the change of
electromagnetic propagation velocity in media of different dielectric constants,
according to Eq. (2).
Also, the amplitude of the pulse at the point (b) decreases with the increase of
soil electrical conductivity, according to Eq. (3).
3.1.2. Calibration of TDR soil water content measurement
There are two approaches that allow calculating the soil water content from
the TDR measurements: empirical and theoretical - using dielectric mixing
models.
The results of TDR measurements of mineral, organic soils as well as
mineral/oganic mixtures are presented in Fig. 2. The coefficients of linear
regression equations fitted to empirical data points differ slightly showing that
mineral composition of the soil significantly influences the TDR calibration
n = a a θ , where θ grav is the soil water content determined by
curve: ( θ grav ) 1 grav
0 +
standard thermogravimetric method ISO 11461. Reconfiguration of the calibration
curve allows the calculation of θ when TDR meter measures n. The accuracy of
the TDR soil water content measurement is about ±2% of the true
thermogravimetrically determined value, Eq. (7).
θ = 0.13
ε − 0.18
(7)
The most popular calibration curve for TDR soil (mineral soils) water content
measurement was given by Topp et al. [90] and is presented below:
17