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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
−2 −2 θ = −5.3⋅10 + 2.92 ⋅10 ⋅ε − 5.5 ⋅10 ⋅ε + 4.3⋅10 ⋅ε −4 2 −6 3 (8) The introduction in the Eq. (7) the correction on soil bulk density, ρ, that accounts for the influence of soil solid phase in the TDR readout decreases the measurement error to about ±1% giving a new calibration curve presented by Eq. (9). ε − 0.57 − 0.58ρ θ = (9) 7.75 + 0.92ρ Fig. 2. Relation between the soil refractive index, n, and its water content, θ, for mineral and organic soil samples as well as their mixtures. A - experimental data, B - regression n(θ). Applying theoretical approach the soil is usually treated as a three phase mixture, and its dielectric constant of soil, ε, can be presented as: α α w α α ( −φ) ⋅ε + ( φ −θ ) ⋅ε ε = θ ⋅ε + 1 (10) where: ε, ε w , ε s and ε a are the dielectric constants of soil as a whole, soil water, soil solids, and air, α is a constant interpreted as a measure of the soil particles geometry. On the base of the measured data collected on various soils, it has been found by Roth et al. [77] that for the three phases dielectric model of the soil the average value of α constant is 0.5. For typical values of α=0.5, ε w =81, ε s =4, s a 18
Page 2 and 3: TDR METHOD FOR THE MEASUREMENT OF W
Page 4 and 5: PREFACE The importance of water for
Page 6 and 7: CONTENTS 1. Status of water as an s
Page 8 and 9: 12.2. New prototype version of FOM/
Page 10 and 11: that stimulate the phenomena and pr
Page 12 and 13: The solution to the problem of elec
Page 14 and 15: 3. ELECTRICAL MEASUREMENTS METHODS
Page 16 and 17: v = c 1 2L = = ( θ ) n ∆t ε (2)
Page 20 and 21: ε a =1 and φ=0.5 the Eq. (10) has
Page 22 and 23: The experiment description is given
Page 24 and 25: Results obtained with the discussed
Page 26 and 27: Notice that at C s = 0, ie when dis
Page 28 and 29: water orientation polarization [s],
Page 30 and 31: Fig. 8. Frequency dispersion of the
Page 32 and 33: The measurements were performed in
Page 34 and 35: 5. EVALUATION OF DIELECTRIC MIXING
Page 36 and 37: ε α = ∑V α iε i (29) i where
Page 38 and 39: where SAND and CLAY are percents of
Page 40 and 41: Below the transition value, the soi
Page 42 and 43: To verify the observations concerni
Page 44 and 45: 6. TEMPERATURE EFFECT ON SOIL DIELE
Page 46 and 47: chamber and the freezer, only one w
Page 48 and 49: The user interface on the main comp
Page 50 and 51: soil sample volumetric water conten
Page 52 and 53: close vicinity of the analyzed obje
Page 54 and 55: process of measurement is non-destr
Page 56 and 57: dielectric permittivity change caus
Page 58 and 59: The values of the refractive index
Page 60 and 61: The relative, ∆n/∆T, and absolu
Page 62 and 63: Fig. 25. The temperature effect on
Page 64 and 65: The temperature effect of TDR soil
Page 66 and 67: The purpose of the above discussion
Page 68 and 69:
( ,25 + T )( 45 + T ) 49 9 f rel =
Page 70 and 71:
8. THE ACCURACY OF SOIL WATER CONTE
Page 72 and 73:
gravimetric method and the soil ref
Page 74 and 75:
where: a 0 ÷ a 6 are coefficients
Page 76 and 77:
values of p F calculated for the re
Page 78 and 79:
eason of this is the fact that wate
Page 80 and 81:
9 ∆θ = 0,2 ⋅10 ⋅ ∆t = 0.00
Page 82 and 83:
values of θ rel for lower values o
Page 84 and 85:
9. COMPARISON OF OPEN-ENDED COAX AN
Page 86 and 87:
ε" = EC ε d " + (82) 2π fε wher
Page 88 and 89:
dielectric permittivity, the dielec
Page 90 and 91:
The applied lumped capacitance mode
Page 92 and 93:
stainless steel wires soldered to t
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ε’ Open Coax and ε b-TDR , exce
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− − The difference of the imagi
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Integrated Circuit) recently introd
Page 100 and 101:
10.2. Electromechanical microwave s
Page 102 and 103:
RF1. Two PIN diodes in series in ea
Page 104 and 105:
The significant aspect of the proto
Page 106 and 107:
visible for lower values of soil el
Page 108 and 109:
In the TDR method the signal propag
Page 110 and 111:
description of functional and techn
Page 112 and 113:
MASTER modules are wireless transce
Page 114 and 115:
time the Real Time Clock module wak
Page 116 and 117:
ase station distinguishes A SLAVE d
Page 118 and 119:
sensor, format the data received fr
Page 120 and 121:
12. SOIL WATER STATUS MONITORING DE
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D-LOG/mpts is a moisture/pressure/t
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For compatibility reasons it uses t
Page 126 and 127:
D-LOG/mts (Fig. 59) is a TDR (Time-
Page 128 and 129:
12.4. FP/m, FP/mts - Field Probe fo
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inside of a separate polyethylene b
Page 132 and 133:
differential water capacity and the
Page 134 and 135:
− maximum number of LP/p probes t
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12.7. LP/t - Laboratory Probe for s
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From the collected data set, after
Page 140 and 141:
25. de Loor G.P.: Dielectric proper
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TDR. Symposium on Time Domain Refle
Page 144 and 145:
100. Whalley W.R.: Considerations o
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Fig. 14. Fig. 15. Fig. 16. Fig. 17.
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Fig. 38. The three-rod TDR probe -
Page 150 and 151:
Fig. 67. A set of LP/ms and LP/p LO
Page 152:
Table 17. An example of the executa
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