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The applied lumped capacitance model of the probe assumes the presence of capacitance at the end of the probe. Its value depends on the complex dielectric permittivity of the material the probe was pressed to (Fig. 37). The complex value of the admittance of the probe end, Y L * , is: Y * * L f 0 ( f 0 + 0 = jω C + jω C ε = jω C + C ε ') ω C ε" (86) where: C f represents the part of admittance that is independent from the dielectric sample, C 0 is a part of admittance for air as dielectric. Fig. 37. A - Open-ended coaxial probe in the form of coaxial line open to the space with material of unknown dielectric permittivity ε * , B - modeling the discontinuity of electromagnetic field by lumped capacitances C f and C 0 Before performing measurements on unknown materials the Open-Ended Coax probe should be calibrated on media with known dielectric properties, usually distilled water, methanol or air to find the values of C f and C 0 . The described measurement method and the lumped capacitance model were verified by measuring the dielectric permittivity of teflon. It was found that the C f value is negligible and C 0 calculated from (86) equals to the C 0 coefficient of the correction polynomial obtained during calibration. Consequently the model (86) may be simplified to: * * Y L = ω C ε = jω C ε + ω C ε" (87) 0 0 ' 0 The value of C 0 describes the geometry of the probe and the frequency range of its application. The larger dimensions of the probe, the bigger its value and the better accuracy in applications for the materials of small values of dielectric permittivity, and simultaneously the high frequency range of measurement is 89
limited. This limitation comes from the fact that in case of materials of high dielectric permittivity and at high frequencies, the changes of the dielectric permittivity of the material causes very small changes of the phase shift of the reflection coefficient. This drastically decreases the accuracy of measurement and the need to apply calibration media of small value of dielectric permittivity. 9.3. Materials and methods Dielectric permittivity of three soils was compared. Table 16 presents the basic physical parameters of the tested soils. Table 16. Localization and selected physical parameters of tested soil samples No. 611 Olempin 606 Localization Wólka Kątna Soil type Gleba brunatna Gleba murszowa Density [gcm -3 ] Soil texture ISSS sand silt clay 2-0,02 mm 0,02-0,002 < 0,002 mm mm Specific surface [cm 2·g -1 ] 1.59 94 5 1 9 1.42 97 2 1 23 619 Sahryń Czarnoziem 1.16 87 12 1 69 Dry soils were mixed with appropriate amount of distilled water to achieve four samples for each analysed soil with different water content values and taking care to get the homogeneous distribution of water in the soil samples. Twelve containers with soil samples covered with plastic foil (to minimize evaporation) were left for 24 hours in room temperature for water distribution in the samples in natural way. The gravimetric water content, θ, and bulk density, ρ, were determined according to ISO standard 16586 [44] for each soil sample directly after the dielectric permittivity measurements were completed. The values of ρ in Table 1 are the mean for all applied water contents for each tested soil. Soil texture was determined by standard Bouyoucos method (Turski [93]). The values of soil specific surface, S, was determined with water vapour adsorption method (Ościk [73]). Time Domain Reflectometry (TDR) and Open-Ended Coax Probe methods of determination of the complex dielectric permittivity of porous materials were applied. TDR measurements were performed using the oscilloscope frame HP54120 with the TDR unit HP54121T, featured by 20 ps rise-time step pulse. The TDR probe was based on the standard coaxial connector (type N jack) with three 90
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TDR METHOD FOR THE MEASUREMENT OF W
Page 4 and 5:
PREFACE The importance of water for
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CONTENTS 1. Status of water as an s
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12.2. New prototype version of FOM/
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that stimulate the phenomena and pr
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The solution to the problem of elec
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3. ELECTRICAL MEASUREMENTS METHODS
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v = c 1 2L = = ( θ ) n ∆t ε (2)
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The TDR probe consists of two waveg
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ε a =1 and φ=0.5 the Eq. (10) has
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The experiment description is given
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Results obtained with the discussed
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Notice that at C s = 0, ie when dis
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water orientation polarization [s],
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Fig. 8. Frequency dispersion of the
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The measurements were performed in
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5. EVALUATION OF DIELECTRIC MIXING
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ε α = ∑V α iε i (29) i where
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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 92 and 93: stainless steel wires soldered to t
Page 94 and 95: ε’ Open Coax and ε b-TDR , exce
Page 96 and 97: − − The difference of the imagi
Page 98 and 99: 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
Page 122 and 123: D-LOG/mpts is a moisture/pressure/t
Page 124 and 125: 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
Page 130 and 131: 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
Page 136 and 137: 12.7. LP/t - Laboratory Probe for s
Page 138 and 139: From the collected data set, after
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25. de Loor G.P.: Dielectric proper
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TDR. Symposium on Time Domain Refle
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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 -
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Fig. 67. A set of LP/ms and LP/p LO
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Table 17. An example of the executa
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