ekS - Instytut Agrofizyki im. Bohdana DobrzaÅskiego PAN w Lublinie ...

More documents

Recommendations

Info

dielectric permittivity, the dielectric constant, is the indicator of its water content, θ. For the probe length L, the apparent dielectric constant of the material, ε, and indirectly its water content can be calculated [59] using the formula in Fig. 35, where: (t b -t a ) is the measured time the pulse covers the distance 2L, c is the velocity of light in free space. Most commonly used EC determination is based on Giese and Tiemann [33] approach: Z0 EC = (84) 120πLZ L where Z 0 is probe impedance, L is probe length, and Z L is the measured resistive load impedance across the probe embedded into the soil: Z0 Z L = (85) [( 2V 0 / V f ) −1] Lossy media attenuate the TDR reflected pulse and this makes the method practically useless for electrical permittivity of materials exceeding 4 dS/m, which is a high value for arable soils (Marschall and Holmes [63]). In saline soils the signal reflected from the end of the probe rods is completely attenuated (no amplitude at point b in Fig. 35). The measurement of the amplitude of the pulse reflected from the end of the probe, taken simultaneously to the pulse travel time, enables to determine the electrical conductivity of the soil, σ, and its salinity defined after Rhoades and Ingvalson [75] as the electrical conductivity of the extract of a saturated soil paste. 9.2. Open-Ended Coaxial Probe method The measurement of dielectric properties of materials in frequency domain, with the application of microwave frequencies, needs expensive instrumentation. The basic elements of a simplified Vector Network Analyzer (VNA), working only in reflection mode, for determination of reflection parameter S 11 is presented in Fig. 36. The device consists of a very stable frequency synthesizer generating sinusoidal waveforms of variable frequency feeding a sensor – Open-Ened Coax probe connected to the output/input port of the VNA. For a defined frequency the directional couplers sense the amplitudes and the phases of the waveforms produced by the generator and reflected from the probe. The detected difference in amplitude and phase depends on the dielectric properties of the tested material 87
at the end of the open-ended coaxial probe. The signals from the directional couplers are mixed with the signals from another generator, heterodyne, which frequency is adjusted by the phase detector. The reason for this frequency conversion is to have the constant frequency, not dependent on the synthesized frequency, of the signals reaching the measuring detector. The detected signals are characterized by constant frequency but their amplitudes and phase shifts do not change. The whole process of waveforms generation and detections is controlled by a microprocessor that changes the frequency of the synthesizer and registers the detected signals. All elements used in the presented system work in broad frequency range, from kHz to GHz, and they must have linear characteristics. Moreover, the measuring system accomplishes continuous recalibration to maintain parameters stable in time and temperature. Fig. 36. Basic elements of the frequency domain reflectometer for determination of complex dielectric permittivity of porous media Fundamental to use the Open-Ended Coax probe (Fig. 37) is an accurate model relating the reflection coefficient at its end to the permittivity of the material contacting with the probe. The lumped capacitance circuit model described by Stuchly and Stuchly [89] is applied in the presented example. 88
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 18 and 19:
The TDR probe consists of two waveg
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 90 and 91: The applied lumped capacitance mode
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
Page 140 and 141:
25. de Loor G.P.: Dielectric proper
Page 142 and 143:
TDR. Symposium on Time Domain Refle
Page 144 and 145:
100. Whalley W.R.: Considerations o
Page 146 and 147:
Fig. 14. Fig. 15. Fig. 16. Fig. 17.
Page 148 and 149:
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
show all

ekS - Instytut Agrofizyki im. Bohdana DobrzaÅskiego PAN w Lublinie ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?

ekS - Instytut Agrofizyki im. Bohdana DobrzaÅskiego PAN w Lublinie ...