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| Titel |
The dielectric calibration of capacitance probes for soil hydrology using an oscillation frequency response model |
| VerfasserIn |
D. A. Robinson, C. M. K. Gardner, J. Evans, J. D. Cooper, M. G. Hodnett, J. P. Bell |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 2, no. 1 ; Nr. 2, no. 1, S.111-120 |
| Datensatznummer |
250000398
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| Publikation (Nr.) |
 copernicus.org/hess-2-111-1998.pdf |
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| Zusammenfassung |
Capacitance probes are a fast, safe and relatively
inexpensive means of measuring the relative permittivity of soils, which can then be used
to estimate soil water content. Initial experiments with capacitance probes used empirical
calibrations between the frequency response of the instrument and soil water content. This
has the disadvantage that the calibrations are instrument-dependent. A twofold calibration
strategy is described in this paper; the instrument frequency is turned into relative
permittivity (dielectric constant) which can then be calibrated against soil water
content. This approach offers the advantages of making the second calibration, from soil
permittivity to soil water content. instrument-independent and allows comparison with
other dielectric methods, such as time domain reflectometry.
A physically based model,
used to calibrate capacitance probes in terms of relative permittivity (εr)
is presented.
The model, which was developed from circuit analysis, predicts, successfully, the
frequency response of the instrument in liquids with different relative permittivities,
using only measurements in air and water. lt was used successfully to calibrate 10
prototype surface capacitance insertion probes (SCIPS) and a depth capacitance probe. The
findings demonstrate that the geometric properties of the instrument electrodes were an
important parameter in the model, the value of which could be fixed through measurement.
The relationship between apparent soil permittivity and volumetric water content has been
the subject of much research in the last 30 years. Two lines of investigation have
developed, time domain reflectometry (TDR) and capacitance. Both methods claim to measure
relative permittivity and should therefore be comparable. This paper demonstrates that the
IH capacitance probe overestimates relative permittivity as the ionic conductivity of the
medium increases. Electrically conducting ionic solutions were used to test the magnitude
of this effect on the determination of relative permittivity. The response was modelled so
that the relative permittivity, independent of ionic conductivity, could be determined in
solutions with an electrical conductivity of up to 0.25 S m-1. It was found
that a solution EC of less than 0.05 S m-1 had little impact on the
permittivity measurement. |
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