The dielectric calibration of capacitance probes for soil hydrology using an oscillation frequency response model

摘要

Capacitance probes are a fast, safe and relativelyinexpensive means of measuring the relative permittivity of soils, which can then be usedto estimate soil water content. Initial experiments with capacitance probes used empiricalcalibrations between the frequency response of the instrument and soil water content. Thishas the disadvantage that the calibrations are instrument-dependent. A twofold calibrationstrategy is described in this paper; the instrument frequency is turned into relativepermittivity (dielectric constant) which can then be calibrated against soil watercontent. This approach offers the advantages of making the second calibration, from soilpermittivity to soil water content. instrument-independent and allows comparison withother dielectric methods, such as time domain reflectometry. A physically based model,used to calibrate capacitance probes in terms of relative permittivity (ε_r) is presented.The model, which was developed from circuit analysis, predicts, successfully, thefrequency response of the instrument in liquids with different relative permittivities,using only measurements in air and water. lt was used successfully to calibrate 10prototype surface capacitance insertion probes (SCIPS) and a depth capacitance probe. Thefindings demonstrate that the geometric properties of the instrument electrodes were animportant parameter in the model, the value of which could be fixed through measurement.The relationship between apparent soil permittivity and volumetric water content has beenthe subject of much research in the last 30 years. Two lines of investigation havedeveloped, time domain reflectometry (TDR) and capacitance. Both methods claim to measurerelative permittivity and should therefore be comparable. This paper demonstrates that theIH capacitance probe overestimates relative permittivity as the ionic conductivity of themedium increases. Electrically conducting ionic solutions were used to test the magnitudeof this effect on the determination of relative permittivity. The response was modelled sothat the relative permittivity, independent of ionic conductivity, could be determined insolutions with an electrical conductivity of up to 0.25 S m-1. It was foundthat a solution EC of less than 0.05 S m-1 had little impact on thepermittivity measurement.