FAQ6 CONCENTRATION MODELS

The relationship between the permittivity distribution and the capacitance measured between a pair of electrodes must be considered carefully if accurate permittivity/concentration images are to be obtained. There are a number of models which can be used to improve the accuracy of the concentration measurement. It is very important to use the correct permittivity model (parallel/series/Maxwell etc) if accurate concentration values are to be obtained from the permittivity image. Further information on capacitance/ permittivity models (including Maxwell's method) is given in the paper by Yang and Byars (1999). Back to top

If the two dielectric materials exist as discrete stratified permittivity layers between the two electrodes, then two component capacitances, each due to one of the dielectric materials, and effectively connected in parallel, will exist between the electrodes. The sum of these capacitances will therefore accurately reflect the relative proportions (or concentration) of the 2 materials present. In this case, the mixture concentration is found by assuming that the dielectric materials combine to form two capacitances in parallel. Back to top

If the materials exist as alternating bands of permittivity between the electrodes, the capacitances measured between the electrodes will be constituted from component capacitances which are effectively connected in series. In this case, the reciprocal rule for adding up capacitances in series must be used to obtain the component permittivities and concentration from the measured capacitances. Back to top

If there is a combination of the two basic parallel and series material distributions, more complex relationships, such as the method described by Maxwell, must be used to define the permittivity/ concentration/ capacitance relationships. Back to top

Last updated 17-05-2002

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