E-Zine December 2012
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Noting that the measured capacitance is dependent upon the size of the plates and the effective distance between the plates, variation in these parameters can cause measurement problems. The distance between the plates is held constant by the dimensional stability of the sensor. However, in nonconductive materials, dielectric constant variations can cause the effective distance between the plates to vary. In conductive materials, relatively low electrical conductivity of the material can reduce the effectiveness of the material side of the capacitor. This means that the level measurement can change when the electrical properties of the material change --- even when the level remains constant.
Conductive materials should maintain sufficient electrical conductivity such that the material effectively forms one plate of the capacitor. This is generally accomplished when the material exceeds a given electrical conductivity specified by the supplier. Knowing that the electrical conductivity of the material in a given application will exceed this value infers that the covered portion of the probe will form an effective capacitor plate and provide immunity to changing electrical conductivity in the application. Stated differently, measurement error caused by changing electrical conductivity is reduced when the material exceeds the electrical conductivity requirement of the sensor. Achieving this objective is practical in many applications involving conductive materials. Note that different sensors have different electrical conductivity requirement such that sensor selection can influence level measurement error.
For nonconductive materials, dielectric constant variations of the material cause the effective distance of the capacitor (and hence its capacitance) to vary. Therefore, measurement error caused by changing dielectric constant will be reduced when the material dielectric constant does not vary. Achieving this objective may not be practical in some applications because temperature and composition variations affect the dielectric constant of the material and result in significant level measurement error.
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Excerpted from The Consumer Guide to Capacitance and Radar Contact Level Gauges