Sensors and A/D ratios
Measurements of dielectric properties often involve the use of simple parallel plate electrodes. However, their separation can change with pressure, or expansion and contraction of the material between them. The ratio of electrode area A and the distance D between them—the A/D ratio—therefore may not be well known. As the cell constant or scaling factor between conductance and conductivity, or capacitance and permittivity, uncertainty in A/D causes inaccuracies in determining dielectric material properties.
Reference :
Lambient TechnologiesTM Application Note 3.06—Sensors, A/D Ratio and Base Capacitance, 1 Page
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