The principle is schematically shown in right figure. The centered electrode is surrounded by the guard electrode for forming a parallel field at the centered electrode. The conductive material is parallel to the end of the probe surface.The capacitance C, between the centered electrode and material is ideally described by C=E0A/g, where E0 is the dielectric constant of vacuum, A the area of the centered electrode, and g the gap. The reciprocal of the capacitance is directly proportional to the gap. g. High stabillty wide-range and high resolution of PS sensor are based on this simple principle and optimized electronics.