A capacitive proximity switch works with a high frequency oscillating circuit that creates an electromagnetic field on the active sensor surface by means of a capacitor. When a substantial or fluid substance nears this field a capacitance unbalance occurs and leads to a variation of the amplification in the oscillating circuit. If this amplification exceeds a threshold value, a switching signal is generated.
The operating distance is the separation of an object from the active sensor surface at which a switching signal is produced. The operating distance depends on the diameter of the sensor surface. Therefore larger sensors are required for longer operating distances. Many EGE sensors have an adjustable operating distance. Capacitative proximity switches react to conductive materials and non-conductive materials with a dialectrical constant Epsilon >1. The operating distance depends on the material. Constant measurement of different materials against earthed steel ST37 yields a changed operating distance. The following table lists the approximate values for the material dependent reduction factors. In practical applications there could be variations, because of the mounting conditions, for example.
The water content of an object or a liquid has a decisive influence on the operating distance. A high humidity content increases the operating distance considerably. If the proximity switch is moistened with conductive materials, its function can be impaired when a conductive film builds up that electrically connects the sensor electrode with a metallic conducting side. Capacitive proximity switches can detect filling products right through non-electrically conducting container sides. The filler will certainly be detected if its dialectric constant is at least as large as that of the container sides.