How does a level switch based on the conductivity principle function?
The level switch can only be used to detect the level of conductive liquids like water whose conductivity is more than the specified value. For standard applications this should greater than 25 micro siemens. A part insulated conductivity probe is installed on the vessel. The length of the probe is defined by the point level at which the alarm is desired. The sense electrode is connected to a low voltage AC supply derived from the evaluation unit (about 12 V). For metallic tanks, the tank body is connected to ground, which in turn connects to the low voltage circuit ground through a suitable resistance. The voltage across this resistance is monitored by the circuitry in the evaluation unit. When the level is below the tip of the probe, there is no flow of electric current and therefore no voltage across the resistance. When the liquid covers the probe tip there is a flow of current via the conductive liquid and the resistance. The voltage across the series resistance is used to switch
