Self-controlled PTC sensor for reliable overfill protection of liquids

The paper deals with the design of sensors with automatic self-control. A PTC thermistor was selected, which is used as a level sensor to prevent overfill of liquids in tanks. In self-heated mode, the I-U characteristic is basically dependent on the unloading resistance value R_T of the PTC sensor itself, the thermal resistance R_W and the temperature T_M of the medium that is surrounding it. This I-U characteristic of the sensor in thermally steady state in still air is modelled. Problems in detecting the liquid level occur for a soiled sensor surface or liquids with high temperature and/or high viscosity (e.g. crude oil). A solution to this worst-case scenario is to determine R_W continuously from the I-U characteristic of the PTC thermistor in function. The constant monitoring of the sensor parameters enables an automatic sensor self control, e.g. to identify slow changes of the sensor parameters due to corrosion and dust layers on the sensor surface or ageing.