Filter elements are economically used only if their dirt holding capacity is fully exploited. A reliable indication of the optimal time to replace the element both on the filter itself and in the plant control or monitoring system is therefore vital. In a return line filter, the flow resistance increases as a function of operating time owing to the dirt that is retained in the filter element and the differential pressure rises accordingly. The service life can now be additionally extended thanks to an intelligent monitoring device for differential pressure. - The newly developed PiS 3300-2.9 2SP LED maintenance indic- ator is a self-checking, microprocessor-controlled pressure switch for MAHLE return line filters with two alarm outputs, high-intensity LEDs for all-round visibility and pulse and cold start suppression.