Using so-called “self-cleaning” gas filters is worth considering for sample gas systems where secondary contamination primarily consisting of larger particles are to be expected. The functional principle of these filters is that of the ‘Cross Flow’ process. Here a partial stream is continuously adequate for analysis is continuously extracted from the main sample gas flow. Inside the housing, the main stream is forced along the filter element in a spiral. The dirt particles from the partial stream deposited on the surface of the element are carried along by the main stream and discharged. The continuous discharge of dirt deposit results in a very long filter element life, thus low maintenance. The BF2-S was developed specifically for these applications.