PROCESS FOR KEEPING CLEAN THE MEASURING TUBES OF EMISSION-MEASURING APPARATUSES, USE OF THE PROCESS AND GAS SAMPLE TUBE FOR CARRYING IT OUT

1. Method for keeping clean the measuring tubes of emission-measuring apparatuses of flue gas purifying installations, wherein the stream of gas for analysis contains as gaseous or vapour impurities at least one component (A) and one component (B), and component (A) reacts with component (B) to form liquid or solid deposits on the inside of the measuring tubes, characterised in that - the stream of gas for analysis (r3 ) taken from the flue gas at a single sampling point is sub-divided into at least two parallel branch streams (r31 , r32 ) and the two branch streams are heated to a temperature above the condensation point of the vapour impurities (A, B) contained in the gas for analysis, - the first branch stream (r31 ) is passed through a first filter (A0 ) which selectively filters out component (A) in the stream of gas for analysis, - the second branch stream (r32 ) is passed through a second filter (B0 ) which selectively filters out component (B) from the stream of gas for analysis, - and the filtered first branch stream is conducted to an emission-measuring apparatus (AG1, AG2) which determines the amount of component (B), and the filtered second branch stream to one which determine the amount of component (A).