A description the fig1 to 4 show a measuring device 2 of the system according to a first embodiment of the present invention the measuring device is built by means of a flange 22 into the wall 6 of the operating engine in the present case into the working area 4 of an internal combustion engine the flange 22 comprises a feed line 12 for a gas aerosol mixture which is drawn in from the working area 4 by the pumping action of a convection pump 8 and which is again fed back into the working area 4 by means of a recovery line 20 in the present embodiment the recovery line 20 is arranged close to the feed line 12 and therefore the suction point 12 a and the recovery point 20 a of the gas aerosol mixture are located close to each other the two lines 12 20 u2014 as seen from the suction point and recovery point respectively and starting out from the flange u2014 initially comprise a substantially horizontal section which u2014 after a bent section u2014 leads into the actual convection pump area the latter comprises in the feed line 12 thereof a heating device 42 with a resistive heating element 42 a and a heat exchanger 42 b and in the recovery lines 20 thereof a cooling device 44 with a heat exchanger 44 a in the recovery line and a further heat exchanger 44 b that conducts the heat to the environment by means of the heating device 42 with the resistive heating element 42 a and the heat exchanger 42 b the.