In dual measurement, methane (CH4) is measured in a second combustion chamber. For this purpose, long-chain atoms are reduced with the aid of a thermal cutter so that only methane remains in the sample gas for measurement. The concentration of non-methane hydrocarbons (NMHC) is then determined from the difference.
Since this measurement principle mainly ionizes the carbon atoms (C atoms), higher compounds with several C atoms (e.g. C3H8) also give a higher current at the detector. If the proportion of the various HC compounds is to be specified, this can be determined by means of so-called response factors. These are strongly dependent on the design of the measuring chamber and the flow rate of sample gas, which is why response factors are determined individually for each flame ionization detector.