Verfahren und Gerät zur Gasanalyse

Gas analysis apparatus comprises a radiation source (13) arranged to transmit radiation through a gas (12) to be analysed to a radiation sensor (32). Datum means (50-60) intercepts the radiation and transmits it to the sensor at a reference or datum value. The datum means comprises a mirror to reflect the radiation directly to the sensor thereby providing a zero absprption datum point. The datum means may also comprise one or more gas cells (54, 56,58) containing a high concentration of the gas to be analysed and thereby giving a high scale value datum point. The mirror and gas cells are mounted on a datum wheel (20) which is continuously rotated . Infra-red radiation from a source (13) is transmitted at two frequencies chosen for each gas to be analysed , by virtue of a rotary filter wheel 16 having pairs of filter discs (M1,R1) and (M2,R2) and (M3,R3).