This International Standard specifies the principles, the essential performance characteristics and the calibration of automated systems for measuring carbon dioxide, carbon monoxide and oxygen in the flues of stationary sources. This International Standard specifies extractive and non-extractive systems in connection with several types of instrumental analyser. The following techniques have provided the basis for practical instrumentation: — paramagnetism (O2); — magnetic wind (O2); — differential pressure (Quinke) (O2); — magnetodynamics; — zirconium oxide (O2); — electrochemical cell (O2 and CO); — infrared absorption (CO and CO2). Other equivalent instrumental methods may be used provided they meet the minimum requirements proposed in this International Standard. The measuring system may be calibrated with certified gases, in accordance with this International Standard, or comparable methods.