The primary method used for measuring carbon monoxide in US is based on nondispersive infrared photometry (NDIR).The analyser has a hot filament sorce of infrared radiation, a chopper , a sample cell, reference cell and a detector.The reference cell is filled with a non- infrared- absorbing gas,and the sample cellis continuously flushed with ambient air containnig unknown amount of CO.The detector cell is divided into two compartments by a flexible membrane, with each filled with CO.Movement of the membrane causes a change in elektrical capacidance in control circuit whose signal is processed and fed to recorder.
