contd....

NDIR analyzers enable accurate measurements of CO and CO₂in the exhaust gases.

NDIR instruments are seldom used for measurement of hydrocarbons except in the garage type analyzers as the IR absorbance to different hydrocarbons varies substantially. The unsaturated hydrocarbons are primarily responsible for photochemical smog but they do not have an adequate absorption in the IR wavelength range that is specific to the saturated hydrocarbons and vice versa. Sensitivity and response of NDIR to the exhaust HC is typically only half of the probable true value. NO absorbs only weekly in the infrared region. Moreover, CO, CO₂and water vapours interfere seriously; hence NDIR analyzers are also not used for NO measurement.