Q1(c). NDIR determination of carbon monoxide

Answer in brief

Measure selective absorption near the CO band at 4.6–4.7 μm against a reference signal.

Key points

• A broadband IR source sends modulated radiation through a gas cell of known path length. A narrow-band optical filter selects a CO absorption region.
• The detector converts transmitted intensity into an electrical signal. A reference wavelength or matched reference cell compensates source drift and optical changes.
• After zeroing with CO-free gas, calibrate with certified CO mixtures. Within the calibrated range, A=log (I₀/I)=ε bc.
• Control gas flow, temperature and pressure. Filtering/drying or appropriate compensation reduces interference from particles, water vapour and other IR-absorbing gases.