Environmental monitoring of methane utilizing multispectral NDIR gas sensing for compensation of spectral impact from water vapor in air

A multispectral nondispersive infrared (NDIR) sensor was developed for simultaneous detection of methane and water vapor in air. The NDIR sensor is capable of measuring optic transmission in the CH4 absorption spectra at 3.375 $\mu\mathrm{m}$ and the H 2 O absorption spectra at 2.7 $\mu\mathrm{m}$ . Data from a third channel, 3.95 $\mu\mathrm{m}$ , is used as reference value for “zero-level” calibration. The actual CH4 concentration is retrieved by adjusting the data obtained in the $\text{CH}_{4}$ spectra with respect to the concentration sensed in the $\mathrm{H}_{2}\mathrm{O}$ spectra. A calibration procedure was developed and tested, which involves matching of the absorbed light energy in the CH4 and the $\mathrm{H}_{2}\mathrm{O}$ spectrum in humid reference environments. A compensation algorithm for elimination of humidity impact was developed and validated in environments with variable $\text{CH}_{4}$ and H 2 O concentrations. By implementing the multispectral approach, and the developed algorithm, an uncertainty of 15–25 ppm relative the reference concentrations was achieved. For a concentration range valid for environmental monitoring applications this should be compared to an uncertainty of 180–200 ppm for the non-corrected CH4 concentration.