A laser Raman system for obtaining N2 and CO2 concentrations in probe-sampled combustion gases

A system for obtaining N2 and CO2 concentrations in probe-sampled combustion gases by means of vibrational Raman scattering of an argon laser beam passed through a small cell located in the sample transfer line has been designed and tested. The detectors consisted of conventional photomultiplier tubes windowed by narrow bandpass (approximately 1 nm) interference filters and Rayleigh rejection filters. For N2, the center wavelength of the filter was 584.6 nm with a half-width of 1.2 nm, and for CO2, the wavelength was 554.1 nm with a half-width of 1.3 nm. Laboratory tests with calibration gases indicated excellent system linearity with respect to species concentration and partial pressure. The detection threshold was found to be approximately 1 percent at a sample pressure of 0.5 atm. The factors which determine this limit were photomultiplier noise levels and out-of-band leakage of the Raman interference filters.