OXYGEN CONCENTRATION AND TEMPERATURE-MEASUREMENTS IN N-2-O-2 MIXTURESUSING ROTATIONAL COHERENT ANTI-STOKES-RAMAN SPECTROSCOPY

The accuracy and precision of oxygen concentration and temperature mea sured by dual-broadband rotational Coherent Anti-Stokes Raman Spectros copy (CARS) were investigated in nitrogen-oxygen mixtures at atmospher ic pressure and temperatures between 290 and 1410 K. The relative stan dard deviation of temperatures evaluated from pure oxygen rotational C ARS spectra was found to be around 5%, and the mean temperature was th e same as for nitrogen CARS spectra, except for temperatures above 100 0 K, where the temperature was 120 K below the correct value. The in s itu calibrated oxygen concentrations were within 10% of the correct va lue, with a standard deviation of around 1.2% for the mixtures of 12 a nd 20% oxygen in nitrogen. For the lowest oxygen concentrations consid ered in this study (2 and 4%), the systematic errors in the evaluated concentrations were very large, and the standard deviation of repeated single-shot measurements was above 2%. However, employing weighting i n the spectral fitting routine reduced the errors in the concentration and the single-shot standard deviation was lowered to 0.5%. Finally, it was shown that spectral interference (from oxygen) in a rotational CARS spectrum of nitrogen generally had little impact on the temperatu re evaluated from fitting the spectra to theoretical nitrogen spectra.