Influence of the pressure and temperature on laser induced breakdown spectroscopy for gas concentration measurements

Laser induced breakdown spectroscopy (LIBS) is an effective technique to obtain quantitative concentration information for the fuel injection and combustion processes. Practical applications of fuel injection and combustion usually involve a wide range of ambient pressures and temperatures, but understanding of the influence of these parameters on LIBS is far from adequate. The objective of this paper is therefore to investigate the pressure and temperature effects on LIBS over a wide range for gas concentration measurements, and further correct these effects to improve the measurement accuracy. The calibrations at various ambient pressures show that the peak intensity ratios (PIRs) exhibit better linear correlations with the equivalence ratios than the area intensity ratios (AIRs) for H656/N746. The intensities of H656 and N746 are remarkably influenced by the gas pressure due to the combined effects of varied gas density and self-absorption. While the PIR of H656/N746 depends greatly on gas pressures from 0.3 to 1.0 bar, the gas pressure effects on the PIR of H656/N746 are very slight at gas pressure above 1.0 bar. The effects of gas temperature on the PIR of H656/N746 are investigated at both constant gas pressure and constant gas density. The results show that the temperature differences between the calibrations and test conditions would significantly degrade the measurement accuracy, especially at low gas pressures. Then, a novel method to correct the gas temperature effects on LIBS for gas concentration measurements is proposed, based on a linear correlation between the PIR of H656/N746 and the gas temperature, with the gas temperature estimated by the full width at half maximum (FWHM) of H656.