A sensor for measurements of temperature and water concentration using a single tunable diode laser near 1.4 μm

Gas temperature and water concentration are measured by a tunable diode laser sensor using a single laser near 1.4 μm. Temperature is determined from the ratio of absorption of a H2O line pair near 7079.176 and 7079.855 cm−1, which are chosen using some selected criteria. The second harmonic detection of wavelength modulation spectroscopy (WMS-2f) is employed to improve the sensor sensitivity and accuracy. Normalization of the 2f signal with the 1f signal magnitude is used to remove the need for calibration and correct for transmission variation due to beam steering, mechanical misalignments, soot, and windows fouling. An optimum laser modulation depth is selected to make the WMS-2f signals of both lines as big as possible for the target test conditions. The influence of total pressure and water mole fraction is evaluated through theoretical simulations. The validation of the sensor is conducted in a controlled static cell, yielding an accuracy of 1.06% temperature and 1.9% for H2O concentration. Burner experiments demonstrate the accuracy (1.2% for temperature and 3.2% for H2O concentration) and potential utility of the system for combustion diagnosis.