Measuring spectral parameters of water vapor at low temperature based on tunable diode laser absorption spectroscopy

Abstract

Accurate and reliable spectral line parameters of gas are very important for measuring gas concentration and temperature.The mainstream spectrum database (e.g.HITRAN) includes the values from theoretical computation based on different models,which have some inevitable deviations from the corresponding actual values.To address this problem,we develop a low-temperature spectral experimental platform for simulating low temperature and low pressure environment so as to accurately measure gas absorption spectral parameters.The spectral experimental platform uses the static cooling technology combined with the Dewar insulation system to maintain the quartz cell at a constant temperature.Through adjusting the electric heating and liquid helium refrigeration,we can achieve temperature change and stability.Temperature of the low temperature absorption cell can be adjusted in a range of 100-350 K with a precision lower than 0.3 K and the temperature gradient in the cell is lower than 0.01 K/cm.The length of quartz cell is 100 cm,and a reflector can be used to increase optical path for absorption.The window diameter is 76 mm,and the spectral resolution is better than 0.001 cm-1.We use a tunable diode laser spectrometer to measure absorption spectra of pure water vapor with the platform at different temperatures (230-340 K) and different pressures (10-1000 Pa).Voigt profile is the leastsquares fit to the measured spectra by using a multi-spectrum fitting routine.A filter is used to reduce electronic noise of detector signal.As spectral lines in the band of 7240-7246 cm-1 are often used in low temperature wind tunnel flow field measurements,a distributed feedback (DFB) diode laser with a wavelength of 1381 nm is used in the experiment, and five water vapor lines are selected and measured.Firstly,from the linear fitting of line area and the full width at half maximum of collisional broadening (or pressure broadening) we obtain line strengths and self-broadening half-width coefficients at different temperatures.Secondly,from nonlinear fitting of line strengths and self-broadening half-width coefficients at different temperatures we obtain the values of line strengths and self-broadening half-width coefficients at the reference temperature (296 K).In the end,comparison between our experimental results and HITRAN2012 database values shows that the maximum discrepancy between the HITRAN database and the experimental result is 10.96%.A transparent uncertainty analysis is given for the measurement values.Uncertainties of our measured line strengths are in a 1.11%-2.98% range (95% confidence level,k=2),which is smaller than those of HITRAN2012 database values (uncertainties are in a range of 5%-10%).The accurate spectral parameters are obtained experimentally,and of great significance for improving the spectrum measurement accuracy of water vapor in low temperature environment in the future.