RESUMO
Tunable diode laser absorption spectroscopy technology (TDLAS), with its advantages of high selectivity and accuracy, provides a reliable approach to the on-line detection of escaping ammonia. Firstly, the present paper introduces the TDLAS principle, experimental system and the analyses of system noise. Then with the concentration of 90 x 10(-6) and 30 x 10(-6) NH3 for example, we used TDLAS system to collect their second harmonic original spectrum with all kinds of noise interference. To improve the signal spectrum, five types of digital filtering methods were respectively used to filter the original spectrum. Finally we did the NH3 experiments of concentration gradient and the long time monitoring: NH3 experiment of 20 x 10(-6). The analysis indicated that the averaging-wavelet filtering is validated to be more accurate than the other filtering methods in the noise reduction, which can improve the precision of the monitoring system from 10 x 10(-6) to 1.25 x 10(-6) and the SNR also increases by 14 times. It provides an effective pretreatment during the monitoring of escaping ammonia of extremely low concentration.
RESUMO
The theoretical analysis of the direct absorption spectroscopy, the continuous modulation spectroscopy and the quasi-continuous modulation spectroscopy was shown and the corresponding experiments were carried out in order to choose the adequate scheme of the laser modulation spectroscopy to satisfy different requirements of the detection. CO2 gas with different concentrations was detected under the same experimental conditions by using the three different modulation techniques with the same laser. Technical characteristics, signal features and detection limits were compared respectively. Results showed that the detection limit of the quasi-continuous modulation spectroscopy was approaching to that of the continuous modulation spectroscopy. However the linear distortion of the detection signal was obvious, because of the effects of laser energy intermittent and parasitic amplitude modulation on the line shape. Therefore the quasi-continuous modulation spectroscopy is not suitable for the pressure and flow measurements, which closely depend on the line shape. This work has provided reference for selections of the laser modulation spectroscopy.
RESUMO
Numerous harmonic components such as multiple frequency, sum frequency and difference frequency of multiple modulation signals were found in quasi-continuous-wave (QCW) diode laser modulation absorption spectroscopy. Then, the authors analyzed these harmonic components' existence in terms of non-linear interactions of laser and gas absorption line. And the signals' characteristics were studied experimentally. The results shows that there are some sum frequency and difference frequency components that have larger amplitudes compared to the second harmonic wavelength modulation spectroscopy signal (2f-WMS) commonly used in tunable diode laser spectroscopy (TDLAS), and it may improve the detection sensitivity of QCW modulation spectroscopy.
RESUMO
Multiple gas components monitoring in situ by near infrared (NIR) spectroscopy is of great significance in the environmental monitoring. In the present paper, the apectral characteristics of the three types of the volatile organic compounds (VOCs)-propane, propylene and methylbenzene were analyzed, the linear relationship of propylene between concentration and absorbance was considered, and the NIR spectra from 1620 to 1750 nm including the characteristic absorption of the three VOCs were acquired, A linear regression model of chemical metrology was created by partial least-squares method and it predicted the concentration of propane and isobutene in the validation set. The results of the experiment indicated that NIR spectroscopy could easily, accurately and quantitatively determine the content of the multiple gas components, and can be used for monitoring the multiple components of the VOCs in situ.
