| Recently the concept of low carbon and environmental protection makes the environment topic, especially the detection of CO, become hot. Traditional methods for trace gas detection, like chemical analysis and gas chromatographic method, cannot realize real-time online detection and have low detection sensitivity. Based on Lambert-Beer Law, this paper explored the trace CO detection by TDLAS (Tunable Diode Laser Absorption Spectroscopy). Main sections are listed as follows:Key technology of TDLAS testing system was studied and harmonic detection technique was derived mathematically. According to the analysis of the characters of different harmonic waves, second-harmonics was selected. For better modulation effect, the modulation factor was chosen as2.2. Based on the national spectroscopic database (HITRAN2008) and corresponding software (JavaHAWKS), the spectrum for CO and other disturbances was obtained. Considering the laser performance and the strength of the disturbances, the peak density for CO was determined at1567nm by analyzing the spectrum.In accordance with the selected spectrum, TDLAS based trace CO detection system was designed and the block diagram was given, consisting of light source and driver unit, optical path unit and detection and receiving unit. The components controlling the system performance were determined and debugged, including light source, laser driver and collimator, photoelectric detector, lock-in amplifier, data acquisition card, and so on. Based on the Lab VIEW platform and modular programming concept, a reliable and intelligent CO detection system with well favorable man-computer interface was developed.After the TDLAS system built, the system performance was studied and modified experimentally. Tuning characteristic of tunable diode laser on temperature and current was debuged, and then the optimal temperature and current were set as25℃and60mA respectively. The detection system was calibrated at normal temperature and pressure and modified considering the effect of temperature and pressure. The stability and uncertainty of the detection system was analyzed. It was showed that the lower limit of detection was40ppm, the stability variance was2ppm and uncertainty1.75%. The results demonstrated that the proposed system could achieve good online test for trace CO. |
