痕量气体可调谐激光二极管吸收光谱分析系统在线自校准技术研究

可调谐激光二极管吸收光谱技术(TDLAS)是最有潜力的痕量气体在线监测技术之一。受测量原理的限制，其测量结果受温度和气压影响很大，目前多采用现场安装传感器来测量温度和气压信息，以对该误差进行修正。提出了利用在线参考气室对TDLAS系统中温度、气压变化引起的测量误差进行在线自校准;该参考气室内包含标准浓度的被测气体，并带有能变形的压力膜盒，工作的时候，该气室被放置在被测气体工作环境中，能自适应地调整室内气压和温度;在一次扫描过程中同时测得工作光路和参考光路的吸收光谱，并求得二者的吸收谱线强度比，即可得到校准后的被测痕量气体的浓度，无需考虑温度和气压影响;还介绍了这一自校准系统的原理、设计、实验和现场应用。

Study on Online Self-Calibration Technique for Trace Gas Analyzer Based on Tunable Diode Laser Absorption Spectroscopy

After decades of development, the tunable diode laser absorption spectroscopy (TDLAS) became one of the most promising techniques for online trace gas analyzing in process industry. Limited by its principle, the measurement result of TD-LAS system is seriously affected by temperature and gas pressure variation. For this reason, most TDLAS systems employed temperature and pressure sensors, which can provide information for partly correcting the error. Theoretically, the gas absorp-tion theory itself is not perfect enough to give an analytical relation between the measurement error and the temperature & pres-sure variation. Practically, temperature and pressure sensors are not available in some harsh working condition. To address these problems, an online self-calibration technique with a reference gas cell is proposed to compensate temperature and pressure varia-tion induced measurement error in a TDLAS system. More specifically, a reference gas cell filled with known proportion target gas is placed on site, surrounded by working gas to be measured. The main body of the gas cell is made from a stainless tube, one end is a silica glass window and the other end is a reflector. A pressure bellows is connected to the middle of the stainless tube by a branch conduit. The pressure bellows can adaptively deform to keep the pressure balance between the inside and out-side gas. Thereby, the temperature and pressure inside the reference cell are equal to that of the gas outside. To ensure the simi-larity between the reference gas cell and working gas cell, they share the same laser diode source and signal processing circuit. In one working cycle, the TDLAS system obtains the absorption spectrum of both gas cells synchronously. Then the concentration of the trace gas can be easily obtained by calculating the absorption intensity proportion of both absorption spectra without con-sidering the affection of temperature and pressure. The principle, design, and experiments of this technique were presented in this paper.