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在20 ℃室温下,以N2为背景气体,配置50、100、150、200、250、300、350、400、450、500 ppmv的10组CH4标准气。为了测量2f信号的峰峰值和CH4浓度之间的关系,将已配置好的10组不同标准浓度的CH4以5 min时长为间隔,依次通入气室。系统每 5 s记录一次检测数据,每一种浓度的CH4进行60次检测。
如图3所示,实验中得到的2f信号的峰峰值记为amp(2f),数据表明CH4对激光能量的吸收与其浓度呈线性关系,线性拟合得到公式(11)(式中CCH4为被测CH4浓度):
继而推导出公式(12):
根据公式(12)可以将检测到的2f信号的峰峰值换算为相应的CH4浓度,其线性相关度为0.999 8,拟合曲线如图4所示。
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为了测量CH4检测仪的检测下限,配置250 ppmv的CH4通入检测气室,通过双光路差分检测结构消除光源功率波动、电路温漂等影响,锁相放大器探测到的2f信号的峰峰值如图5所示;CH4浓度为0 ppmv时,其2f信号噪声如图6
所示。数据表明,在CH4浓度为250 ppmv时,2f信号的峰峰值(SA)为1.501 V, 0 ppmv的噪声信号波动幅值(NA)为0.06 V,系统的检测下限为:250 ppmv×NA/SA≈10 ppmv。 -
为了计算系统的检测误差,在与标定实验相同的条件下,以5 min时长为间隔,依次对50、100、150、200、250、300、350、400、450、500 ppmv这10种浓度的CH4进行检测,根据拟合公式计算相应的浓度,表1给出了不同浓度的CH4检测数据。数据分析表明,CH4检测仪最大绝对误差为6 ppmv,最大相对误差在±10%范围内,系统的检测精度优于10%。
No. CCH4/ppmv 50 100 150 200 250 300 350 400 450 500 1 52 102 150 201 251 300 351 398 454 496 2 51 103 151 201 253 301 351 400 452 501 3 48 98 152 202 249 300 354 401 450 503 4 50 99 148 204 255 304 352 401 451 504 5 49 97 149 199 254 300 351 400 451 505 6 47 100 144 197 251 301 351 399 453 500 7 50 101 154 199 251 299 352 398 451 499 8 53 101 152 201 251 297 349 398 452 504 9 51 102 151 201 252 301 348 395 448 505 10 55 98 151 200 247 301 351 401 449 504 Maximum absolute error 5 ppmv 3 ppmv 6 ppmv 4 ppmv 5 ppmv 4 ppmv 4 ppmv 5 ppmv 4 ppmv 5 ppmv Maximum relative error 10.00% 3.00% 4.00% 2.00% 2.00% 1.33% 1.14% 1.25% 0.89% 1.00% Table 1. Test results with N2 as background
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为了测定检测系统稳定性,可以通过长时间向气室内输入一定标准浓度的CH4气体,观测检测系统输出值与时间的关系来衡量。将浓度为250 ppmv的CH4通入检测气室内,每1 s记录一次检测仪输出的浓度数值,在恒温条件下进行时长10 h的稳定度观测实验,分析检测仪输出的气体浓度数值与时间的关系,如图7所示。在持续10 h的长时间检测时,2f信号的峰峰值为1.467 ~ 1.535 V范围内,检测仪输出的CH4浓度值均处在245 ~ 256 ppmv之间,波动小于±2.4%,检测数据的平均值为250.5 ppmv。
引入Allan偏差对检测系统的长期特性进行分析[15],由图8所示的Allan偏差结果可知,积分时间为1 s时的Allan偏差为9.9 ppmv,如果积分时间持续增加,则Allan偏差大大降低,积分时间增加到359 s时Allan,偏差可降至0.06 ppmv,表明了系统具有良好的稳定度。
Research of TDLAS methane detection system using VCSEL laser as the light source
doi: 10.3788/IRLA202049.0405002
- Received Date: 2019-12-27
- Rev Recd Date: 2020-01-11
- Publish Date: 2020-04-24
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Key words:
- VCSEL laser /
- low power /
- TDLAS /
- WMS /
- methane concentration detection
Abstract: Compared with the DFB laser, the TDLAS laser gas detecting system using the VCSEL laser as the detecting light source has the advantage of low power consumption. According to the characteristic of the low power TDLAS gas detection signal, combined with the characteristics of modulation VCSEL laser source, a laser driver, signal acquisition and processing were designed, and a low-power TDLAS methane (CH4) gas detector based on WMS was designed. The absorption peak of CH4 molecule near 1653.7 nm was selected as the absorption line, and the second harmonic (2f) signal was extracted by a lock-in amplifier. The response of different concentrations of CH4 detection was studied experimentally, and the 2f signal's amplitude was recorded and linearly fitted. The results show that the CH4 concentration has a good linear relationship with its 2f signal's amplitude, and the linearity is 0.9998. Accuracy of the detection system is better than 10% in the range of 50 - 500 ppmv, and the detection limit is 10 ppmv. With the long-term detection of 250 ppmv of CH4 for 10 h, the fluctuations are less than ± 2.4%. Allan deviation analysis was introduced. When the initial integration time was 1 s, the Allan deviation was 9.9 ppmv. When the integration time reached 359 s, the Allan deviation was 0.06 ppmv, which indicated the good stability of the system.