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通过对MPGC工作温度进行控制,验证了研究的温度控制系统的性能,实验结果如图4所示。
从图4可以看出,控制时间达到15 s时,MPGC的工作温度达到稳定状态,没有超调。在稳定状态下,温度波动小于±0.08 ℃。在达到设定的工作温度阈值之前,采用PD算法。然后,在稳定状态下使用PID算法。由于采用了上述积分分离PID控制算法,MPGC的工作温度上升较快,无超调现象。从而避免了加热超调恢复缓慢的问题。
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图5为5种不同浓度(20、30、40、50、60 ppmV)的二次谐波信号(1 ppm=10−6)。在检测系统响应实验中,通过减去非吸收性的背景信号,可以得到二次谐波信号。然后分别求出二次谐波信号的峰值与气体浓度之间的关系。
图5示出,采用气体稀释系统,配置5种不同浓度的CO2气体,然后分别由本系统检测,得到不同浓度的13CO2和12CO2对应的二次谐波信号的峰值,进而计算CO2同位素。
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为了准确测量CO2同位素,有必要利用已知的气体浓度校准检测系统。将上述配置的5种不同浓度的CO2注入MPGC,测量时间为5 min。13CO2和12CO2的二次谐波信号峰值如图6所示(1ppb=10−9)。
图6分别测量了13CO2和12CO2的二次谐波信号的峰值。利用获得的实验数据,通过线性拟合得到以下公式:
$$^{{\rm{12}}}C = 11.198 \times \max 2{\rm f}{(^{12}}C) - 49.429$$ (1) $$^{13}C = 0.388 \times \max 2{\rm f}{(^{13}}C) - 0.563$$ (2) 式中:max2f(12C)和max2f(13C)分别为12CO2和13CO2二次谐波信号的峰值。根据上述两个公式,推导出二次谐波信号的峰值,可以计算出12CO2和13CO2的浓度,响应线性可达0.999 6,从而可以准确得到碳同位素。
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在测量13CO2/12CO2同位素比值时,测量数据随时间漂移。为了测试检测系统的测量精度和长期稳定性,将一定浓度的CO2泵入MPGC。根据检测数据计算艾伦方差结果如图7所示。
实验结果表明,在积分时间为1 s的情况下,CO2同位素检测精度达到0.610‰。此外,当积分时间增加到92 s时,相应的检测精度可以显著降低至0.013 9‰。由于92 s之前白噪声是主要成分,因此检测精度随着积分时间的增加而减小。92 s之后,漂移成为主导噪音,检测精度开始上升。图中绿色虚线描述了白噪声主导下的系统响应。
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实验中采用两个流量控制器分别在2.5 SCCM和177.5 SCCM控制CO2气体和N2的流量,从而形成混合气体泵入MPGC。经2 h检测后,实测数据如图8所示。
在2 h的测试实验中,CO2同位素值在−8.351‰~9.736‰之间,平均值为−9.081‰,最大波动值为0.73‰。在长期实验中,气瓶CO2气体与纯N2载气之间存在流速波动,导致了碳同位素比值的波动。
Development on high precision CO2 isotope measurement system based on infrared TDLAS technology
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摘要: 对天然气分布监测,高精度地检测CO2同位素是非常重要的。采用可调谐二极管激光吸收光谱(TDLAS)技术,通过13CO2/12CO2在4.3 μm处的吸收谱线,实现高精度CO2同位素检测。该检测系统由工作在连续波模式下的中红外间带级联激光器(ICL)、长光程多通池(MPGC)和中红外碲镉汞(MCT)探测器组成。针对13CO2和12CO2两条吸收谱线强度受温度影响的问题,研制了MPGC高精度温度控制系统。实验中,配置5种不同浓度的CO2气体对检测系统进行标定,响应线性度可达0.999 6。结果表明,当积分时间为92 s时,同位素检测精度低至0.013 9‰,具备实际应用价值。Abstract: For natural gas distribution monitoring, it is very important to measure the CO2 isotope with high precision. In this paper, the tunable diode laser absorption spectrum (TDLAS) technology was adopted to realize the high precision CO2 isotope measurement through the absorption spectrum line of 13CO2/12CO2 at 4.3 μm. The measurement system consisted of a mid-infrared interband cascade laser (ICL) operating in a continuous wave mode, a long-path multipass cell (MPGC) and a mid-infrared mercury cadmium telluride (MCT) detector. Aiming at the problem that the intensity of 13CO2 and 12CO2 absorption spectra was affected by the temperature, an MPGC high precision temperature control system was developed. In the experiment, five CO2 gases of different concentrations were configured to calibrate the measurement system, and the response linearity was up to 0.999 6. The results show that when the integral time is 92 s, the isotope measurement precision is as low as 0.013 9‰, which has practical application value.
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