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光声信号的产生和检测是一个光、热、声、电的能量转换过程[22]。光声池中的气体样品受到光子辐射后,在红外区域吸收红外辐射而激发其振动能级至激发态,而后与气体中任一分子相碰撞经过无辐射弛豫过程其能量转移至平动模式,使得分子动能增加而变热。如果给入射光一个强度调制,则封闭在光声腔内的气体将先产生温度调制进而发生频率与光强调制频率相同的周期性胀缩,从而激发出相应的声波。这声波可用传声器直接检测,并将其转化为电信号进行处理[23-24]。
由光声信号产生的这一系列过程可得,光声信号的大小SPA与传声器灵敏度Smic、光声池常数C100、入射光功率Pi、样品气体吸收系数α及其浓度c等参量成正比,可用下式表示:
$ {S_{{\rm{PA}}}} = {S_{{\rm{mic}}}}{C_{100}}{P_{\rm{i}}}\alpha c $
(1) 式中, 光声池常数C100表征光声腔内气体吸收光能转化为声能的能力[22],其值变化对光声信号大小有很大影响,可表示为:
$ {C_{100}} = \frac{{2(\gamma - 1){Q_{100}}{L_{\rm{e}}}}}{{{\pi ^2}{V_{{\rm{cell}}}}{f_{100}}}} $
(2) 式中, γ为比热容比,γ=cp/cV,即气体比定压热容cp和比定容热容cV之比; Vcell为光声池体积; Lc为谐振腔长度。则光声池品质因数Q100和共振频率f100的比值对光声池常数C100的大小起决定作用。
光声池的品质因数是用来描述腔内能量积累和损耗的比例关系[22],可由下式计算得到:
$ {Q_{100}} = \frac{{{R_{\rm{c}}}}}{{{l_{\rm{v}}} + (\gamma - 1){l_{{\rm{th}}}}\left( {1 + 2{R_{\rm{c}}}/{L_{\rm{c}}}} \right)}} $
(3) 式中, Rc为谐振腔半径,lv和lth分别为粘性边界层厚度和热边界层厚度,表示为:
$ \left\{ {\begin{array}{*{20}{l}} {{l_{\rm{v}}} = \sqrt {2\eta /(\rho \omega )} }\\ {{l_{{\rm{th}}}} = \sqrt {2\kappa /\left( {\rho \omega {c_p}} \right)} } \end{array}} \right. $
(4) 式中, η为样品气体的粘滞系数,ω=2πf为调制角频率,κ为样品气体的热导率。可看出,光声池品质因数的大小受池内气体密度影响,且成正比的关系。
光声池的共振频率是描述其谐振腔工作状态的重要参量[22],与谐振腔几何尺寸的关系可表示为:
$ {f_{100}} = \frac{{{v_{\rm{s}}}}}{{2\left[ {{L_{\rm{c}}} + 16{R_{\rm{c}}}/(3\pi )} \right]}} $
(5) 以共振频率调制光源可使谐振腔工作于共振态,此时光声信号有极大值[25]。其中vs为腔内气体的声速[22],表达式为:
$ {v_{\rm{s}}} = \sqrt {\gamma RT/M} $
(6) 式中, R为通用气体常数,T为腔内温度,M为样品气体的摩尔质量,混合样品气体的比热容比γmix和平均摩尔质量Mmix可表示为:
$ {\gamma _{\rm mix }} = \frac{{\sum\limits_{k = 1}^m {{c_{p,k}}} \frac{{{N_k}}}{{{N_{{\rm{all}}}}}}}}{{\sum\limits_{k = 1}^m {{c_{V,k}}} \frac{{{N_k}}}{{{N_{{\rm{all}}}}}}}} $
(7) $ {M_{\rm mix }} = \sum\limits_{k = 1}^m {{M_k}} \frac{{{N_k}}}{{{N_{{\rm{all}}}}}} $
(8) 式中, k为充入光声池中的气体种类数,cp, k和cV, k分别表示第k种气体分子的比定压热容和比定容热容,Mk为第k种气体分子的摩尔质量,Nk/Nall为该气体分子数与气体分子总数之比。根据(5)式、(6)式可得,光声池的共振频率与池内气体的声速成正比,且光声池内声速和共振频率受气体的摩尔质量和腔内温度变化影响。充入缓冲气体后, 光声池内气体分子的碰撞弛豫时间可表示为:
$ \tau = \frac{1}{{N\sigma \sqrt {\frac{{8kT}}{\pi }\left( {\frac{1}{{{m_1}}} + \frac{1}{{{m_2}}}} \right)} }} $
(9) 式中, N为缓冲气体分子数密度,σ为碰撞截面,m1和m2分别为待测气体分子、缓冲气体分子的质量。
综上所述,光声信号与光声池的池常数、品质因数及共振频率等特性参量密切相关,而这些参量又受腔内气体的物理常量影响,所以腔内气体密度、摩尔质量及温度等物理量如果变化,将会直接影响光声信号的大小。
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首先就气体压强对光声光谱检测中光声电信号的影响进行实验分析。实验中采用体积分数为10-4氨气标准气体(背景气为N2)作为待测气体,池内气压大小的变化通过加入体积分数为0.99999的氮气来实现。具体实验步骤如下:首先打开出气口门阀,将机械真空泵与出气口连接进行抽气,直到光声池中气压达到最小值,关闭门阀,停止抽气。然后打开进气口门阀和连接NH3气瓶的进气球阀,调节减压阀大小,使光声池中气压达到0.03MPa后,关闭球阀。接着打开与氮气气瓶连接的进气球阀,充入氮气,同样调节其对应的减压阀大小,实现光声池内气体压强在0.03MPa~0.1MPa范围内变化,每0.01MPa记录对应的光声信号。
调节CO2激光器及偏振衰减器使其输出波长为10.349μm、功率为2W的激光,激光器调制频率为光声池共振频率1026Hz。记录光声信号峰峰值,取10次测量的平均值,则其与气压的关系曲线见图 2中的曲线2。
Figure 2. Effect of pressure on the performance of gas detection based on photoacoustic spectroscopy
图 2中的曲线2显示光声信号随气压的升高而增大,气压达到0.07MPa以上时光声信号增大缓慢,趋于饱和。根据光声基本原理分析,当气压增大时,光声池内气体分子间碰撞更加激烈,无辐射弛豫过程加剧,能量转移速率提高,会有更多的能量转化为热量,光声信号将增大。此外,气压的上升使池内气体密度增大,根据(2)式和(3)式,光声池品质因数和池常数也将增大,进而导致(1)式中光声信号变大。无论从原理分析还是公式推导,均能得到光声信号随气压的升高而增大的关系。光声池的共振频率随气压的变化见图 2中的曲线3,随着气压的升高,光声池共振频率增大,其在0.03MPa~0.1MPa气压范围内的变化量为8Hz,结合表 1中给出的缓冲气体的物理参量及(6)式~(8)式分析,光声池内混合气体分子数增加改变了混合气体的比热容比γmix和平均摩尔质量Mmix,根据表 1计算得出,随着气压的增大,$\sqrt {{\gamma _{{\rm{mix }}}}/{M_{{\rm{mix }}}}} $也将增大,从而使得池内声速增大,因此共振频率也将增大。在实验中根据该曲线对激光调制频率进行修正后测得对应的光声信号与气压的关系, 见曲线1,与频率修正前相比,光声信号增大,这是因为随着气压的增大,光声池的共振频率发生偏移,导致光声信号衰减,而经过频率修正后得到的为对应气压下光声信号的最大值。
Table 1. Physical constants of buffer gases at 20℃ and 1.01×105Pa
gas ρ/(kg·m-3) M/(kg·mol-1) κ/(W·m-1·K-1) cp/(J·kg-1·K-1) cV/(J·kg-1·K-1) γ H2 0.090 0.0020 0.171 14050.0 9935.0 1.414 N2 1.250 0.0280 0.024 1037.9 741.1 1.401 air 1.293 0.0288 0.025 1004.9 718.1 1.400 He 0.180 0.0040 0.143 5200.0 3122.5 1.666 Ne 0.900 0.0200 0.046 1030.0 618.0 1.667 Ar 1.78 0.0400 0.016 520.0 312.0 1.667 NH3 0.771 0.0170 0.019 530.0 400.0 1.325 因此在现场进行气体检测,气压上升时,应该及时调整系统的共振频率,确保光声信号幅值最大。
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在前述实验基础上,改变缓冲气体种类,则可得到不同的缓冲气体对光声信号的影响,为在多种背景气体存在的复杂环境下进行光声光谱气体检测提供了参考依据。
保持与第3.1节中相同的实验条件,改变通入光声池内的缓冲气体种类,向光声池内依次通入缓冲气体空气, Ar, Ne, He, H2,测得不同气压节点处对应的调制频率。实验结果如图 3所示。
由图 3得出,在同一气压下,光声池内共振频率表现为:f100(H2)>f100(He)>f100(Ne) >f100(N2) >f100(air) >f100(Ar),根据(5)式、(6)式,光声池内共振频率与气体的摩尔质量M成反比,因此摩尔质量更小的气体使得光声池内共振频率更高。当通入H2, He, Ne, N2时,共振频率随气压的升高而增大,且越轻的气体频率偏移量越大;通入空气和Ar时,共振频率随气压的升高而减小;其中N2的变化量最小,这是因为NH3的背景气体为N2,通入N2后池内混合气体的平均摩尔质量因气压变化而产生的变化量较小,而通入更轻或更重的缓冲气体会使池内混合气体的平均摩尔质量随气压的升高而减小或增大,从而导致共振频率变化,充入不同缓冲气体后的共振频率的偏移量和缓冲气体的比热容比γ、摩尔质量M有关,结合表 1及(5)式~(8)式,通过计算可得出共振频率的理论值。
根据第3.1节及图 3中曲线得出的结论,对缓冲气体在不同气压下的共振频率进行修正,测得不同缓冲气体对应的光声信号幅值的大小。实验结果如图 4所示。
Figure 4. Effect of various types of buffer gases on the performance of gas detection based on photoacoustic spectroscopy
图 4表明,在不同缓冲气体下,在0.03MPa~0.1MPa范围内改变池内气压,光声信号增大,验证了如前所述的光声信号气压特性。改变缓冲气体种类时,不同缓冲气体对应的光声信号幅值表现为:SPA(air)>SPA(N2)>SPA(Ar)>SPA(Ne)>SPA(He)>SPA(H2)。根据(8)式可得,充入分子质量较大的缓冲气体能够缩短气体的弛豫时间,从而获得更大的光声信号。充入空气时的光声信号最强,这是因为空气中含有H2O,能够明显提高气体分子的碰撞弛豫速率[26]。此外,图 4中充入Ar测得的光声信号值略低于充入N2时的光声信号,这是因为通入Ar时,共振频率随气压的升高而减小,共振频率的减小有利于降低系统的噪声。
缓冲气体对光声光谱法气体检测的影响
Effect of buffer gas on gas detection based on photoacoustic spectroscopy
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摘要: 为了研究气压及缓冲气体种类对光声信号及共振频率的影响,采用光声光谱技术,设计了一套基于光声光谱技术原理的痕量气体检测系统。实验中以NH3标准气作为待测气体,采用向光声池内充入缓冲气体的方法来改变光声池内气压,在气压作为单一变量的条件下得出0.03MPa~0.1MPa气压范围内光声信号及共振频率的变化;采用分别向光声池内充入不同种类缓冲气体的方法,得出不同缓冲气体条件下0.03MPa~0.1MPa气压范围内光声信号及共振频率的变化。结果表明,随着气压的升高,光声信号幅值增大,并且越重的缓冲气体使光声信号增幅越大;气压的升高使得共振频率偏移,共振频率的偏移量与光声池内混合气体分子的摩尔质量成反比。该研究为解决在现场进行气体检测时,气压及背景气体变化的复杂环境对检测结果的影响提供了参考。Abstract: In order to study effect of pressure and buffer gas on photoacoustic signal and resonance frequency, a trace gas detection system was designed based on photoacoustic spectroscopy. Taking NH3 standard gas as an example, filling buffer gas into the photoacoustic cell to change the pressure in the photoacoustic cell, with the pressure as a single variable, the change of photoacoustic signal and resonance frequency was obtained in pressure range from 0.03MPa ~0.1MPa. And then, different kinds of buffer gases were filled into the photoacoustic cell respectively. The change of photoacoustic signal and resonance frequency in pressure range from 0.03MPa ~ 0.1MPa were obtained under different buffer gas conditions. The results show that, with the increase of pressure, the amplitude of photoacoustic signal increases. The heavier the buffer gas, the greater the increase of photoacoustic signal. The increase of pressure makes the resonance frequency shift. The shift of resonance frequency is inversely proportional to molar mass of the mixed gas molecules in the photoacoustic cell. The change of pressure and background gas makes the environment more complex and affects the detection results. This study provides a reference for solving this problem.
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Key words:
- laser technique /
- photoacoustic spectroscopy /
- pressure /
- buffer gas
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Table 1. Physical constants of buffer gases at 20℃ and 1.01×105Pa
gas ρ/(kg·m-3) M/(kg·mol-1) κ/(W·m-1·K-1) cp/(J·kg-1·K-1) cV/(J·kg-1·K-1) γ H2 0.090 0.0020 0.171 14050.0 9935.0 1.414 N2 1.250 0.0280 0.024 1037.9 741.1 1.401 air 1.293 0.0288 0.025 1004.9 718.1 1.400 He 0.180 0.0040 0.143 5200.0 3122.5 1.666 Ne 0.900 0.0200 0.046 1030.0 618.0 1.667 Ar 1.78 0.0400 0.016 520.0 312.0 1.667 NH3 0.771 0.0170 0.019 530.0 400.0 1.325 -
[1] ZHANG J W, DI X T, FAN H, et al. Ammonia trace gas detection based on QEPAS[J]. Electric Machines and Control, 2016, 20(8): 112-118(in Chinese). [2] LI L. Real-time detection of breath ammonia in vivo based on photo-acoustic spectroscopy[D]. Dalian: Dalian University of Technology, 2010: 9-19(in Chinese). [3] KOU X W, ZHOU B, LIU X Ch, et al. Measurement of trace NH3 concentration in atmosphere by cavity ring-down spectroscopy[J]. Acta Optica Sinica, 2018, 38(11): 1130001(in Chinese). [4] WANG H, ZHOU B, REN H W, et al. Research on the detection of escaping ammonia with the interference of high concentration on water vapor based on wavelength modulation on spectroscopy[J]. Journal of Engineering Thermophysics, 2018, 39(4): 911-921(in Chinese). [5] ZHAO Y. Research on ammonia detection based on tunable diode laser absorption spectroscopy[D]. Tianjin: Tianjin University, 2014: 9-12(in Chinese). [6] WU H, DONG L, LIU X, et al. Fiber-amplifier-enhanced QEPAS sensor for simultaneous trace gas detection of NH3 and H2S[J]. Sensors, 2015, 15(10): 26743-26755. doi: 10.3390/s151026743 [7] MA Y, HE Y, TONG Y, et al. Ppb-level detection of ammonia based on QEPAS using a power amplified laser and a low resonance frequency quartz tuning fork[J]. Optics Express, 2017, 25(23): 29356-29364. doi: 10.1364/OE.25.029356 [8] CHEN W G, WAN F, ZHOU Q, et al. Pressure characteristics of dissolved acetylene in transformer oil based on photoacoustic spectroscopy detection[J]. Transactions of China Electrotechnical Society, 2015, 30(1): 112-119(in Chinese). [9] ZHOU Q, PENG S, WANG Q, et al. Study of carbon monoxide detection characteristics with a tunable photoacoustic spectroscopy system[C]//2016 IEEE International Conference on High Voltage Engineering and Application (ICHVE).New York, USA: IEEE, 2016: 1-4. [10] ZHOU Q, LI J, CHEN W, et al. Pressure, temperature and saturation properties of carbon monoxide photoacoustic spectroscopy signal[J]. Science of Advanced Materials, 2018, 10(8): 1154-1162. doi: 10.1166/sam.2018.3323 [11] CHEN Y K, JU Y, HAN L. Study on pressure characteristics of photoacoustic spectroscopy and TDLAS[J]. Spectroscopy and Spectral Analysis, 2017, 37(1): 27-31(in Chinese). [12] ZHAO N. Research of coal mine gas detection system based on photoacoustic spectroscopy[D]. Qinhuangdao: Yanshan University, 2014: 76-85(in Chinese). [13] ZHENG H Q. Detection of trace gas concentration based on photoacoustic spectroscopy[D]. Harbin: Harbin University of Science and Technology, 2017: 38-53(in Chinese). [14] ZHA Sh L. Transformer fault gas detection based on the broadband photoacoustic spectroscopy[D]. Hefei: University of Science and Technology of China, 2017: 97-101(in Chinese). [15] PATIMISCO P, BORRI S, GALLI I, et al. High finesse optical ca-vity coupled with a quartz-enhanced photoacoustic spectroscopic sensor[J]. Analyst, 2015, 140(3): 736-743. doi: 10.1039/C4AN01158A [16] TITTEL F K, SAMPAOLO A, PATIMISCO P, et al. Analysis of overtone flexural modes operation in quartz-enhanced photoacoustic spectroscopy[J]. Optics Express, 2016, 24(6): A682-A692. doi: 10.1364/OE.24.00A682 [17] Ⅲ L J T, KELLY M J, AMER N M. The role of buffer gases in optoacoustic spectroscopy[J]. Applied Physics Letters, 2008, 32(11):736-738. [18] WAKE D R, AMER N M. The dependence of an acoustically nonresonant optoacoustic signal on pressure and buffer gases[J]. A-pplied Physics Letters, 1979, 34(6): 379-381. doi: 10.1063/1.90796 [19] CHEN W G, LIU B J, HU J X, et al. Influential factor analysis on photoacoustic signal of photoacoustic spectroscopy monitoring trace gases[J]. Journal of Chongqing University, 2011, 34(2): 7-13(in Chinese). [20] MOHEBBIFAR M R, KHALILZADEH J, DIBAEE B, et al. Effect of buffer gases on the performance of SO2, trace measurement based on photoacoustic spectroscopy[J]. Infrared Physics & Technology, 2014, 65(5): 61-66. [21] DIBAEE B, PARVIN P, BAVALI A, et al. Effect of colliding partners on the performance of SF6 and SO2 trace measurements in photoacoustic spectroscopy[J]. Applied Optics, 2015, 54(30): 8971-8981. doi: 10.1364/AO.54.008971 [22] YIN Q R. Photoacoustic photothermal technology and application[M]. Beijing: Science Press, 1991:146-157(in Chinese). [23] MIKLOS A, HESS P, BOZOKI Z. Application of acoustic resonators in photoacoustic trace gas analysis and metrology[J]. Review of Scientific Instruments, 2001, 72(4): 1937-1955. doi: 10.1063/1.1353198 [24] AOUST G, LEVY R, RAYBAUT M, et al. Theoretical analysis of a resonant quartz-enhanced photoacoustic spectroscopy sensor[J]. A-pplied Physics, 2017, B123(2): 1-11. [25] CHEN Y, GAO G Zh, CAI T D. Detection technique of ethylene based on photoacoustic spectroscopy[J]. Chinese Journal of Lasers, 2017, 44(5): 0511001(in Chinese). doi: 10.3788/CJL201744.0511001 [26] MA Y F, HE Y, YU X, et al. Research on high sensitivity detection of carbon monoxide based on quantum cascade laser and quartz-enhanced photoacoustic spectroscopy[J]. Acta Physica Sinica, 2016, 65(6): 60701(in Chinese).