3.2. Absorption line selection

where σ(ν) is the absorption cross section (in [cm<sup>2</sup>

AI ¼ ð

strength (in [cm<sup>2</sup>

210 Green Electronics

P0 = 760 Torr.

expressed as [40]

easily obtained:

where <sup>I</sup>2<sup>f</sup> I1f � � Ref

respectively.

3.1. Introduction

and <sup>I</sup>2<sup>f</sup> I1f � �

3. Methane (CH4) monitoring

mixing ratio. The integrated absorbance AI (in [cm�<sup>1</sup>

Að Þν dν ¼

N is the number of absorbing molecules (in [molecules/cm<sup>3</sup>

C ppm � � <sup>¼</sup> <sup>N</sup>

fit the 2f signals of the target gas [41], Eq. (5) can be rewritten as

ð

from the integrated absorbance AI measured at temperature T and pressure P [39]:

NT

CMea ¼ a

CMea ¼

lnð Þ I0ð Þν =Ið Þν dν ¼ NL

� <sup>106</sup> <sup>¼</sup> AIP0<sup>T</sup>

where NL = 2.6868 � 1019 mol/cm<sup>3</sup> represents the Loschmidt number at T0 = 273.15 K and

For gas mixing ratio detection, WMS is often adopted. The intensity of 2f signal can be

When the reference signal and nonlinear least square multiplication method are introduced to

where a is fitting coefficient; CMea and CRef are the mixing ratios of the target gas to be measured and reference gas in the calibration cell, respectively; IRef and IMea denote the intensities of the two split laser beams; and LRef and LMea represent the calibration cell and the measurement optical path length, respectively. In general, the ratio of the 2f and 1f signals can be used to cancel any laser intensity differences. In this case, the mixing ratio from the following equation could be

> I2f I1f � � Mea

I2f I 1f 1f � �

With the increasing attention to environment, energy, and safety, natural gas has gradually replaced coal as the main energy source in China, and its use has been increasing year by year.

IMeaCRe<sup>f</sup> LRe<sup>f</sup> IRe<sup>f</sup> LMea

CRe<sup>f</sup> LRe<sup>f</sup>

Mea represent the 2f/1f ratio value of the reference and target gas signals,

Ref LMea

/molecule]) at frequency ν and C is the gas

NLPT0LS � <sup>106</sup> (4)

I2<sup>f</sup> ∝I0σ0CL (5)

σ νð Þdν ¼ NLS (3)

]); S is the molecule absorption line

(6)

(7)

]) can be written as

ð

/(mol cm)]). Based on Eq. (3), the gas species mixing ratio can be retrieved

The near-infrared absorption band matches with the low loss window of optical fiber and is convenient for long-distance transmission and multipoint distributed detection by using fiber and fiber devices. Therefore, the absorption lines of selected CH4, C2H2, and C2H4 are 1653.72, 1531.59, and 1621.36 nm, respectively. There are three adjacent absorption lines at 1653.72 nm for CH4, which are close to each other and cannot be separated in the atmospheric pressure by consulting the HITRAN 2008 database. In the experiment, they are processed as one absorption line. The C2H4 absorption lines are not included in HITRAN database. A large amount of absorption lines of C2H4 from 1600 to 1650 nm can be found from the PNNL25C (Northwest Pacific National Laboratory) database which have been already experimentally verified in the literature [46]. The parameters of three gases absorption lines are shown in Table 1.
