SONG Lin-li

(School of Information and Communication Engineering, North University of China, Taiyuan 030051, China)

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Methane concentration detection system based on differential infrared absorption

SONG Lin-li

(SchoolofInformationandCommunicationEngineering,NorthUniversityofChina,Taiyuan030051,China)

The infrared absorption method for methane concentration detection is an ideal way to detect methane at present. However, it is difficult to spread this method due to its high cost. In this paper, by using a wideband infrared light emitting diode (LED) accompanied with a PIN photo electric diode, a low-cost methane detection system was designed. To overcome the shortcomings caused by the wide working band, a differential light path was designed. By means of a differential ratio algorithm, the stability and the accuracy of the system were guaranteed. Finally, the validity of the system with the proposed algorithm was verified by the experiment results.

methane detection; infrared absorption; differential light path; differential ratio algorithm

0 Introduction

Methane, a kind of combustible gas without color and taste, is one of the major energy gases[1-2]and a principal component of biogas, natural gas and tunnel gas[3-4], etc., being distributed widely over the nature. The application of methane is so widespread that the research about methane detection was implemented early, from the anti-explosion detection in the traditional mine to the leakage detection of natural gas transmission pipeline, and so on.

According to the operating principle, the methods of methane detection are mainly classified into four types: catalytic combustion, semi-conductive gas-sensing response, fiber leak-checking and infrared absorption[5-6]. The catalytic combustion method is widely used in the mines, and its main disadvantage is the need of periodic calibration, meanwhile, it is likely to be poisoned in high-concentration methane, which makes workers unable to work[7-8]. The semi-conductive gas-sensing response system is the most common handheld testing equipment, which is widely used because of its high sensitivity, compact structure and high portability. Nevertheless, this method is susceptible to interference gases, and is strict in environment temperature and moisture[9]. The fiber leak-checking method has the advantages of wide detection area and high precision, but has poor capability to distinguish different gases, which makes it usually applied to detect single gas[10-13]. The infrared absorption method can achieve methane detection by using the spectral absorption characteristic of gas molecule, which has the advantages of rapid testing speed, strong capability of gas identification, continuous operation, and so on. However, the accuracy of this method depends mainly on the spectrum purity of light source, so that the main way to improve the accuracy depends on using expensive laser with narrow band, which greatly limits its wide application.

In view of the facts, a low-cost methane detection system based on infrared absorption method was designed in this paper. By using a wideband light source, the cost of the system is decreased. Some efforts were made to achieve a warrantable accuracy and stability by means of special light path with differential radio algorithm.

1 System architecture

1.1 Basic principle

The basis of the infrared absorption method is the theory of characteristic spectrum absorption. Therefore, methane detection can be achieved based on the characteristic that only the infrared radiation at specific wavelength can be absorbed by the methane molecule[11]. This method can meet the requirements of gas identification and quantitative analysis well due to its good selectivity, even in mixed environment with interference gases. The Beer-Lambert law[12-13]is the theoretical basis of quantitative calculation for methane concentration. According to the law, if the intensity of incident lightIiand the intensity of transmitted lightItcan be measured, the methane concentrationCmcan be represented as

(1)

whereLis the length of absorption path;αλis the absorption coefficient to be calculated.

1.2 Selection of characteristic wavelength

The methane molecule has four natural vibration frequencies, corresponding to four characteristic absorption wavelengths with strongest absorption capacity, including 3.31 μm, 3.43 μm, 6.52 μm and 7.66 μm, all of which are in mid-infrared band. There are several combined absorption bands and multiple frequency absorption bands in near-infrared band, and the absorption capacity in 1.33 μm and 1.65 μm are the strongest among them. The absorption capacity in mid-infrared band is stronger than that in near-infrared band. However, the laser working in mid-infrared band is expensive and needs to be refrigerated when working. Therefore, light source working in near-infrared band is adopted in this paper and the selected center wavelength is 1 650 nm.

1.3 Differential light path

After the characteristic wavelength is selected, the light path that embodies the differential radio algorithm is designed. Fig.1 shows the principle of differential light path.

Fig.1 Differential light path

The light path consists of near-infrared light source, switch window, gas chamber and infrared detector. An infrared light emitting diode (LED) typed as L1650-06 is selected as the light source, and a PIN photoelectric diode typed as LSIPD-1 is selected as the detector to match with the light source. The switch window is the key unit of the light path and consists of two optic windows: one is filled with air named as Z window, and the other is filled with high concentration methane named as M window. The gas chamber is the detection place of methane with two states: one is idle state with no methane, and the other is detecting state with methane filled in. The differential detection for methane can be achieved through the cooperation between the switch window and the gas chamber.

2 Differential ratio algorithm

As the two windows (Z or M) switch to each other, combined with the two states (idle or detecting) switched, four measurement processes are generated, accompanied with four values of detected light intensity.

For the first measurement process, Z window is aligned with the gas chamber being in idle state, and the detected light intensity is represented asIZ0. For the second measurement process, M window is switched to being aligned with the gas chamber which is still in idle state, and the detected light intensity is represented asIM0. The third measurement process, Z window returns to be aligned with the gas chamber, which is filled with methane gas, and the detected light intensity is represented asIZX. For the last measurement process, M window is aligned with the gas chamber again, which is still filled with methane gas, and the detected light intensity is represented asIMX. So far, one time of measurement is finished.

The absorbed light intensity can be acquired exactly through ratio calculation between the four values of the detected light intensity, with no need for exact wavelength (radiated by modulated light source with narrow band), and then the methane concentration can be calculated exactly. The ratio calculation above mentioned is named as differential ratio algorithm, which is implemented as follows:

1) Take a finite amount of methane with known concentration as gas sample, and then implement one time of measurement. The light intensities ofIZ0,IM0,IZXandIMXcan be acquired.

2) If the difference betweenIM0andIMXis distinct, increase the methane concentration in M window untilIM0≈IMX(relative error is less than 1%).

3) CalculateIZ0-IM0, the result represents the total light intensity at methane characteristic absorption wavelength in light source, which can be used to replaceIiin Eq.(1) with more accuracy. Similarly, the result ofIZX-IMXis the residual light intensity at methane characteristic absorption wavelength in light source, which can be used to replaceItin Eq.(1), with more accuracy. Then the absorption coefficientαλcan be calibrated in accordance with Eq.(1) and rewritten as

(2)

whereCmandLare known.

4) Implement a new measurement for the methane with unknown concentration. A new set of light intensities ofIZ0,IM0,IZXandIMXcan be acquired.

5) Calculate the concentration of methane according to Eq.(1), which is rewritten as

(3)

whereαλwas calibrated just now.

It is shown that, with the differential radio algorithm, the concentration of the methane can be measured exactly.

3 Experiment and analysis

Under the condition of normal temperature and pressure, fill the gas chamber with methane gas with certain concentration, and then implement the differential detection for methane. At the same time, the methane gas is measured by gas analyzer typed as TDL-500. Since the TDL-500 has a good detection accuracy of 10-6, the measured data can be used as the truth value of methane concentration. The measured data with the calculated results are shown in Table 1.

Table 1 Experiment results

As can be seen from Table 1, when the methane concentration is lower, the relative error is higher. The possible reason is that the concentration is low and the methane molecule cannot react adequately with the transmitted light at characteristic wavelength, so the absorbed light intensity is too low to suppress the interference causes by noise. As the concentration increases, the error decreases obviously. When the concentration exceeds 5×10-4, the relative error tends to be stable. Table 1 only lists the data at low concentration. When the concentration exceeds 5×10-3, the relative error will increase again. Therefore, the application range of this method can be confirmed as 5×10-4-5×10-3with good stability and accuracy.

4 Conclusion

Under the guidance of spectral absorption theory, a methane detection system based on differential radio algorithm is designed successfully in this paper. By using a wideband infrared LED, accompanied with a PIN photoelectric diode, the cost of the system decreases. A differential light path with differential ratio algorithm is designed seriously, which can guarantee the stability and accuracy of the system. The experiment results indicate that the measurement range is between 5×10-4and 5×10-3, and the relative error is less than 2.0%.

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基于差分紅外吸收的甲烷濃度檢測系統(tǒng)

宋林麗

(中北大學(xué) 信息與通信工程學(xué)院， 山西 太原 030051)

紅外吸收法是目前甲烷濃度檢測的理想手段， 但其成本較高， 難以推廣應(yīng)用。 本文采用寬帶紅外LED光源并配合PIN光電二極管， 設(shè)計(jì)了一種低成本的甲烷檢測系統(tǒng)。 為了克服因采用寬帶光源帶來的不足， 設(shè)計(jì)了一種差分光路， 并采用差分比例算法來保證系統(tǒng)的穩(wěn)定性和精度。 最后， 通過實(shí)驗(yàn)驗(yàn)證了本系統(tǒng)及提出算法的有效性。

甲烷檢測； 紅外吸收； 差分光路； 差分比例算法

SONG Lin-li. Methane concentration detection system based on differential infrared absorption. Journal of Measurement Science and Instrumentation, 2015, 6(2)： 193-196.

10.3969/j.issn.1674-8042.2015.02.015

SONG Lin-li (solili@163.com)

1674-8042(2015)02-0193-04 doi： 10.3969/j.issn.1674-8042.2015.02.015

Received date： 2015-03-25

CLD number： TQ22; TP274+.5 Document code： A