4. Carbon monoxide (CO) monitoring

#### 4.1. Introduction

3.4. Results and discussion

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in the range of 100 � 100 m.

Figure 5. Concentration curves of experiment results.

The system had been field-tested at the testing ground of China Petroleum Pipeline Bureau. The environment temperature was 35�C, air relative humidity was 45%, and wind speed was 1 m/s during the experiment. We used the gas which was mixed with 90% methane, 5% ethylene, and 5% acetylene to simulate gas pipeline leakage in the experiment. The leakage position was about 2 m below the side of the laser beam. In order to measure the three gases simultaneously, lasers were switched every 10 seconds using an optical switch. Three kinds of gases were detected circularly in the order of CH4 ! C2H2 ! C2H4. The mixing ratios of the gases are exhausted 1 minute each time which was displayed in Figure 5. The reason for fluctuations is that the measured concentrations are the average of the paths along the line of sight. Due to the uncertainty of wind speed and gas diffusion in the measurement field, the concentration on the beam path fluctuates greatly. Meanwhile, this system is also equipped with an alarm limit for each gas, and the veracity of fire alarming system achieved 100%.

The system includes three DFB lasers which have an output power of about 20 mW higher than the other semiconductor lasers. Moreover, the optical fiber loss is less than 0.25 dB/km in this waveband. So the system can connect four pairs of transmitter and receiver units simultaneously. According to the requirements and distribution of gas pipeline, gas gathering device, housing, and other special places in the gas gathering station, the installation scheme including a host control machine and two pairs of transmitter and receiver units was designed and displayed in Figure 6. This system can be used to monitor the leakage of natural gas station

The leakage detection system based on TDLAS can detect methane, ethylene, and acetylene rapidly and effectively in the open environment, and the response time of the three gases is less than 2 s. The accuracy of giving an alarm is 100%, which can be used in natural gas station and valve room gas leakage. Compared to other techniques, this technique has the advantages of safety in nature, no calibration, high accuracy, and little environmental effects. The MDLs for methane, acetylene, and ethylene gas are 100, 40, and 50 ppm-m, respectively, which meet the

requirements for the detection of natural gas leakage in the petrochemical industry.

CO is a kind of toxic, combustible, explosive gas and brings lots of hidden danger to the production and life of human beings. The research of coal spontaneous combustion suggests that a series of gases which could indicate the degree of oxidation and combustion of coal will be produced when coal seam is on fire. Using the relationship between the amount of indicator gases and the rate of change could predict coal seam fire at an early stage. Nowadays, CO is widely used as the main indicator gas for early warning of coal seam fire because the quantity of CO is closely related to the temperature of coal seam and the concentration change is obvious. In addition, the safety production under the mine has attracted much attention. In order to avoid accidents, gas monitoring has become a necessary means. The detection devices of the main gas constituents such as methane and carbon dioxide have been improved and widely used. With the improvement of security awareness, people have higher requirements on the accuracy of gas monitoring [48].

#### 4.2. Absorption line selection

The absorption intensity of CO in the mid-infrared region is two orders of magnitude higher than that of overtone band in the near infrared. With the development of mid-infrared lasers, high sensitivity detection of CO has been obtained by some researchers [49]. But for the longdistance optical fiber transmission signals, the use of the mid-infrared laser is limited because the current optical fiber communication windows are mainly concentrated in the near infrared. The intensity of CO absorption line in the near infrared is weak, and the SNR is poor when low concentration is detected, which requires higher stability of the measurement system. At present, the stability research of high sensitivity detection of CO in the near infrared has not been reported. But there are some reports about measurement techniques such as the stability of DFB lasers [50], the application of signal processing in CO2 and NO2, and other gas measurements [51]. Therefore, it is of great practical significance to study the stability of whole measurement system and realize the high sensitivity detection of CO in the communication windows.

To select a unique gas absorption line usually adopts the following guide rules: (1) strong absorption line strength with good line profile and (2) free of interference from other gases. The second overtone band near 1.566 μm of CO was selected in this work to avoid interferences from other major ambient gases in the mixture. Figure 7 shows the absorption spectrum of CO, CO2, and H2O near the wavelength range of 1.566 μm [52].

#### 4.3. Experimental system design

The experimental system is shown in Figure 8. The system adopts balanced optical path detection method. The 2 \* 1 beam combiner couples the collimated light and the measuring beam to the 1\*3 beam splitter, after that the first beam through a multi-pass absorption cell filled with CO gas, marked as S (measuring light path); the second beam through a high

concentration reference cell with 100% CO, used to determine and control the position of absorption wavelength, marked as H; and the third beam is a reference light through the free space, used to monitor the changes of the laser background, marked as R. The three detection signals are sequential controlled by the switching circuit simultaneously. The wavelength is scanned with 100 Hz sawtooth wave and modulated with 10 kHz sine wave. More details about the electronics setup for the experiment could be found in [53]. Three modulation signals enter the lock-in amplifier through the switching circuit. In the lock-in amplifier, the detector output is mixed with the reference signal (10 kHz) to demodulate the 2f spectral signal. Then the 2f signal is simultaneously processed by a data acquisition card installed on a computer. A new type of multi-pass absorption cell was developed and effectively improved the detection ability of the system. The new absorption cell has the advantages of simple structure, stable performance, effective use of the surface area, and solving the contradiction between the small volume and long-path length. The optical path length of 56.7 m was achieved in the volume of 1 L. At the same time, the optical path is adjustable; the spot array is uniform and in order, so that the optical path calculation is convenient; and the free spectral range is very narrow. The possible interference fringes in the cell are distributed in the high frequency region. By means of the digital averaging method, the influence of interference fringes on the second harmonic signals can be removed effectively and simply. The base length of the multipass absorption cell used in the system is 24.6 cm; the diameter of the mirror is 60 mm. According to the needs of TDLAS system for CO gas measurement, the mirror is coated with a dielectric film with a high reflectivity (typically 0.999) for wavelengths 532 and 1567 nm, wherein 532 nm is the collimated light during the alignment of the optical path. Figure 9

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Figure 8. Schematic diagram of the TDLAS experimental system.

shows the spot distribution of the mirrors at both ends of the absorption cell.

Figure 7. The absorption lines of CO, CO2, and H2O near the wavelength range of 1.566 μm (HITRAN 2008 database).

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Figure 8. Schematic diagram of the TDLAS experimental system.

distance optical fiber transmission signals, the use of the mid-infrared laser is limited because the current optical fiber communication windows are mainly concentrated in the near infrared. The intensity of CO absorption line in the near infrared is weak, and the SNR is poor when low concentration is detected, which requires higher stability of the measurement system. At present, the stability research of high sensitivity detection of CO in the near infrared has not been reported. But there are some reports about measurement techniques such as the stability of DFB lasers [50], the application of signal processing in CO2 and NO2, and other gas measurements [51]. Therefore, it is of great practical significance to study the stability of whole measurement system and realize the high sensitivity detection of CO in the communication

To select a unique gas absorption line usually adopts the following guide rules: (1) strong absorption line strength with good line profile and (2) free of interference from other gases. The second overtone band near 1.566 μm of CO was selected in this work to avoid interferences from other major ambient gases in the mixture. Figure 7 shows the absorption spectrum

The experimental system is shown in Figure 8. The system adopts balanced optical path detection method. The 2 \* 1 beam combiner couples the collimated light and the measuring beam to the 1\*3 beam splitter, after that the first beam through a multi-pass absorption cell filled with CO gas, marked as S (measuring light path); the second beam through a high

Figure 7. The absorption lines of CO, CO2, and H2O near the wavelength range of 1.566 μm (HITRAN 2008 database).

of CO, CO2, and H2O near the wavelength range of 1.566 μm [52].

windows.

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4.3. Experimental system design

concentration reference cell with 100% CO, used to determine and control the position of absorption wavelength, marked as H; and the third beam is a reference light through the free space, used to monitor the changes of the laser background, marked as R. The three detection signals are sequential controlled by the switching circuit simultaneously. The wavelength is scanned with 100 Hz sawtooth wave and modulated with 10 kHz sine wave. More details about the electronics setup for the experiment could be found in [53]. Three modulation signals enter the lock-in amplifier through the switching circuit. In the lock-in amplifier, the detector output is mixed with the reference signal (10 kHz) to demodulate the 2f spectral signal. Then the 2f signal is simultaneously processed by a data acquisition card installed on a computer.

A new type of multi-pass absorption cell was developed and effectively improved the detection ability of the system. The new absorption cell has the advantages of simple structure, stable performance, effective use of the surface area, and solving the contradiction between the small volume and long-path length. The optical path length of 56.7 m was achieved in the volume of 1 L. At the same time, the optical path is adjustable; the spot array is uniform and in order, so that the optical path calculation is convenient; and the free spectral range is very narrow. The possible interference fringes in the cell are distributed in the high frequency region. By means of the digital averaging method, the influence of interference fringes on the second harmonic signals can be removed effectively and simply. The base length of the multipass absorption cell used in the system is 24.6 cm; the diameter of the mirror is 60 mm. According to the needs of TDLAS system for CO gas measurement, the mirror is coated with a dielectric film with a high reflectivity (typically 0.999) for wavelengths 532 and 1567 nm, wherein 532 nm is the collimated light during the alignment of the optical path. Figure 9 shows the spot distribution of the mirrors at both ends of the absorption cell.

Figure 9. Light spot distribution of the mirrors at the both ends of the absorption cell.

#### 4.4. Results and discussion

The CO standard gases of 10 and 200 ppm were measured in the laboratory by using the above described TDLAS system. The stability and detection limit of the system were analyzed. The linearity of the system was tested by measuring the CO standard gas at different concentrations. Figures 10 and 11 display the measurement results of 10 and 200 ppm CO standard gases, respectively. After continuous measurements of 14 h, the average concentrations are 10.57 and 200.36 ppm, and the standard variance is 0.5 and 2.1 ppm, which can be found in Table 2. The standard variance reflects the stability of the system to a certain extent.

The measured concentration fluctuations of 10 and 200 ppm CO standard gas are 4.7 and 1% of the mean value, respectively. This illustrates that different concentration ranges should be divided when measuring low concentration gas with high sensitivity, such as 0–20, 20–50,

CO standard gas (ppm) Mean value (ppm) Standard deviation (ppm) Fluctuation (%)

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10 10.57 0.5 4.7 200 200.36 2.1 1

The results of Allan variance analysis of 10 ppm CO sample gas are shown in Figure 12. The corresponding integration time of the system is 30 s, the Allan variance is 0.067, and the predicted detection limit is 0.25 ppm. Moreover, if we continue to increase the integration time until the intersection with the slope of 1/2, the Allan variance decreases to 0.02, the corresponding detection limit is 0.14 ppm, but the long integration time will affect the sensitivity of the system [54]. Therefore, the integration time should be properly controlled when the requirement of detection limit is not very high. The measurement results of CO gas at different concentrations are shown in Figure 13, and the linear relationship between different concentrations and the peak values of second harmonic signal is displayed in Figure 14. The results illuminate that the measurement concentrations have a good linearity in the range of

50–100 ppm, and so on, and different ranges have different stability indexes.

Figure 11. The measurement results of 200 ppm CO standard gas.

Table 2. Measured deviation of CO standard gas.

10–250 ppm.

Figure 10. The measurement results of 10 ppm CO standard gas.

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Figure 11. The measurement results of 200 ppm CO standard gas.


Table 2. Measured deviation of CO standard gas.

4.4. Results and discussion

218 Green Electronics

The CO standard gases of 10 and 200 ppm were measured in the laboratory by using the above described TDLAS system. The stability and detection limit of the system were analyzed. The linearity of the system was tested by measuring the CO standard gas at different concentrations. Figures 10 and 11 display the measurement results of 10 and 200 ppm CO standard gases, respectively. After continuous measurements of 14 h, the average concentrations are 10.57 and 200.36 ppm, and the standard variance is 0.5 and 2.1 ppm, which can be found in Table 2. The standard variance reflects the stability of the system to a certain extent.

Figure 9. Light spot distribution of the mirrors at the both ends of the absorption cell.

Figure 10. The measurement results of 10 ppm CO standard gas.

The measured concentration fluctuations of 10 and 200 ppm CO standard gas are 4.7 and 1% of the mean value, respectively. This illustrates that different concentration ranges should be divided when measuring low concentration gas with high sensitivity, such as 0–20, 20–50, 50–100 ppm, and so on, and different ranges have different stability indexes.

The results of Allan variance analysis of 10 ppm CO sample gas are shown in Figure 12. The corresponding integration time of the system is 30 s, the Allan variance is 0.067, and the predicted detection limit is 0.25 ppm. Moreover, if we continue to increase the integration time until the intersection with the slope of 1/2, the Allan variance decreases to 0.02, the corresponding detection limit is 0.14 ppm, but the long integration time will affect the sensitivity of the system [54]. Therefore, the integration time should be properly controlled when the requirement of detection limit is not very high. The measurement results of CO gas at different concentrations are shown in Figure 13, and the linear relationship between different concentrations and the peak values of second harmonic signal is displayed in Figure 14. The results illuminate that the measurement concentrations have a good linearity in the range of 10–250 ppm.

Figure 12. The Allan variance of 10 ppm CO.

those situations which have a higher measurement requirement of CO such as alarming of coal spontaneous combustion and mine safety production. But it is only the results of experimental measurement under the laboratory conditions. For the mine environments with high temperature and humidity, the performances of the experimental relevant components need to be

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Hydrogen sulfide (H2S) is an important potential dangerous gas in oil drilling. It is colorless, highly toxic, and acidic; there is a special smell of rotten eggs; the olfactory threshold is 0.00041 ppm. Even low concentrations of H2S can also damage people's sense of smell and have effects on the eye, respiratory system, and central nervous system. It is lethal to detect this kind of gas using a nose [55]. Because there is no smell when the concentration is high (high concentrations of hydrogen sulfide can paralyze olfactory nerves). Hence, sensitive H2S detection is necessary in practical applications. In this part, a 1.578 μm distributed feedback (DFB)

The WMS technique is used in the H2S detection system, as shown in Figure 15. The used multipass absorption cell in this experiment is also homemade with a total optical path length of 56 m and a total volume of 0.8 L. A single-mode pigtailed DFB laser with a central wavelength of

further tested.

5.1. Introduction

5. Hydrogen sulfide (H2S) monitoring

Figure 14. The linear relationship between different concentrations.

laser is used to detect H2S of low concentration [56].

5.2. Wavelength modulation spectroscopy system

Figure 13. The 2f signals with different concentrations of CO.

The system of CO high sensitivity detection based on TDLAS technology combined with the new type of multi-pass absorption cell basically realizes the high sensitivity detection of CO in the near infrared. The system exhibits good stability and high linearity after long-term measurement experiments. According to the Allan variance analysis, the detection limit of the system is 0.25 ppm with an integration time of 30 s. The system meets the requirements for Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy http://dx.doi.org/10.5772/intechopen.72948 221

Figure 14. The linear relationship between different concentrations.

those situations which have a higher measurement requirement of CO such as alarming of coal spontaneous combustion and mine safety production. But it is only the results of experimental measurement under the laboratory conditions. For the mine environments with high temperature and humidity, the performances of the experimental relevant components need to be further tested.
