Research on Multi-point Measurement System of Mine Gas Based on TDLA

Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology combines the advantages of wavelength modulation technology and direct absorption spectroscopy technology, and the system achieves highly sensitive and full-range detection of methane gas concentrations. In the system, only the pure optical methane probe is positioned under the mine, which establishes connectivity with the host computer on the surface through the optical fiber. This setup enables the simultaneous detection of 16 channels of methane gas through time-sharing multiplexing technology. Experimental results demonstrate a measurement error of less than ±6.0×10−4 at low concentrations (0∼1%) and within ±6% of the true value at high concentrations under normal temperature and pressure conditions. The full-range linear fit attains 0.9998. According to the one-fold standard deviation, the minimum detection lower limit of the system stands at 68.36 ppm and the measured gas concentrations across different channels of the methane probes remain consistent inhe same environment. This technology provides a new solution for continuous, accurate, and multi-point monitoring of mine gas.


Introduction
China, with its complex coal seam endowment and mining complexities, reports the most serious coal mine accidents in the world.The million-ton mortality rate in China's coal mines is 5-10 times higher than in developed countries such as the United States, Australia, and South Africa.Among these accidents, gas disasters are the most prominent in China's coal mine accidents [1].The occasional mine gas accidents have placed substantial pressure on coal mine managers, especially in lives and property safety, causing serious threats.Therefore, to avoid mine gas accidents, it becomes crucial to study the monitoring technology of mine gas concentration.
Methane gas is the major component of mine gas and typically ranges from 50% to 90% in content.It is lighter than air, colorless, odorless, and combustible.When oxygen and methane are mixed in the air and the methane concentration reaches about 5%-15%, it becomes explosive upon contact with fire.Traditional methane detection methods, including catalytic combustion [2], gas chromatography [3], optical interferometry [4], spectroscopy [5], etc, cannot meet the requirements of full-range, real-time on-line monitoring.Moreover, the detection limit needs further enhancement.In recent years, with the rapid development of narrow linewidth semiconductor laser technology, the tunable diode laser absorption spectroscopy (TDLAS) technology has gradually come into light, which has the advantages of high sensitivity, real-time, dynamic, and multicomponent simultaneous measurements, and finds extensive applications in natural disaster prevention and environmental monitoring.
This paper proposes a distributed fiber optic methane monitoring system based on the in-depth study of TDLAS principles and wavelength modulation technology.Combining the advantages of direct absorption spectroscopy and wavelength modulation technology, it is well-suited for the harsh environment downhole, and at the same time, it employs time division multiplexing technology for multi-point simultaneous measurement, which greatly reduces system costs.

TDLAS technology principle
The Beer-Lambert law forms the basic theory of TDLAS technology.According to this law, when a beam of monochromatic light is transmitted through a certain thickness of the absorbing medium, the intensity of the transmitted light is weakened due to the absorption of a part of the light energy by the medium.This attenuation becomes more apparent with the higher concentration of the absorbing medium and greater thickness of the medium.The change in laser intensity can be expressed by Equation (1) [6][7]: In the equation, v k is the absorption coefficient at optical frequency; 0 I is the incident light intensity; t I is the emitted light intensity; P is the gas pressure, atm; ( ) S T is the spectral line intensity, cm-2atm- 1; ( ) v  is the lineshape function, cm; rad X is the percentage of the target gas; L is the optical path length of the absorbing pool, cm.The lineshape function is normalized over the frequency range and satisfies Equation (2): Combining Equations ( 1) and (2) and integrating over the entire frequency domain, it can be described by Equation (3): Wavelength modulation is achieved by superimposing a high-frequency sinusoidal signal on the lowfrequency scanning signal of the laser.The instantaneous frequency of the laser can be calculated as the following Equation (4) [8,9]: where c v is the laser frequency corresponding to the low-frequency scanning signal and a is the modulation amplitude of the modulating signal.We let 2 ft    , and the expansion in the form of Fourier set numbers can be expressed as: where ( ) H v is each harmonic component, whose magnitude can be measured by the lock-in amplifier.These harmonic components maintain a proportional relationship with the gas concentration.Theoretically, it is possible to conduct gas concentration inversion by using each harmonic signal pair [10].Considering the amplitude and characteristics of these harmonic signals, the second harmonic signal concentration inversion is commonly used.Wavelength modulation absorption spectroscopy detection technology effectively mitigates the low-frequency noise in the circuit and optical path and improves the measurement sensitivity.However, its applicability is confined to occasions with a low absorption rate.When the gas absorption rate is more than 0.05, there will be obvious nonlinearities, which will cause notable measurement errors.

Systematic design
The system is mainly composed of a monitoring host and a methane detector module, connected via optical fiber.The monitoring host is comprised of a laser, a fiber coupler for signal acquisition and processing module, an analog switch for photoelectric detection module, a current module, a temperature module, and a local communication display module.The methane detector is a light absorption pool probe composed of purely optical components.A monitoring host can be connected with as many as 16 methane detectors.The schematic diagram of the system principle is illustrated in Figure 1.

Sensing probe design
The multi-point gas monitoring system has the capability to interface with a maximum of 16 sensing probes.These probes are positioned at different detecting points in the mine and connected to the host computer on the surface via optical fiber.The sensing probes are mainly composed of armored optical fibers, lenses, mirrors, and related components.They have external dimensions of about 35 mm in diameter and 80 mm in length.The principle and physical diagram of the sensing probe is shown in Figure 2. Physical drawing.As shown in Figure 2(a), the laser beam is transmitted from the armored optical fiber to the methane sensing probe.Following collimation by the lens and reflection by the mirrors, the beam carrying information about methane gas concentration is conveyed from the optical fiber to the photodetector.The optical absorption path of the system is increased by the use of three mirrors, approximately 15 cm in length.All components are sealed in a stainless steel housing to ensure the probe's resilience against vibrations and shocks.The base of the sensing probe is a waterproof and breathable metallurgical powder filter, along with a steel mesh connected to the outside.The setup facilitates effective gas exchange with the outside world while preventing dust and water from infiltrating the gas chamber and safeguarding the lens against contamination.This ensures the accuracy of measurements, prolongs the probe's operational lifespan, and improves the robustness for applications in harsh environments.

Systematic full-scale concentration testing
Tests are conducted by using standard concentrations of methane gas (0%, 0.5%, 1.5%, 2.0%, 8.5%, 20%, 35% and 70%) at ambient temperature and pressure.Each set of experimental gases is vented for 3 minutes and stabilized for five tests, with the average value taken.Figure 3 shows the test results conducted with varying concentrations of standard methane gas.It can be inferred that in the concentration range of 0~1%, the maximum error occurs at 0.5%, with a relative error of 4.07%.However, the absolute error is only 2×10 -4 , meeting the measurement requirements of gas concentrations in the range of 0~1%, where the acceptable error is less than ±6.0×10 -4 ; in the concentration range of 1%~100%, all measurement errors remain within ±6% of the true value.Through fitting the measurement results with one-dimensional linear regression, a goodness of fit value of 0.9998 is obtained, indicating that the measurement system exhibits excellent linearity in the full-scale range and that the accuracy of the measurement complies with existing measurement standards.

Limit of measurement
Although the wavelength modulation technique in the original low-frequency scanning signal atop the high-frequency sinusoidal wave can effectively suppress the impact of low-frequency noise on the measurement, complete elimination of system background noise remains unattainable due to the overlapping of the optical path of the background noise, mechanical strain noise, and the harmonic signals.This results in irregular fluctuations of the sidebands in the harmonic signals of non-absorption.This is an important factor affecting the lower limit of the measurement of the system and its overall stability.
The second harmonic signal for a concentration of 0.5% methane gas is shown in Figure 4.It can be seen that the sidebands of the second harmonic signal exhibit skewness, indicating the presence of residual amplitude modulation in the signal.The standard deviation σ of the non-absorbing sidebands is determined to be about 0.392 mV, with the peak-to-peak value of the second harmonic signal at 28.673 mV.According to the calculations, the theoretical minimum detection limit of methane gas measured by the system is 68.36 ppm.

Channel consistency
In order to test the consistency of the system with multiple channels, three channels are randomly selected.The corresponding sensing probes are placed at ambient temperature and pressure.Tests are conducted by using standard concentrations of methane gas (0.5%, 1.5%, 5%, 10%, 20%, and 35%) for each group of experimental gases.Each group is ventilated for 3 minutes, stabilized, and tested five times, with the average value taken.  1 shows the methane concentration test in different channels under the same environment.The results indicate that the gas concentrations measured by three different channels of methane probes are nearly identical.For the methane concentration within the range of 0~1%, the error is less than ±6.0×10 - 4 ; when the concentration is more than 1%, the measured error is less than ±6% of the true value.The results demonstrate compliance with the existing methane measurement standards.The system exhibits satisfactory accuracy, stability, and responsiveness, meeting the need for accurate methane gas detection.

Conclusions
A multi-point gas monitoring system is designed based on TDLAS technology.The experiments show that the combined direct absorption and wavelength modulation techniques at room temperature and pressure result in a measurement error of less than ±6.0×10 -4 for low concentrations (0~1%) and less than ±6% of the true value for high concentrations.The linear fit of the full scale is 0.9998.Through analysis of the second harmonic signal of 0.5% methane gas concentration, the minimum detection limit of the system for methane gas is determined to be 68.36 ppm, according to the one-fold standard deviation.In addition, experimental verification under consistent environmental conditions confirms that the methane probes of different channels yield basically the same concentration of gas.The above experimental results affirm that the designed multi-point gas monitoring system meets the demand for continuous, accurate, and multi-point detection of gas in mining operations, and will play a crucial role in mine gas monitoring.

Figure 1 .
Figure 1.Schematic diagram of system structure.The system adopts a tunable semiconductor DBF laser with a central wavelength of 1653.72 nm as the light source.The temperature module is used to stabilize the wavelength of the laser output; The scanning signal generated by the signal generator module is overlaid onto the driving current of the laser through the current module to enable the laser to produce the scanned wavelength.The laser signal is divided into multiple signals of the same frequency through the fiber optic coupler.These signals then reach the methane detectors at different monitoring points, respectively, where they are absorbed by the methane to carry the information of methane concentration at each monitoring point.The optical signals carrying the information are converted into electrical signals by the detectors.Subsequently, the data from each monitoring point is relayed to the data acquisition and processing module by 16 analog switches through time division multiplexing.Finally, the methane concentration information for each monitoring point is displayed in the local communication display module after analysis.

Figure 2 .
Figure 2. Sensing probe structure.(a) Schematic diagram; (b)Physical drawing.As shown in Figure2(a), the laser beam is transmitted from the armored optical fiber to the methane sensing probe.Following collimation by the lens and reflection by the mirrors, the beam carrying information about methane gas concentration is conveyed from the optical fiber to the photodetector.The optical absorption path of the system is increased by the use of three mirrors, approximately 15 cm in length.All components are sealed in a stainless steel housing to ensure the probe's resilience against vibrations and shocks.The base of the sensing probe is a waterproof and breathable metallurgical powder filter, along with a steel mesh connected to the outside.The setup facilitates effective gas exchange with the outside world while preventing dust and water from infiltrating the gas chamber and safeguarding the lens against contamination.This ensures the accuracy of measurements, prolongs the probe's operational lifespan, and improves the robustness for applications in harsh environments.

Figure 3 .
Figure 3. Full-scale test results.Figure3shows the test results conducted with varying concentrations of standard methane gas.It can be inferred that in the concentration range of 0~1%, the maximum error occurs at 0.5%, with a relative error of 4.07%.However, the absolute error is only 2×10 -4 , meeting the measurement requirements of gas concentrations in the range of 0~1%, where the acceptable error is less than ±6.0×10 -4 ; in the

Figure 4 .
Figure 4. Second harmonic signal of methane gas (0.5%).The standard deviation σ of the non-absorbing sidebands is determined to be about 0.392 mV, with the peak-to-peak value of the second harmonic signal at 28.673 mV.According to the calculations, the theoretical minimum detection limit of methane gas measured by the system is 68.36 ppm.

Table 1 .
Concentration Test of Different Channels in the Same Environment.