A multi-stage coupled amplification photoacoustic spectroscopy greenhouse gas measurement system and method

Abstract

The application discloses a multi-stage coupling amplification photoacoustic spectrum greenhouse gas measuring system and method, and belongs to the technical field of trace gas measurement. The measuring system comprises a micro multi-pass variable-diameter T-shaped photoacoustic cell, a gas distribution system, a DFB laser, and a data acquisition and processing system. Reflectors one and two are arranged at the two side windows of the micro multi-pass variable-diameter T-shaped photoacoustic cell. The gas distribution system introduces the to-be-measured gas into the gas chamber of the micro multi-pass variable-diameter T-shaped photoacoustic cell. The DFB laser generates modulated laser, which enters the micro multi-pass variable-diameter T-shaped photoacoustic cell through the collimating lens from the incident hole of the reflector one, and is reflected back and forth between the reflectors two, so that multi-stage coupling amplification photoacoustic signals are generated, and then the signals are demodulated by a lock-in amplifier, and enter the data acquisition and processing system. Through multiple reflections in the micro multi-pass variable-diameter T-shaped photoacoustic cell, the interaction between the laser and the gas is enhanced, so that the monitoring of the methane gas concentration is more accurate and reliable.

Description

Technical Field

[0001] This invention belongs to the field of trace gas measurement technology, specifically relating to a multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system and method. Background Technology

[0002] Currently, high-precision detection technologies for trace gases (such as methane) mainly include electrochemical gas sensors, gas-sensing methods, thermocatalytic methods, and gas chromatography. These methods can achieve detection accuracy in the PPM range, but they are all based on non-optical detection principles. Compared to optical detection methods, non-optical methods have a narrower dynamic range and longer acquisition, sampling, and processing times, making them unsuitable for online real-time detection. Optical methods are mainly based on direct absorption spectroscopy, but their detection capability is directly related to the optical path length. Therefore, in practical measurements, to achieve a smaller detection limit, a longer optical cavity is usually required, which also increases the overall gas consumption of the instrument.

[0003] Most methane detection equipment on the market uses TDLAS technology, which is a method for detecting methane gas using tunable diode laser absorption spectroscopy. However, this method is expensive and bulky. In contrast, PAS-based trace gas detection technology effectively improves the system's detection performance by enhancing the gas's absorption of laser energy, resulting in a lower system cost and smaller size.

[0004] Currently, methane detection technology is in a stage of continuous development and innovation. Improving sensitivity and reducing the cost of detection systems are particularly important, while also placing higher demands on the portability and integration of detection systems.

[0005] Current methane detection technologies have achieved very low detection limits, but further improvements in detection sensitivity are still needed. Most current methane detection research focuses on a single direction, failing to simultaneously achieve high sensitivity and portability. To achieve high sensitivity, some studies emphasize using high-precision instruments and complex techniques to accurately detect methane content. However, this approach often requires large equipment and specialized operators, making it unsuitable for mobile or portable applications. On the other hand, some research focuses on developing portable methane detection devices to meet practical application needs. These devices are typically small, lightweight, and easy to carry and operate. However, to achieve portability, they usually sacrifice some degree of accuracy and sensitivity. Therefore, improvements to the above detection systems are necessary. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system and method, which solves the problems in the prior art.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] A multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system includes a miniature multi-pass variable diameter T-shaped photoacoustic cell, a gas distribution system, a DFB laser, a collimating lens, a data acquisition card, and a computer; a reflector with an entrance hole is respectively provided at the two side windows of the miniature multi-pass variable diameter T-shaped photoacoustic cell.

[0009] The gas distribution system introduces the gas to be tested into and fills the gas chamber of the miniature multi-channel variable diameter T-shaped photoacoustic cell. The DFB laser generates a modulated laser beam, which enters the miniature multi-channel variable diameter T-shaped photoacoustic cell through the entrance hole of the first reflector through a collimating lens. The beam is reflected back and forth between the second reflector and the second reflector, forming multiple light spots on the mirror surface. The laser beam then exits from the entrance hole of the first reflector, generating a multi-stage coupled and amplified photoacoustic signal within the miniature multi-channel variable diameter T-shaped photoacoustic cell. This signal is then received by a high-sensitivity microphone, processed, and acquired by a data acquisition card. Finally, the data is processed and calculated by a computer.

[0010] Furthermore, the measurement system also includes: a function generator, a lock-in amplifier, an adder, and a laser controller; the modulation of the light source is achieved by the function generator generating a low-frequency triangular wave for scanning the absorption line of methane gas, and the lock-in amplifier generating a high-frequency sine wave for output modulation. These are superimposed by the adder and then input to the laser controller to modulate the wavelength of the laser generated by the DFB laser, so that the output wavelength of the laser is located at the center of a certain absorption spectral line of the gas under test.

[0011] Furthermore, the measurement system also includes a microphone and a preamplifier. An acoustic signal that can be received by a highly sensitive microphone is generated in the miniature multi-pass variable diameter T-shaped photoacoustic cell. The acoustic signal is converted into an electrical signal by the microphone, passes through the preamplifier, and is then demodulated by the lock-in amplifier to extract the second harmonic signal.

[0012] Furthermore, the gas concentration measurement involves calibrating the photoacoustic signal generated in the miniature multi-channel variable diameter T-shaped photoacoustic cell using a standard gas of known concentration and performing linear fitting. The concentration of the gas to be measured is then calculated by inverting the amplitude of the photoacoustic signal using a computer.

[0013] Furthermore, the T-shaped photoacoustic cell is equipped with an air inlet and an air outlet. The air inlet is connected to the gas distribution system, and the measurement system is also equipped with a micro air pump, which is connected to the air outlet and can extract the measured gas from the T-shaped photoacoustic cell.

[0014] Furthermore, an air inlet valve is installed between the air inlet and the air distribution system; and an air outlet valve is installed between the air outlet and the micro air pump.

[0015] Furthermore, the T-shaped photoacoustic cell is manufactured using 3D printing technology.

[0016] Furthermore, the gas under test absorbs and modulates the light source to generate a heat source H(r,t), and produces a photoacoustic signal. The photoacoustic signal can be represented by the mathematical model of sound pressure p(r,t):

[0017]

[0018] In the formula, Here, r is the Laplace operator, and r is the displacement vector. For heat capacity ratio, C p C v These are the isobaric heat capacity and the isochoric heat capacity, respectively, where v is the speed of light in the gas, and H(r,t) is the heat power density.

[0019] Furthermore, in cylindrical coordinates, the resonant frequency f in the normal mode j j Amplitude A j (ω) can be expressed as:

[0020]

[0021]

[0022] In the formula, ω j Let be the resonant angular frequency of mode j. For p j The complex conjugate of V c Q is the volume of the photoacoustic cell resonator. j Let P0 be the quality factor of mode j, P0 be the laser power, and c be the gas concentration; integral The degree of coupling between light intensity distribution and normal modes is represented by I. j express;

[0023] In ω=ω j Under resonant conditions, the generated sound pressure is the strongest, and the r in the T-shaped photoacoustic cell at this time is obtained. M The sound pressure at that location is:

[0024]

[0025] In the formula, Let C be the photoacoustic cell constant. cell express;

[0026] After the acoustic detector is introduced, the detected photoacoustic signal S is generated in the T-shaped photoacoustic cell. PA for:

[0027] S PA =s m C cell αcP0

[0028] In the formula, sm This refers to the sensitivity of the acoustic detector.

[0029] A multi-stage coupled amplified photoacoustic spectroscopy method for measuring greenhouse gases, using the aforementioned measurement system, includes the following steps:

[0030] S1, open the air inlet valve, and use the gas distribution system to fill the miniature multi-port variable diameter T-shaped photoacoustic cell with methane gas;

[0031] S2 generates a low-frequency triangular wave for scanning the absorption line of methane gas through a function generator, and a high-frequency sine wave for output modulation through a lock-in amplifier. The two waves are then superimposed by an adder and input to the laser controller to modulate the wavelength of the laser generated by the DFB laser, so that the output wavelength of the laser is at the center of a certain absorption line of the methane gas to be measured.

[0032] S3, the modulated laser enters the 3D-printed T-shaped photoacoustic cell chamber through the entrance hole of the collimating lens and the two sides of the mirror, generating a multi-level coupled and amplified photoacoustic signal in the micro multi-pass variable diameter T-shaped photoacoustic cell; then it is received by a high-sensitivity microphone, and after signal processing, it is acquired by a data acquisition card. Finally, the data is processed and calculated by a computer to obtain the gas concentration.

[0033] S4. Open the outlet valve and start the micro air pump to extract the measured methane gas from the T-type photoacoustic cell.

[0034] The beneficial effects of this invention are:

[0035] 1. This invention adds a set of custom reflective lenses outside the micro multi-pass variable diameter T-shaped photoacoustic cell window. Through multiple reflections within the micro multi-pass variable diameter T-shaped photoacoustic cell, the interaction between laser and methane is enhanced, effectively improving the absorption effect of methane on laser, making the monitoring of methane gas concentration more accurate and reliable. Furthermore, by utilizing the acoustic signal acquired by a high-sensitivity microphone, the system can quickly and accurately obtain methane gas concentration information.

[0036] 2. This invention utilizes 3D printing technology, characterized by low cost, high efficiency, and strong customizability, to manufacture a miniature multi-pass variable-diameter T-shaped photoacoustic cell. This reduces material waste and simplifies the manufacturing process, significantly lowering the manufacturing cost of the testing equipment. Compared to traditional photoacoustic cell structures, the 3D-printed miniature multi-pass variable-diameter T-shaped photoacoustic cell is compact and lightweight, making it easy to carry and install, and also convenient to integrate into other systems, improving the system's portability and operability. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a schematic diagram of the overall structure of the measurement system of the present invention;

[0039] Figure 2 This is the mirror reflection light path diagram of the present invention;

[0040] In the diagram, 1-Adder, 2-Laser controller, 3-DFB laser, 4-Function generator, 5-Lock-in amplifier, 6-Collimating lens, 7-Preamplifier, 8-High-sensitivity microphone, 9-Data acquisition card, 10-Reflector 1, 11-Reflector 2, 12-Miniature multi-channel variable diameter T-type photoacoustic cell, 13-Air inlet, 14-Air outlet, 15-Air outlet valve, 16-Air inlet valve, 17-Gas distribution system, 18-Computer, 19-Miniature air pump. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] Example 1

[0043] like Figure 1 As shown, a multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system includes a miniature multi-pass variable diameter T-shaped photoacoustic cell 12, a gas distribution system 17, a DFB laser 3, a collimating lens 6, a data acquisition card 9, and a computer 18; a reflector 10 with an entrance hole and a reflector 21 with an entrance hole are respectively provided at the two side windows of the miniature multi-pass variable diameter T-shaped photoacoustic cell 12.

[0044] The methane gas to be tested is introduced into and fills the gas chamber of the miniature multi-pass variable-diameter T-shaped photoacoustic cell 12 through the gas distribution system 17. The DFB laser 3 generates a modulated laser beam, which enters the 3D-printed miniature multi-pass variable-diameter T-shaped photoacoustic cell gas chamber 12 through the entrance aperture of the first reflector 10 via the collimating lens 6. The beam is reflected back and forth between the second reflector 11 and the surface of the third reflector 11, forming multiple light spots on the mirror surface, such as... Figure 2As shown. Finally, the laser beam exits from the entrance aperture of the reflector 10, generating multi-stage coupled and amplified photoacoustic signals within the miniature multi-pass variable-diameter T-shaped photoacoustic cell 12. These photoacoustic signals are received by a high-sensitivity microphone, processed, acquired by the data acquisition card 9, and finally processed and calculated by the computer 18.

[0045] In this embodiment, laser modulation is achieved through a laser modulation module, which includes a function generator 4, a lock-in amplifier 5, an adder 1, and a laser controller 2. The light source modulation is primarily achieved by the function generator 4 generating a low-frequency triangular wave for slowly scanning the methane gas absorption line, and the lock-in amplifier 5 generating a high-frequency sine wave for output modulation. These are superimposed by the adder 1 and input to the laser controller 2 to modulate the wavelength of the laser generated by the DFB laser, ensuring that the laser's output wavelength is at the center of a specific absorption line of the methane gas being measured. Signal generation is mainly performed in the miniature multi-pass variable-diameter T-shaped photoacoustic cell 12.

[0046] Within the miniature multi-pass variable-diameter T-shaped photoacoustic cell, when the modulated light source is incident on the gas to be measured, photons interact with gas molecules, causing a change in the temperature of the gas molecules. This photothermal effect causes a change in pressure around the gas molecules, thereby generating sound waves. Through the photoacoustic effect, light energy is converted into sound energy, thus generating an acoustic signal that can be received by a high-sensitivity microphone within the miniature multi-pass variable-diameter T-shaped photoacoustic cell 12. The acoustic signal is processed mainly by the high-sensitivity microphone 8, which converts the acoustic signal into an electrical signal. The electrical signal is first amplified by the preamplifier 7, and then demodulated at a specific frequency by the lock-in amplifier 5 to extract the second harmonic signal. Finally, the photoacoustic signal generated within the miniature multi-pass variable-diameter T-shaped photoacoustic cell 12 is calibrated using a standard gas of known concentration, and a linear fit is performed to obtain the functional relationship between the photoacoustic signal and the concentration of the gas to be measured. For a gas of unknown concentration, the amplitude of the photoacoustic signal output by the system can be substituted into the calibrated functional relationship between the photoacoustic signal and the concentration to calculate the concentration. This process is automatically completed by the computer 18.

[0047] In this embodiment, the miniature multi-port variable diameter T-shaped photoacoustic cell 12 is provided with an air inlet 13 and an air outlet 14. The air inlet 13 is connected to the gas distribution system 17, which can fill the methane gas to be measured into the miniature multi-port variable diameter T-shaped photoacoustic cell 12 through the air inlet 13. The measurement system is also provided with a miniature vacuum pump 19, which is connected to the air outlet 14 and can extract the measured methane gas from the miniature multi-port variable diameter T-shaped photoacoustic cell 12.

[0048] In addition, an air inlet valve 16 is provided between the air inlet 13 and the air distribution system 17 to control the air intake switch; an air outlet valve 15 is provided between the air outlet 14 and the micro air pump 19 to control the air outlet switch.

[0049] It is worth mentioning that in this embodiment, the T-shaped photoacoustic cell is manufactured using 3D printing technology, which reduces material waste and simplifies the manufacturing process, significantly reducing the manufacturing cost of the testing equipment. Compared with the traditional photoacoustic cell structure, the 3D-printed T-shaped photoacoustic cell is compact and lightweight, making it not only easy to carry and install, but also easy to integrate into other systems, improving the system's portability and operability.

[0050] The principle of this invention is as follows: Within a closed, miniature, multi-channel, variable-diameter T-shaped photoacoustic cell 12, the methane gas to be measured absorbs and modulates the light source to generate a heat source H(r,t), which excites a photoacoustic signal within the cell. This photoacoustic signal can be represented by the sound pressure p(r,t), and its mathematical model is as follows:

[0051]

[0052] In the formula, Here, r is the Laplace operator, and r is the displacement vector. The heat capacity ratio (where C) p C v (These are isobaric heat capacity and isochoric heat capacity, respectively), v is the speed of light in the gas, and H(r,t) is the heat power density.

[0053] In cylindrical coordinates, the resonant frequency f in normal mode j j Amplitude A j (ω) can be expressed as:

[0054]

[0055]

[0056] In the formula, ω j Let be the resonant angular frequency of mode j. For p j The complex conjugate of V c Q is the volume of the photoacoustic cell resonator. j Let P0 be the quality factor of mode j, P0 be the laser power, and c be the gas concentration; integral The degree of coupling between light intensity distribution and normal modes is represented by I. j express.

[0057] In ω=ω j Under resonant conditions, the generated sound pressure is the strongest, and the result is obtained at this time in the micro multi-pass variable diameter T-shaped photoacoustic cell 12. M The sound pressure at that location is:

[0058]

[0059] In the formula, C is the photoacoustic cell constant, referred to as C in the following text. cell express.

[0060] Introducing a high-sensitivity microphone (sensitivity s) m After that, the detected photoacoustic signal S is generated in the miniature multi-pass variable diameter T-shaped photoacoustic cell 12. PA It can be represented as:

[0061] S PA =s m C cell αcP0

[0062] Example 2

[0063] A method for detecting the concentration of greenhouse gas (methane gas) using the measurement system in Example 1 includes the following steps:

[0064] S1, methane gas is filled into the miniature multi-channel variable diameter T-shaped photoacoustic cell 12;

[0065] Open the air inlet valve 16 and use the gas distribution system 17 to fill the miniature multi-port variable diameter T-shaped photoacoustic cell 12 with methane gas.

[0066] S2, Light source modulation;

[0067] A low-frequency triangular wave is generated by function generator 4 to slowly scan the absorption line of methane gas, and a high-frequency sine wave is generated by lock-in amplifier 5 to modulate the output light. The two waves are then superimposed by adder 1 and input to laser controller 2 to modulate the wavelength of the laser generated by DFB laser 3, so that the output wavelength of the laser is located at the center of a certain absorption line of the methane gas to be measured.

[0068] S3, Light source incident detection;

[0069] The modulated laser light enters the 3D-printed T-shaped photoacoustic cell gas chamber 12 through the entrance hole of the reflector 10 via the collimating lens 6, and reflects back and forth between the reflector 11 and the surface of the micro multi-pass variable diameter T-shaped photoacoustic cell 12, generating multi-stage coupled and amplified photoacoustic signals. These photoacoustic signals are received by a high-sensitivity microphone, processed, and then acquired by the data acquisition card 9. Finally, the computer 18 performs data processing and calculations, calibrates the photoacoustic signals generated in the micro multi-pass variable diameter T-shaped photoacoustic cell 12 using a standard gas of known concentration, performs linear fitting, and calculates the concentration of the gas to be measured by inverting the amplitude of the photoacoustic signal using the computer 18.

[0070] S4, gas release;

[0071] Open the gas outlet valve 15 and start the micro air pump 19 to extract the detected methane gas from the micro multi-port variable diameter T-type photoacoustic cell 12.

[0072] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0073] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system, characterized in that, It includes a miniature multi-channel variable diameter T-type photoacoustic cell (12), a gas distribution system (17), a DFB laser (3), a collimating lens (6), a data acquisition card (9), and a computer (18); the miniature multi-channel variable diameter T-type photoacoustic cell (12) is equipped with a reflector with an entrance hole on each side window (10) and a reflector with an entrance hole (11). The gas distribution system (17) introduces the gas to be tested into and fills the gas chamber of the miniature multi-channel variable diameter T-type photoacoustic cell (12). The DFB laser (3) generates a modulated laser, which enters the miniature multi-channel variable diameter T-type photoacoustic cell (12) through the entrance hole of the first reflector (10) through the collimating lens (6) and reflects back and forth with the second reflector (11), forming multiple light spots on the mirror surface. Then the laser is emitted from the entrance hole of the first reflector (10), and a multi-level amplified photoacoustic signal is generated in the miniature multi-channel variable diameter T-type photoacoustic cell (12). Then it is received by the acoustic detector and demodulated by the lock-in amplifier. The signal is then acquired by the data acquisition card (9), and finally the data is processed and calculated by the computer (18). The gas under test absorbs and modulates the light source to generate a heat source. It generates photoacoustic signals, which can be expressed using sound pressure. The mathematical model is expressed as follows: In the formula, For the Laplace operator, It is a displacement vector. For heat capacity ratio, , These are isobaric heat capacity and isochoric heat capacity, respectively. The speed at which light travels through a gas. It is the heat power density; Normal mode in cylindrical coordinates The resonant frequency below amplitude It can be represented as: In the formula, For simplified mode The resonant angular frequency, for The complex conjugate, Let V be the volume of the photoacoustic cell resonator. For pattern The quality factor, Let c be the laser power and c be the gas concentration; integral The degree of coupling between light intensity distribution and normal modes is represented by... express; exist Under the resonant conditions, the generated sound pressure is the strongest, and the sound pressure at rM in the micro multi-pass variable diameter T-shaped photoacoustic cell (12) at this time is: In the formula, Let be the photoacoustic cell constant, using express; After the acoustic detector is introduced, the detected photoacoustic signal is generated in the miniature multi-pass variable diameter T-shaped photoacoustic cell (12). for: In the formula, This refers to the sensitivity of the acoustic detector.

2. The multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system according to claim 1, characterized in that, The measurement system also includes: a function generator (4), a lock-in amplifier (5), an adder (1), and a laser controller (2); the modulation of the light source is achieved by the function generator (4) generating a low-frequency triangular wave for scanning the gas absorption line, and the lock-in amplifier (5) generating a high-frequency sine wave for output modulation. The sine waves are superimposed by the adder (1) and input to the laser controller (2) to modulate the wavelength of the laser generated by the DFB laser (3), so that the output wavelength of the laser is located at the center of a certain absorption spectral line of the gas to be measured.

3. The multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system according to claim 2, characterized in that, The measurement system also includes a microphone (8) and a preamplifier (7). A sound signal that can be received by a highly sensitive microphone is generated in the miniature multi-channel variable diameter T-shaped photoacoustic cell (12). The sound signal is converted into an electrical signal by the microphone (8), and after passing through the preamplifier (7), it is demodulated by the lock-in amplifier (5) to extract the second harmonic signal.

4. A multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system according to claim 1 or 3, characterized in that, The gas concentration measurement is as follows: the photoacoustic signal of the gas to be measured generated in the micro multi-channel variable diameter T-type photoacoustic cell (12) is calibrated using a standard gas of known concentration, and linear fitting is performed. The concentration of the gas to be measured is calculated by inverting the amplitude of the photoacoustic signal using a computer (18).

5. The multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system according to claim 3, characterized in that, The miniature multi-port variable diameter T-type photoacoustic cell (12) is provided with an air inlet (13) and an air outlet (14). The air inlet (13) is connected to the gas distribution system (17). The measurement system is also provided with a miniature vacuum pump (19). The miniature vacuum pump (19) is connected to the air outlet (14) and can extract the measured methane gas from the miniature multi-port variable diameter T-type photoacoustic cell (12).

6. The multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system according to claim 5, characterized in that, An air inlet valve (16) is provided between the air inlet (13) and the air distribution system (17); an air outlet valve (15) is provided between the air outlet (14) and the micro air pump (19).

7. The multi-stage coupled amplified photoacoustic spectroscopy greenhouse gas measurement system according to claim 1, characterized in that, The miniature multi-channel variable diameter T-shaped photoacoustic cell (12) is manufactured using 3D printing technology.

8. A multi-stage coupled amplified photoacoustic spectroscopy method for measuring greenhouse gases, using the measurement system described in claim 6, characterized in that, Includes the following steps: S1, open the air inlet valve (16) and use the gas distribution system (17) to fill the miniature multi-port variable diameter T-type photoacoustic cell (12) with methane gas; S2, a low-frequency triangular wave for scanning the absorption line of methane gas is generated by the function generator (4), and a high-frequency sine wave for output modulation is generated by the lock-in amplifier (5). The waves are then superimposed by the adder (1) and input to the laser controller (2) to modulate the wavelength of the laser generated by the DFB laser (3), so that the output wavelength of the laser is at the center of a certain absorption line of the methane gas to be tested. S3, the modulated laser enters the gas chamber of the 3D-printed micro multi-channel variable diameter T-shaped photoacoustic cell (12) through the entrance hole of the first reflector (10) through the collimating lens (6), and reflects back and forth between the second reflector (11) and generates a multi-level coupled and amplified photoacoustic signal in the micro multi-channel variable diameter T-shaped photoacoustic cell (12); then it is received by a high-sensitivity microphone, and after signal processing, the signal is acquired by the data acquisition card (9), and finally the data is processed and calculated by the computer (18) to obtain the concentration of the gas to be measured; S4, open the outlet valve (15) and start the micro air pump (19) to extract the measured methane gas from the micro multi-channel variable diameter T-type photoacoustic cell (12).

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