Non-contact photoacoustic gas sensing device and detection method

Abstract

The invention belongs to the technical field of photoacoustic spectrum sensing, and discloses a non-contact photoacoustic gas sensing device and a detection method. The non-contact photoacoustic gas sensing device comprises a tunable excitation light source, a first photoacoustic cell, a second photoacoustic cell, a collimating lens and a cantilever beam sonic sensor. The first photoacoustic cell is used for introducing gas to be detected, the second photoacoustic cell is used for introducing reference gas with known concentration, the first photoacoustic cell and the second photoacoustic cell are mutually independent in space, and light beam energy transfer is realized through the collimating lens. A light beam emitted by the tunable excitation light source sequentially penetrates through the first photoacoustic cell and the second photoacoustic cell, light intensity attenuation is caused after gas in the first photoacoustic cell absorbs part of light energy, the attenuated light beam is absorbed by reference gas in the second photoacoustic cell and generates a photoacoustic effect, and vibration of the cantilever beam acoustic wave sensor is caused. By comparing the ratio of the variable quantity of the photoacoustic signal to the reference signal and combining the Beer-Lambert law, the concentration of the gas to be detected can be inverted, and remote, non-contact and high-sensitivity gas detection is realized.

Description

Technical Field

[0001] This invention belongs to the field of photoacoustic spectroscopy and trace gas detection technology, and relates to a non-contact photoacoustic gas sensing device and detection method. Background Technology

[0002] Photoacoustic spectroscopy, as a trace gas sensing method, has been successfully applied in various key fields due to its high sensitivity and selectivity, such as respiratory disease diagnosis, transformer fault monitoring, environmental monitoring, and fire alarm and explosion environment monitoring. The basic principle of photoacoustic spectroscopy is to use an acoustic sensor to capture the amplitude of the photoacoustic signal generated after gas molecules absorb light energy. Researchers have been pursuing higher performance photoacoustic spectroscopy and have proposed various enhanced photoacoustic spectroscopy techniques in recent years. Among them, the most representative are quartz-enhanced photoacoustic spectroscopy and cantilever beam-enhanced photoacoustic spectroscopy, both of which can achieve high-sensitivity detection of trace gases. The quartz-enhanced photoacoustic spectroscopy system uses a quartz tuning fork with a high resonant frequency (32.7 kHz) and a high Q value (>10000) as an acoustic transducer, converting the photoacoustic signal into an electrical signal through the piezoelectric effect. However, quartz-enhanced photoacoustic spectroscopy is susceptible to the effects of certain gas relaxation processes, resulting in a weaker photoacoustic signal.

[0003] Cantilever beam-enhanced photoacoustic spectroscopy (CES) can detect gases with varying relaxation rates, and has therefore found wide application. However, when applied to special and demanding environments such as breath gas diagnostics, dissolved gas analysis in transformer oil, and explosion atmosphere monitoring, CES still faces the following key challenges.

[0004] Breathable gases may contain moisture, mucus, or other bioaerosols, and prolonged exposure can lead to sensor contamination, affecting diagnostic accuracy and hygiene safety. Gases produced by the decomposition of transformer oil may have corrosive or oily residues; direct contact can corrode the cantilever beam surface, causing sensor failure. Explosive gases or their decomposition products (such as H2S) are corrosive. More importantly, in explosive environments, any electrical spark can trigger a safety accident, and placing sensitive electronic or optical components directly in such environments poses a significant safety hazard. Furthermore, in some industrial settings, cantilever beam-enhanced photoacoustic spectroscopy may encounter fine particulate matter or dust. Fabry-Perot interferometry-based cantilever beam-enhanced photoacoustic spectroscopy sensors rely on the free vibration of a tiny gap between the cantilever beam and the substrate. Once this gap is blocked by dust, oil mist, or bioaerosols, it will severely affect the vibration amplitude of the cantilever beam, significantly reducing the detection sensitivity of the cantilever beam-enhanced photoacoustic spectroscopy system. Therefore, this technology is not suitable for detecting gases containing dust, oil mist, or complex particulate matter. Based on this, the literature Gong, Z.; Wu, G.; Xing, J.; Wu, X.; Mei, L. Noncontact Fiber-Optic Cantilever-Enhanced Photoacoustic Spectroscopy. Anal. Chem. 2024, 96, 15008–15013. cleverly combines a sound-permeable polyethylene film with a cantilever beam acoustic sensor to solve the defect of easy clogging of the tiny gap between the cantilever beam and the substrate, and protect the cantilever beam from corrosion by corrosive gases. However, this solution is not a strictly non-contact photoacoustic gas sensing method, and its safety is not high enough.

[0005] In summary, existing cantilever beam-enhanced photoacoustic spectroscopy technology, while pursuing high sensitivity, fails to address core issues such as sensor susceptibility to corrosion and contamination, and the need for long-distance non-contact detection in demanding applications like breathalyzer diagnostics, transformer fault monitoring, and explosion environment monitoring. Therefore, there is an urgent need in this field for a novel, highly stable photoacoustic sensing technology with non-contact and corrosion-resistant capabilities. Summary of the Invention

[0006] To address the aforementioned problems, this invention proposes a non-contact photoacoustic gas sensing device and detection method. This gas sensor completely separates the sensing and detection units, solving the problem that traditional photoacoustic gas sensors must contact the gas to be measured. Furthermore, it enables high-sensitivity, long-distance, real-time, in-situ monitoring of trace gases, showing great promise for applications in the field of photoacoustic spectroscopy.

[0007] The technical solution of this invention:

[0008] A non-contact photoacoustic gas sensing device, comprising:

[0009] A tunable excitation source 1;

[0010] The first photoacoustic cell 2 serves as the gas cell to be tested, and is equipped with an air inlet 4 and an air outlet 5 for introducing and containing the gas to be tested; the light-inlet end of the first photoacoustic cell 2 is provided with an incident window 6, and the light-outlet end is provided with an exit window 7.

[0011] The second photoacoustic cell 3 serves as a detection reference cell, and is equipped with an air inlet 4 and an air outlet 5 for introducing and containing a reference gas of known concentration; the light-inlet end of the second photoacoustic cell 3 is provided with an incident window 6, and the light-outlet end is provided with a cantilever beam acoustic wave sensor 9.

[0012] A collimating lens 8 is provided between the first photoacoustic cell 2 and the second photoacoustic cell 3.

[0013] The light beam emitted by the tunable excitation light source 1 passes through the first photoacoustic cell 2, is converged by the collimating lens 8 located between the first photoacoustic cell 2 and the second photoacoustic cell 3, and then enters the second photoacoustic cell 3.

[0014] The non-contact photoacoustic gas sensing device achieves the separation of the first photoacoustic cell 2 as an independent gas sampler and the second photoacoustic cell 3 as an independent signal detector.

[0015] The collimating lens 8 is used to collimate and couple the excitation beam after it has been attenuated by the first photoacoustic cell 2, so that it can enter the second photoacoustic cell 3 efficiently.

[0016] The wavelength tuning range of the tunable excitation light source 1 can simultaneously cover the absorption spectra of both the gas to be tested and the reference gas.

[0017] The reference gas introduced into the second photoacoustic cell 3 is a gas of known concentration. This gas is non-corrosive and has extremely low or non-existent abundance in nature. The reference gas has different components from the gas to be tested and is non-corrosive, which is used to protect the cantilever beam acoustic sensor 9. The gas to be tested and the reference gas have similar absorption spectra, and within the selected wavelength range, the absorption spectra of the two gases are similar and do not interfere with each other, including but not limited to the combination of ammonia and acetylene absorbed in the 1530-1532 nm band.

[0018] The light beam emitted by the tunable excitation light source 1 is transmitted over long distances through optical fiber or free space optical path, so that the first photoacoustic cell 2 can be placed at a distance as needed, thereby realizing non-contact long-distance gas measurement.

[0019] The second photoacoustic cell 3 continuously introduces a reference gas of known concentration to ensure that the concentration of the reference gas inside remains constant.

[0020] A non-contact photoacoustic gas sensing and detection method, comprising the following steps:

[0021] The gas to be tested is introduced into the first photoacoustic cell 2, while a reference gas of known concentration is continuously introduced into the second photoacoustic cell 3 to maintain a constant reference gas concentration. The wavelength of the tunable excitation light source 1 is adjusted so that the emitted excitation beam passes through the first photoacoustic cell 2, and the beam intensity is attenuated due to absorption by the gas to be tested. The attenuated excitation beam enters the second photoacoustic cell 3 through the collimating lens 8, is absorbed by the reference gas, and causes a photoacoustic effect, which in turn causes the cantilever beam acoustic wave sensor 9 to vibrate. The vibration change A of the cantilever beam acoustic wave sensor 9 is demodulated, and the reference vibration amplitude A0 is obtained, which is the vibration amplitude measured when the reference gas is introduced into the first photoacoustic cell 2. The reference vibration amplitude is used to eliminate the influence of system drift and light source fluctuation. The vibration change is proportional to the beam intensity entering the second photoacoustic cell 3. Based on the ratio of the demodulated vibration change A to the reference vibration amplitude A0, the constant of the second photoacoustic cell 3 is normalized, and the concentration C of the gas to be tested in the first photoacoustic cell 2 is calculated according to the Beer-Lambert law. T =-1 / (α T ×L1)×ln(A / A0), where α T L1 represents the molar absorption cross section of the gas to be measured; L2 represents the effective operating length of the first photoacoustic cell 2.

[0022] The system constants include the combined gain coefficients of multiple factors such as light source power, photoacoustic cell geometry, beam intensity, and cantilever beam response. These system constants are eliminated by normalization using the comparison value A / A0.

[0023] The beneficial effects of this invention are as follows: This invention achieves corrosion protection and high stability by completely isolating the sensitive cantilever beam acoustic sensor from the gas to be measured (especially corrosive or hazardous gases). A non-corrosive gas is continuously introduced into the second photoacoustic cell as a reference medium, completely eliminating the risk of sensor corrosion or contamination and greatly improving system lifespan and long-term stability. This invention achieves strong resistance to system drift by employing a light intensity ratio inversion method based on the Beer-Lambert law. Normalization of the ratio of vibration quantity to reference vibration quantity effectively eliminates systematic errors such as power fluctuations of the tunable excitation source and sensitivity drift of the cantilever beam sensor, ensuring detection accuracy. This invention achieves non-contact, long-distance gas detection by utilizing the characteristic of light beam transmission between the two photoacoustic cells, achieving physical separation between the sensing unit (second photoacoustic cell) and the gas sampling unit (first photoacoustic cell), allowing the first photoacoustic cell to be flexibly deployed in distant or inaccessible areas. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a non-contact photoacoustic gas sensing device according to the present invention.

[0025] In the figure: 1. Tunable excitation light source; 2. First photoacoustic cell; 3. Second photoacoustic cell; 4. Air inlet; 5. Air outlet; 6. Entrance window; 7. Exit window; 8. Collimating lens; 9. Cantilever beam acoustic sensor. Detailed Implementation

[0026] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0027] like Figure 1 As shown, the present invention provides a non-contact photoacoustic gas sensing device, comprising a tunable excitation light source 1, a first photoacoustic cell 2, a second photoacoustic cell 3, and a collimating lens 8 disposed between the first photoacoustic cell 2 and the second photoacoustic cell 3. The first photoacoustic cell 2 is used to contain the gas to be measured and serves as a gas sampling unit; the second photoacoustic cell 3 is used to contain a reference gas and serves as a signal detection unit.

[0028] The laser beam generated by the tunable excitation source 1 passes sequentially through the first photoacoustic cell 2, the collimating lens 8, and enters the second photoacoustic cell 3. The first photoacoustic cell 2 and the second photoacoustic cell 3 are spatially independent, achieving complete separation of sensing and detection functions. The first photoacoustic cell 2 has an inlet 4 and an outlet 5 at each end for introducing and discharging the gas to be measured. An incident window 6 is installed at the end of the first photoacoustic cell 2 to allow the light beam emitted by the tunable excitation source 1 to pass through. The gas to be measured absorbs some of the light energy, causing light intensity attenuation. This attenuated light beam is shaped by the collimating lens 8 before entering the second photoacoustic cell 3. The second photoacoustic cell 3 also has an inlet 4 and an outlet 5 for continuously introducing a reference gas of known concentration. An outlet window 7 is installed at the light-incident end of the second photoacoustic cell 3 to ensure high light transmittance and maintain the cell's airtightness; a cantilever beam acoustic sensor 9 is installed at the end of the second photoacoustic cell 3 to receive photoacoustic signals. The cantilever beam acoustic wave sensor 9 uses a high-sensitivity micro-silicon cantilever structure fabricated with MEMS technology, which can convert the periodic pressure wave generated by the absorption of laser light by the reference gas into a detectable vibration signal.

[0029] A non-contact photoacoustic gas sensing method is disclosed. In its operation, the output wavelength of the tunable excitation light source 1 covers the absorption spectra of both the target gas and the reference gas. When the target gas (e.g., acetylene) is introduced into the first photoacoustic cell 2, the intensity of the light beam is weakened due to absorption as it passes through the cell. After being shaped by the collimating lens 8, the attenuated light beam enters the second photoacoustic cell 3, where it interacts with the reference gas to induce a photoacoustic effect, exciting the cantilever beam acoustic sensor 9 to vibrate. The method detects the change in the amplitude A of the cantilever beam vibration and compares it with the reference vibration amplitude. (i.e., the response when a pure reference gas is introduced into the first photoacoustic cell) undergoes normalization processing. In this invention, the normalization ratio A / A0 of the photoacoustic signal is used to eliminate systematic errors caused by light source fluctuations, temperature changes, etc. This normalization process ensures that the final calculated concentration of the gas to be measured is only related to the absorption characteristics of the gas and is not affected by other external factors. The concentration of the gas to be measured in the first photoacoustic cell 2 is calculated according to the Beer-Lambert law: C T =-1 / (α T ×L1)×ln(A / A0), where α T L1 represents the molar absorption cross section of the gas to be measured; L2 represents the effective length of the first photoacoustic cell.

[0030] Furthermore, the reference gas used in the second photoacoustic cell 3 is a non-corrosive, stable specific gas, and its concentration is maintained constant through continuous ventilation. This design effectively prevents the cantilever beam acoustic sensor 9 from directly contacting the gas being measured, thereby avoiding corrosion or contamination and significantly improving system lifespan and long-term stability.

[0031] Furthermore, the beam of the tunable excitation light source 1 can be transmitted via optical fiber or free-space optical path, enabling the first photoacoustic cell 2 to be remotely deployed, achieving non-contact long-distance gas measurement. This invention has a simple, flexible, and expandable structure, making it particularly suitable for in-situ gas monitoring in hazardous, corrosive, or high-temperature environments.

[0032] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A non-contact photoacoustic gas sensing device, characterized by, The non-contact photoacoustic gas sensing device comprises: a tunable excitation light source (1); a first photoacoustic cell (2) as a cell for the gas to be measured, which is provided with an air inlet hole (4) and an air outlet hole (5) for the introduction and accommodation of the gas to be measured; the light inlet end of the first photoacoustic cell (2) is provided with an incident window sheet (6), and the light outlet end is provided with an emission window sheet (7); a second photoacoustic cell (3) as a detection reference cell, which is provided with an air inlet hole (4) and an air outlet hole (5) for the introduction and accommodation of a reference gas with a known concentration; the light inlet end of the second photoacoustic cell (3) is provided with an incident window sheet (6), and the light outlet end is provided with a cantilever beam acoustic wave sensor (9); a collimating lens (8) is arranged between the first photoacoustic cell (2) and the second photoacoustic cell (3); the light beam emitted by the tunable excitation light source (1) passes through the first photoacoustic cell (2), is converged by the collimating lens (8) between the first photoacoustic cell (2) and the second photoacoustic cell (3), and then enters the second photoacoustic cell (3).

2. A method of non-contact photoacoustic gas sensing detection, characterized in that, The steps are as follows: The to-be-tested gas is introduced into a first photoacoustic cell (2), and a reference gas with a known concentration is continuously introduced into a second photoacoustic cell (3) to maintain the concentration of the reference gas constant; the wavelength of the tunable excitation light source (1) is adjusted so that the excitation light beam emitted by the tunable excitation light source (1) passes through the first photoacoustic cell (2), and the intensity of the light beam is attenuated due to the absorption of the to-be-tested gas; the attenuated excitation light beam enters the second photoacoustic cell (3) through the collimating lens (8), is absorbed by the reference gas to generate a photoacoustic effect, and causes the vibration of the cantilever beam acoustic wave sensor (9); the vibration change amount A of the demodulated cantilever beam acoustic wave sensor (9) is obtained, and the reference vibration amplitude A0 is obtained, which is the vibration amplitude measured when the reference gas is introduced into the first photoacoustic cell (2); the vibration change amount A is proportional to the intensity of the light beam entering the second photoacoustic cell (3); based on the ratio of the demodulated vibration change amount A to the reference vibration amplitude A0, the system constant of the second photoacoustic cell (3) is normalized, and the concentration C of the to-be-tested gas in the first photoacoustic cell (2) is calculated according to the Beer-Lambert law T = -1 / (α T ×L1) × ln(A / A0), wherein α T is the molar absorption cross section of the to-be-tested gas; L1 is the effective action length of the first photoacoustic cell (2).

3. The method of non-contact photoacoustic gas sensing detection according to claim 2, wherein, The gas to be measured and the reference gas have similar absorption spectral lines, and the absorption spectral lines of the two are similar and do not interfere with each other, including but not limited to ammonia and acetylene combination absorbing in the 1530-1532 nm waveband.

4. The method of non-contact photoacoustic gas sensing detection according to claim 2, wherein, The system constant includes the comprehensive gain coefficient of the light source power, the photoacoustic cell geometric design, the light beam intensity, and the cantilever beam response, which is normalized and eliminated by the contrast value A / A0.

5. The method of non-contact photoacoustic gas sensing detection according to claim 2, wherein, The light beam emitted by the tunable excitation light source (1) is transmitted by an optical fiber or a free space optical path for long-distance transmission, so that the first photoacoustic cell (2) is placed at a long distance as needed, thereby realizing the measurement of the non-contact long-distance gas to be measured.

6. The method of non-contact photoacoustic gas sensing detection according to claim 2, wherein, The second photoacoustic cell (3) is continuously introduced with a reference gas with a known concentration to ensure that the concentration of the reference gas inside is always constant.

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