A photoacoustic gas sensing system and measurement method based on T-type differential Helmholtz structure

Abstract

The present invention relates to a photoacoustic gas sensing system and measurement method based on a T-type differential Helmholtz structure. The system includes: a laser emission module for emitting laser light; a gas mixing module for preparing C₂H₂ gas of varying concentrations; an MTRDH photoacoustic module for storing C₂H₂ gas within a cavity based on the Helmholtz structure of the MTRDH photoacoustic cell. When the C₂H₂ gas absorbs the laser light, it generates an anti-phase photoacoustic signal. Combined with the T-type photoacoustic cell structure, the acoustic pressure is concentrated at the end of the resonant tube; and a gas detection module for extracting a 2f signal related to the gas concentration based on the acoustic pressure, and using the 2f signal for concentration detection. The present invention can achieve gas detection with concentrated acoustic pressure distribution, improved signal-to-noise ratio (SNR), and reduced noise-to-noise ratio (NNEA).

Description

Technical Field

[0001] The present invention relates to the technical field of industrial gas detection, and in particular to a photoacoustic gas sensing system and a measurement method based on a T-type differential Helmholtz structure. Background Art

[0002] Acetylene is a common chemical raw material used in industrial production, used in the manufacture of polyethylene, propylene, and other chemicals. It is also an important solvent, used to extract and dissolve certain gases. Acetylene is also a flammable and explosive gas. When mixed with oxygen in air within a certain range, it can form an explosive mixture. Acetylene has a very wide explosion limit, ranging from 2.1% to 80% (volume ratio), which increases the danger of its leakage. Therefore, there is an urgent need to develop a highly sensitive C2H2 gas detection sensor in industrial production.

[0003] Common gas detection methods include semiconductor and catalytic combustion. However, electrochemical sensors are more susceptible to poisoning and contamination than other sensors, and catalytic combustion sensors can only detect combustible gases. In recent years, spectral sensors have been widely used in gas concentration detection due to their high selectivity, high sensitivity, and non-invasiveness. PAS technology is one of the main technologies for gas detection. It is an indirect detection technology. Gases of a certain concentration have a high absorption of light of a specific wavelength, and detection is achieved by measuring the acoustic wave signal generated by the photoacoustic effect. Photoacoustic spectroscopy also has the advantages of low gas sample requirements, ability to detect multiple gas components, fast response time, and high selectivity.

[0004] Currently, the acoustic structures of photoacoustic spectroscopy (PAS) sensors are classified into T-type cells and Helmholtz cells. Methods for enhancing the photoacoustic signal include optimizing the cell's buffer and resonant cavity structures, increasing optical power, and increasing the effective optical path. Currently, the design of photoacoustic cells focuses on optimizing a single structure, with limited research on the integration of different structures.

[0005] T-shaped photoacoustic cells are characterized by their small size and concentrated acoustic pressure distribution. While the aforementioned method for enhancing the photoacoustic signal in a T-shaped cell has shown some performance improvements, further improvement in the signal-to-noise ratio (SNR) is difficult due to the high airflow noise associated with the cell. Compared to a T-shaped cell, the differential structure of a Helmholtz cell can amplify the photoacoustic signal. However, the acoustic pressure distribution at the resonant frequency is not concentrated, and the airflow noise is relatively high in a single-connected tube configuration. This significantly reduces the SNR in low-concentration gas detection, limiting significant sensitivity improvements.

[0006] Regarding the design of differential Helmholtz sensors using photoacoustic spectroscopy, existing technical solutions have proposed a differential Helmholtz photoacoustic cell (DHPAC) based on multiple reflection enhancement as a photoacoustic spectroscopy sensor. A window is installed on the wall of one of the resonant cavities of the Helmholtz photoacoustic cell. A pair of prisms are placed at each end of the window, causing infrared light to reflect back and forth four times, quadrupling the effective optical path length. The 2f signal is enhanced by 3.71 times compared to a single optical path length. The MDL of this PAS sensor is 14 ppb. Regarding the design of T-type photoacoustic sensors, existing technical solutions have developed a T-type photoacoustic cell. Gold-plated covers are used at both ends of the cell body instead of traditional windows. Fiber optic collimators are embedded in the covers to cause the light beam to reflect multiple times within the absorption cell, increasing the equivalent absorption path of the sample gas. Under the conditions of a phase-locked integration time of 1 s and a single detection time of 5 s, the proposed method improves the signal-to-noise ratio (SNR) by 14.6 times. However, the above design cannot simultaneously achieve the gas detection of exponential enhancement of the photoacoustic signal, centralized sound pressure distribution, and suppression of common-mode noise. Moreover, the T-type photoacoustic cell has large airflow noise, making it difficult to improve the SNR by optimizing the structure of the buffer cavity and resonant cavity of the photoacoustic cell, increasing the optical power, increasing the effective optical path, etc. The sound pressure distribution at the resonant frequency of the Helmholtz photoacoustic cell is not centralized, and the airflow noise is relatively large in the form of a single connecting tube. The SNR is greatly reduced in low-concentration gas detection, limiting the significant improvement in sensitivity.

[0007] Optimizing the structure of the photoacoustic cell's buffer and resonant cavities only slightly increases the photoacoustic signal and improves the quality factor. This optimization process also results in a T-shaped cell that is too large. The sound pressure distribution in a Helmholtz cell is more dispersed, failing to fully exploit the unique advantages of a structured photoacoustic sensor. Existing Helmholtz cells employ a single connecting tube, resulting in a non-concentrated sound pressure distribution and ineffective suppression of airflow noise.

[0008] Currently, the sensitivity of photoacoustic spectroscopy can be improved by increasing optical power. Using fiber amplifiers is the most common method. Through external amplification, the output power of the laser can be significantly increased. However, the applicable wavelength range of fiber amplifiers is only a small part of the near-infrared wavelength range, making this method limited and unsuitable.

[0009] Increasing the effective optical path can also improve the photoacoustic signal. Existing T-shaped photoacoustic cells use a gold-plated cover plate to cause multiple reflections of light within the buffer cavity, increasing the path for gas absorption. However, contact-type reflections within the cell and absorption of light by the material can lead to solid-state photoacoustic effects, generating noise that reduces the system's signal-to-noise ratio (SNR) and increases the noise-to-noise ratio (NNEA). Therefore, improving the photoacoustic signal through contact-type reflections within the cell is currently ineffective. Summary of the Invention

[0010] In order to solve the problems existing in the above-mentioned prior art, the purpose of the present invention is to propose a photoacoustic gas sensing system and measurement method based on a T-type differential Helmholtz structure. While achieving exponential enhancement of the photoacoustic signal, the sound pressure distribution is concentrated at the end of the resonant cavity, and the dual-connected tube structure significantly suppresses common-mode noise.

[0011] To achieve the above object, the present invention provides the following solutions:

[0012] A photoacoustic gas sensing system based on a T-type differential Helmholtz structure, comprising:

[0013] A laser emission module, used for emitting laser;

[0014] Gas mixing module, used to prepare C2H2 gas with different concentrations;

[0015] An MTRDH photoacoustic module is used to store the C2H2 gas in a cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C2H2 gas absorbs the laser, it generates an anti-phase photoacoustic signal, and combines the T-type photoacoustic cell structure to concentrate the sound pressure at the end of the resonant tube;

[0016] The gas detection module is used to extract a 2f signal related to the gas concentration based on the sound pressure, and use the 2f signal to perform concentration detection.

[0017] Optionally, the laser emission module includes:

[0018] The signal generating unit is used to generate a low-frequency triangle wave and two high-frequency sine waves with the same frequency, modulate one high-frequency sine wave with one low-frequency triangle wave to form a driving signal, and use the other high-frequency sine wave to form a reference signal;

[0019] A current and temperature control driving unit, configured to receive the driving signal and drive the laser emitting unit;

[0020] The laser emitting unit is used for emitting laser.

[0021] Optionally, the MTRDH photoacoustic module includes:

[0022] The MTRDH photoacoustic unit stores the C2H2 gas in the cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C2H2 gas absorbs the laser, it generates an anti-phase photoacoustic signal, and combines the T-type photoacoustic cell structure to concentrate the sound pressure at the end of the resonant tube;

[0023] The vacuum pump unit is used to discharge the exhaust gas generated in the MTRDH photoacoustic unit along the gas outlet.

[0024] Optionally, the MTRDH photoacoustic cell includes: a first buffer cavity, a second buffer cavity is arranged at a position parallel to the first buffer cavity, for storing the C2H2 gas, a first window is installed at both ends of the first buffer cavity and the second buffer cavity, a high-reflectivity right-angle prism is installed outside the first window to reflect the laser back and forth, so that the C2H2 gas is absorbed multiple optical paths, the first buffer cavity and the second buffer cavity are connected by a first double-connected tube structure, and the maximum value of the acoustic standing wave is transferred to the tube belly of the buffer cavity, the second double-connected tube includes: a first connecting tube and a second connecting tube, a first air inlet and a first air outlet are designed in the middle of the first connecting tube and the second connecting tube, for diverting the airflow equally into two different buffer tubes to form common-mode noise, and a first resonant tube and a second resonant tube are respectively arranged in the first buffer cavity and the second buffer cavity to form a T-type photoacoustic cell structure to concentrate the sound pressure at the ends of the first resonant tube and the second resonant tube.

[0025] Optionally, the MTRDH photoacoustic cell further comprises: a third buffer cavity, a fourth buffer cavity is provided at a position parallel to the third buffer cavity for storing the C2H2 gas, a second window is provided at both ends of the third buffer cavity and the fourth buffer cavity, a chopper with a reflector is provided at a preset angle outside the second window of the third buffer cavity, a reflector is provided outside the second window of the fourth buffer cavity, and the chopper with the reflector is driven to rotate, so as to convert the laser light passing through the third buffer cavity into a modulated laser light of a target frequency, and reflect the laser light that does not pass through the third buffer cavity to the fourth buffer cavity via the reflector to generate a laser beam. A modulated laser with the same target frequency is generated, and the third buffer cavity and the fourth buffer cavity are connected by a second double-connected tube structure to transfer the maximum value of the acoustic standing wave to the tube belly of the buffer cavity. The second double-connected tube includes: a third connecting tube and a fourth connecting tube. A second air inlet and a second air outlet are designed in the middle of the third connecting tube and the fourth connecting tube to equally divide the airflow into two different buffer tubes to form common-mode noise. A third resonant tube and a fourth resonant tube are respectively provided in the third buffer cavity and the fourth buffer cavity to form a T-type photoacoustic pool structure to concentrate the sound pressure at the ends of the third resonant tube and the fourth resonant tube.

[0026] Optionally, the MTRDH photoacoustic cell further comprises: a fifth buffer cavity, a sixth buffer cavity is provided at a position parallel to the fifth buffer cavity for storing the C2H2 gas, a third window is provided at both ends of the fifth buffer cavity and the sixth buffer cavity, a target chopper is provided outside the second window in a direction perpendicular to the buffer cavity, and the rotation speed of the target chopper is controlled so that when the first laser passes into the fifth buffer cavity, the second laser is projected onto the target chopper, and when the second laser passes into the sixth buffer cavity, the first laser is projected onto the target chopper, and the alternating frequency is set so that the laser cross The sound is passed into two buffer chambers alternately, and the fifth buffer chamber and the sixth buffer chamber are connected by a third double-connected tube structure to transfer the maximum value of the standing acoustic wave to the tube belly of the buffer chamber. The third double-connected tube includes: a fifth connecting tube and a sixth connecting tube. A third air inlet and a third air outlet are designed in the middle of the fifth connecting tube and the sixth connecting tube to divert the airflow into two different buffer tubes in equal amounts to form common-mode noise. A fifth resonant tube and a sixth resonant tube are respectively provided in the fifth buffer chamber and the sixth buffer chamber to form a T-type photoacoustic pool structure to concentrate the sound pressure at the ends of the fifth resonant tube and the sixth resonant tube.

[0027] Optionally, the gas detection module includes:

[0028] a differential microphone unit, configured to receive the sound pressure;

[0029] a subtraction circuit unit, configured to calculate the sound pressure;

[0030] a phase-locked amplifier unit, configured to perform frequency locking using a reference signal and extract the 2f signal based on a calculation result;

[0031] A DAQ data acquisition unit, configured to acquire the 2f signal;

[0032] A Labview monitoring unit is used to perform concentration detection using the 2f signal.

[0033] To achieve the above objectives, the present invention further provides a photoacoustic gas measurement method based on a T-type differential Helmholtz structure, comprising:

[0034] preparing C2H2 gases of different concentrations, and storing the C2H2 gases in a cavity based on a Helmholtz structure in an MTRDH photoacoustic cell;

[0035] Emitting laser light, when the C2H2 gas absorbs the laser light, it generates an anti-phase photoacoustic signal, and combines the T-type photoacoustic cell structure to concentrate the sound pressure at the end of the resonant tube;

[0036] A 2f signal related to the gas concentration is extracted according to the sound pressure, and the gas concentration data is acquired using the 2f signal.

[0037] The beneficial effects of the present invention are:

[0038] This invention combines the advantages of both T-shaped and Helmholtz photoacoustic cells, concentrating the sound pressure distribution at the resonant frequency at the end of the resonant tube. The differential dual-connected tube structure significantly suppresses common-mode noise and further enhances the photoacoustic signal. Two right-angle prisms are used to achieve multiple reflections of the laser light, increasing the optical path and boosting the photoacoustic signal. Compared to conventional T-shaped and Helmholtz photoacoustic sensors, this sensor has higher sensitivity and SNR, and a lower normalized noise equivalent absorption coefficient (NNEA).

[0039] The MTRDH photoacoustic cell of the present invention is a photoacoustic cell with a new structure obtained by combining a T-type photoacoustic cell and a Helmholtz photoacoustic cell. Two resonant tubes are added to the structure of the Helmholtz photoacoustic cell to form a double-T-shaped structure. A right-angle prism with a reflectivity of 94% is installed to achieve multiple reflections of the laser, reducing airflow noise, significantly increasing the photoacoustic signal, greatly improving the system's SNR and detection sensitivity, and further reducing NNEA.

[0040] This method leverages the signal amplification mechanism of anti-phase light in an MTRDH photoacoustic cell. Unlike traditional photoacoustic signal amplification methods, two light beams of the same frequency but opposite phases are introduced into the two buffer chambers of the MTRDH photoacoustic cell, generating two in-phase photoacoustic signals. These two signals are then superimposed and detected by a differential microphone. This method can exponentially amplify the photoacoustic signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0042] Figure 1 Schematic diagram of a photoacoustic gas sensing system based on a T-type differential Helmholtz structure according to an embodiment of the present invention;

[0043] Figure 2 Schematic diagram of the MTRDH photoacoustic cell principle according to an embodiment of the present invention;

[0044] Figure 3 Schematic diagram of the first principle of inverse photomechanical modulation based on the MTRDH photoacoustic cell according to an embodiment of the present invention;

[0045] Figure 4 Schematic diagram of the second principle of inverse photomechanical modulation based on the MTRDH photoacoustic cell according to an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0047] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0048] like Figure 1 As shown, a photoacoustic gas sensing system based on a T-type differential Helmholtz structure includes: a laser emission module for emitting laser light; a gas mixing module for preparing C2H2 gas of different concentrations; an MTRDH photoacoustic module for storing C2H2 gas in a cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C2H2 gas absorbs the laser light, it generates an anti-phase photoacoustic signal, and combines the T-type photoacoustic cell structure to concentrate the sound pressure at the end of the resonant tube; and a gas detection module for extracting a 2f signal related to the gas concentration based on the sound pressure, and using the 2f signal to detect the concentration.

[0049] Furthermore, the laser emission module includes: a signal generating unit, used to generate a low-frequency triangular wave and two high-frequency sine waves with the same frequency, modulate one high-frequency sine wave into a low-frequency triangular wave to form a driving signal, and use another high-frequency sine wave to form a reference signal; a current and temperature control driving unit, used to receive the driving signal to drive the laser emission unit; and a laser emission unit, used to emit laser.

[0050] Specifically, the sensor detection system first uses a signal generator to generate a low-frequency triangle wave and a high-frequency sine wave as the drive signal and reference signal. The current and temperature control drive unit includes a current driver and a temperature controller. The laser emission unit uses a C2H2 laser, which is driven by the current driver and temperature controller. The reference signal is fed into a phase-locked amplifier for frequency locking. The C2H2 laser output optical power used is approximately 17mW.

[0051] The low frequency is generally below 1Hz, as the main function of the low-frequency triangle wave is to sweep the second harmonic of the absorption peak. The sweep frequency should not be too fast, as this will cause overlapping and crosstalk of the second harmonic. The high frequency is generally above 500Hz, as the resonant frequency of the photoacoustic cell is generally 1000Hz and above. Therefore, the second harmonic frequency of the high-frequency sine wave should be above 1000Hz.

[0052] Furthermore, the MTRDH photoacoustic module includes: an MTRDH photoacoustic unit, which stores C2H2 gas in the cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C2H2 gas absorbs the laser, it generates an inverse photoacoustic signal, and combines the T-type photoacoustic cell structure to concentrate the sound pressure at the end of the resonant tube; a vacuum pump unit, which is used to discharge the exhaust gas generated in the MTRDH photoacoustic unit along the outlet; among them, the multi-pass T-type-resonance enhanced differential Helmholtz (MTRDH) is a new type of photoacoustic cell that combines T-type and Helmholtz photoacoustic cells.

[0053] Furthermore, the MTRDH photoacoustic cell includes: a first buffer cavity, a second buffer cavity is arranged at a position parallel to the first buffer cavity, for storing C2H2 gas, a first window is installed at both ends of the first buffer cavity and the second buffer cavity, a high-reflectivity right-angle prism is installed outside the first window to reflect the laser back and forth, so that the C2H2 gas is absorbed multiple times of the optical path, and the first buffer cavity and the second buffer cavity are connected by a first double-connected tube structure to transfer the maximum value of the acoustic standing wave to the tube belly of the buffer cavity. The second double-connected tube includes: a first connecting tube and a second connecting tube, a first air inlet and a first air outlet are designed in the middle of the first connecting tube and the second connecting tube, for diverting the airflow equally into two different buffer tubes to form common-mode noise, and a first resonant tube and a second resonant tube are respectively arranged in the first buffer cavity and the second buffer cavity to form a T-type photoacoustic cell structure to concentrate the sound pressure at the ends of the first resonant tube and the second resonant tube.

[0054] Specifically, the MTRDH photoacoustic cell in the present invention is different from the traditional PA absorption cell. It generates two anti-phase photoacoustic signals through the Helmholtz structure, and combines the characteristics of the T-type photoacoustic cell structure to concentrate the sound pressure at the end of the resonant tube. In the designed MTRDH photoacoustic cell, the laser only passes through one of the buffer cavities, and the other buffer cavity serves as a reference. Two windows are installed at both ends of the buffer cavity, and the transmittance is greater than 90%. Two high-reflectivity right-angle prisms with a reflectivity of 94% are installed at both ends outside the window. The laser is reflected back and forth four times between the two right-angle prisms to achieve four times the optical path absorption of the gas, which is used to detect C2H2 with a concentration of 0 to 2000 ppm. The principle diagram of the MTRDH photoacoustic cell is shown as follows. Figure 2 As shown. The MTRDH photoacoustic cell consists of two T-shaped photoacoustic cells of the same size connected by two connecting tubes. A differential microphone is installed at the resonant tube of each T-shaped photoacoustic cell. The specially designed dual connecting tube structure can also transfer the maximum value of the acoustic standing wave to the belly of the buffer tube. In addition, the air inlet and outlet are designed in the middle of the two connecting tubes, so the air flow is equally diverted into two different buffer tubes, forming common-mode noise. The Helmholtz differential structure can generate acoustic signals with opposite phases. Sending the signal collected by the microphone to the subtraction circuit can eliminate common-mode noise, thereby greatly improving the sensitivity of photoacoustic spectroscopy detection.

[0055] The PA unit was modeled and simulated using COMSOL Multiphysics software. The magnitude of the airflow noise was determined by simulating the gas flow velocity at the resonance point of the photoacoustic cell. The normal inflow velocity at the air inlet was set to 2 m / s, the turbulence intensity IT was set to 0.05, and the time was set to 20 s. The airflow velocity at the resonance point of the traditional Helmholtz double-tube photoacoustic cell was stable at 0.019 m / s. The airflow velocity at the resonance point of the MTRDH photoacoustic cell of the present invention was greatly reduced and stabilized at 1.1 × 10 -7 m / s, thus verifying that the designed MTRDH photoacoustic cell has the characteristic of lower airflow noise. The resonant frequency and sound pressure at the resonance point of the photoacoustic cell were simulated, and the frequency domain sound pressure analysis was performed in the range of 1100-1500Hz. The resonant frequency of the traditional double-connected tube Helmholtz photoacoustic cell is 1361.6Hz, and the absolute sound pressure is 10 8 The MTRDH photoacoustic cell is embedded in the structure of a T-type photoacoustic cell. Due to the change in volume, the resonant frequency is slightly reduced to 1341.1Hz, but the absolute sound pressure value is increased by nearly an order of magnitude to 10 9 order of magnitude.

[0056] The performance of the MTRDH photoacoustic cell was experimentally verified. A dynamic gas distribution method using a gas mixing system was used to prepare gas samples. C₂H₂ concentrations ranging from 0 to 2000 ppm were introduced into the MTRDH photoacoustic cell, and C₂H₂ concentrations were measured continuously for 50 seconds at each concentration. When the sensor system was configured for single-ended acoustic pressure output, the standard deviation noise was 1.03 mV, resulting in a calculated SNR of 2602 and a corresponding minimum detection limit (MDL) of 769 ppb. When the sensor system was configured without a prism, it behaved as a single-pass PA photoacoustic cell. Observing the 2f signal waveform at 2000 ppm of C₂H₂, the waveform amplitude was 1.76 V. When a prism was added, it behaved as a multi-pass PA photoacoustic cell, with a waveform amplitude of 5.6 V. The calculated standard deviation noise was 0.29 mV, resulting in a calculated SNR of 19310 and a corresponding MDL of 105 ppb.

[0057] Therefore, compared with the PA photoacoustic cell with single-ended output, the PA signal of the MTRDH photoacoustic cell is the superposition of the signals output at both ends. The SNR is increased by 7.4 times compared to the single-ended output, and the detection limit is lower. It can be seen that the superiority of the MTRDH photoacoustic cell used in the present invention. When the system noise is detected under the condition of passing pure nitrogen, it is found that the standard deviations of the signals measured by the two microphones are σ1=1.28mV and σ2=1.03mV, and the standard deviation of the differential signal obtained after signal subtraction is σ3=0.29mV. It can be seen that the differential structure of the MTRDH photoacoustic cell designed by the present invention has a strong anti-noise ability; wherein, photoacoustic (PA) is when a pulsed laser is irradiated into a gas sample, the gas pressure will produce periodic expansion and contraction, thereby generating an acoustic signal, and this acoustic signal generated by light excitation is called a photoacoustic signal.

[0058] The following is a performance comparison (including SNR, LoD, NNEA, etc.) between the MTRDH photoacoustic cell designed in Table 1 and existing common types of photoacoustic cells. The MTRDH photoacoustic cell has unique advantages in SNR, LoD, and NNEA under the same configuration. The SNR of the MTRDH photoacoustic cell is higher than that of all traditional types of photoacoustic cells, the LoD is lower than that of non-resonant photoacoustic cells, and the NNEA is lower than that of all traditional types of photoacoustic cells, which expands its application potential in various fields; among them, photoacoustic spectroscopy (PAS) is a new spectral analysis and detection technology based on the photoacoustic effect. The signal-to-noise ratio (SNR) is an indicator that measures the ratio of signal intensity to background noise intensity. The limit of detection (LoD) is the lowest detectable concentration that can be distinguished from noise when the sample is extracted, processed, and detected according to the requirements of the analytical method. The normalized noise equivalent absorption coefficient (NNEA) is an important parameter that characterizes the detection sensitivity of the sensor.

[0059] Table 1

[0060]

[0061] Furthermore, the MTRDH photoacoustic cell includes: a third buffer cavity, a fourth buffer cavity is arranged at a position parallel to the third buffer cavity for storing C2H2 gas, a second window is installed at both ends of the third buffer cavity and the fourth buffer cavity, a chopper with a reflector is installed at a preset angle outside the second window of the third buffer cavity, a reflector is arranged outside the second window of the fourth buffer cavity, and the chopper with the reflector is driven to rotate to convert the laser light passing through the third buffer cavity into a modulated laser light of a target frequency, and the laser light that does not pass through the third buffer cavity is reflected to the fourth buffer cavity via the reflector to generate a phase shift. A modulated laser with the same target frequency uses a second double-connected tube structure to connect the third buffer cavity and the fourth buffer cavity, transferring the maximum value of the acoustic standing wave to the tube belly of the buffer cavity. The second double-connected tube includes: a third connecting tube and a fourth connecting tube. A second air inlet and a second air outlet are designed in the middle of the third connecting tube and the fourth connecting tube, which divide the airflow into two different buffer tubes in equal amounts to form common-mode noise. A third resonant tube and a fourth resonant tube are respectively set in the third buffer cavity and the fourth buffer cavity to form a T-shaped photoacoustic pool structure to concentrate the sound pressure at the ends of the third resonant tube and the fourth resonant tube.

[0062] Furthermore, the MTRDH photoacoustic cell includes: a fifth buffer cavity, a sixth buffer cavity is arranged at a position parallel to the fifth buffer cavity for storing C2H2 gas, a third window is installed at both ends of the fifth buffer cavity and the sixth buffer cavity, a target chopper is arranged outside the second window in a direction perpendicular to the buffer cavity, and the rotation speed of the target chopper is controlled so that when the first laser passes into the fifth buffer cavity, the second laser is shot at the target chopper, and when the second laser passes into the sixth buffer cavity, the first laser is shot at the target chopper, and the alternating frequency is set so that the laser is alternately passed into the two buffers. In the cavity, a third double-connected tube structure is used to connect the fifth buffer cavity and the sixth buffer cavity to transfer the maximum value of the acoustic standing wave to the tube belly of the buffer cavity. The third double-connected tube includes: a fifth connecting tube and a sixth connecting tube. A third air inlet and a third air outlet are designed in the middle of the fifth connecting tube and the sixth connecting tube to divert the airflow into two different buffer tubes in equal amounts to form common-mode noise. A fifth resonant tube and a sixth resonant tube are respectively arranged in the fifth buffer cavity and the sixth buffer cavity to form a T-type photoacoustic pool structure to concentrate the sound pressure at the ends of the fifth resonant tube and the sixth resonant tube.

[0063] Specifically, the MTRDH photoacoustic cell designed in the present invention can also utilize modulated antiphase light to further enhance the intensity of the photoacoustic signal. Antiphase light refers to two beams of light with a 180° phase difference. The MTRDH photoacoustic cell comprises two symmetrical upper and lower T-shaped photoacoustic cells, each with a buffer cavity equipped with a window with a transmittance greater than 90% at each end. When laser light is passed into only one of the buffer cavities, photoacoustic signals with opposite phases are generated in the upper and lower T-shaped photoacoustic cells. When laser light is passed into the other buffer cavity, that is, into both buffer cavities simultaneously, if the two laser beams have the same phase, the photoacoustic signals generated by the gas stimulated by the two laser beams will cancel each other out; if the two laser beams have opposite phases, the photoacoustic signals generated by the gas stimulated by the two laser beams will superimpose each other. Adjusting the phase of light can be achieved through physical modulation and optical interference techniques. Common physical modulations include electro-optical modulation and acousto-optic modulation. In the MTRDH photoacoustic cell, the antiphase light generated by these modulation techniques is simultaneously introduced into two different buffer cavities, both of which can generate in-phase photoacoustic signals.

[0064] The present invention introduces two methods of mechanically modulating reversed-phase light, which are introduced into the developed MTRDH photoacoustic cell to enhance the photoacoustic signal. Figure 3 and Figure 4 As shown. The first method is to load a chopper with a reflector at the laser incident point, with an angle of 45° to the window, and use a motor to drive the chopper to rotate. A reflector is installed on the left side of the second buffer cavity, also at an angle of 45° to the window. The light entering the first buffer cavity is converted into a modulated laser with a frequency of 682Hz after passing through the chopper. The laser that does not pass through the chopper is reflected by the reflector on the chopper to the reflector on the left side of the second buffer cavity, and then injected into the second buffer cavity through the reflector, with a modulation frequency of 682Hz. The anti-phase light injected alternately at 682Hz generates an in-phase photoacoustic signal in the MTRDH photoacoustic cell. After the two signals are superimposed, they can be detected by a differential microphone. This method can increase the photoacoustic signal to twice the original, while the noise after superposition is only 1.414 times the original, further improving the SNR of the system.

[0065] The second method is to load a large enough chopper blade at the laser incident point and place it parallel to the window, such as Figure 4As shown, a motor drives the chopper blade. Lasers of identical wavelength and power are introduced into the photoacoustic cell. The position of the chopper blade is adjusted so that when laser 1 enters the cell, laser 2 strikes the chopper blade; and when laser 2 enters the cell, laser 1 strikes the chopper blade, thereby achieving alternating laser incidence in the two buffer cavities. The chopper blade's rotational speed is controlled to maintain an alternating laser injection frequency of 682 Hz. After mechanical modulation, the anti-phase light injected at 682 Hz can also generate in-phase photoacoustic signals in the MTRDH photoacoustic cell. Superposition can double the photoacoustic signal, while the noise superposition is only 1.414 times the original, further improving the system's signal-to-noise ratio (SNR).

[0066] Furthermore, the gas detection module includes: a differential microphone unit for receiving sound pressure; a subtraction circuit unit for calculating the sound pressure; a phase-locked amplifier unit for locking the frequency using a reference signal and extracting a 2f signal based on the calculation result; a DAQ data acquisition unit for collecting the 2f signal; and a Labview monitoring unit for using the 2f signal for concentration detection.

[0067] Because this embodiment uses a Helmholtz structure, the sound pressure signals detected by the two MEMS microphones are in opposite phases. The subtraction circuit mainly performs a subtraction operation on the signals detected by the two microphones through an operational amplifier circuit to achieve phase subtraction of the differential signals.

[0068] This embodiment discloses a photoacoustic gas sensing system based on a T-type differential Helmholtz structure, including: a laser, a signal generator, a T-type differential Helmholtz photoacoustic cell, a right-angle prism (15mm×15mm×15mm) and a right-angle prism (5mm×5mm×5mm), a subtraction circuit, a lock-in amplifier, a data acquisition card, a current driver, a temperature controller, and a Labview host computer. A schematic diagram of the gas sensor detection system is shown in FIG. Figure 1 shown.

[0069] The sensor detection system of the present invention first generates a low-frequency triangle wave and a high-frequency sine wave as a driving signal and a reference signal through a signal generator, drives a C2H2 laser through a current driver and a temperature controller, and the reference signal is sent to a phase-locked amplifier for frequency locking. The C2H2 laser used has an output light power of about 17mW. After the laser is injected into the photoacoustic cell, it is reflected four times by two right-angle prisms. Two differential microphones are used to obtain the sound signal, which is sent to the phase-locked amplifier after a subtraction circuit operation. The phase-locked amplifier extracts the 2f signal related to the gas concentration and sends it to the Labview monitoring platform embedded in a personal computer (PC). C2H2 gas of different concentrations is prepared by a gas mixing system using standard C2H2 gas and pure nitrogen (N2). An air inlet and an air outlet are respectively set in the middle of the two connecting pipes to realize dynamic measurement of the gas.

[0070] The proposed sensor utilizes a dual-connected tube structure, shifting the maximum sound pressure to the center of the buffer chamber. The T-shaped design allows for more concentrated sound pressure distribution within the two cavities, concentrating it at the end of the resonant tube. A microphone then collects the acoustic signal at this point, significantly enhancing the photoacoustic signal. The newly proposed mechanism of mechanically modulating antiphase light in an MTRDH photoacoustic cell multiplies the photoacoustic signal without altering the original Helmholtz structure.

[0071] In this invention, the dual-connector structure, with air inlets and outlets installed on the connecting pipes, generates symmetrical airflow, which in turn generates common-mode noise. The differential structure of the Helmholtz photoacoustic cell precisely eliminates this common-mode noise, thus resolving the drawback of the T-type photoacoustic cell, which produces high airflow noise. While suppressing noise, the photoacoustic signal output by the MTRDH photoacoustic cell is equivalent to the superposition of two T-type photoacoustic cells, significantly improving the system's SNR and further reducing the NNEA to only 1.98×10 -9 .

[0072] In summary, the sensor developed by the present invention has the advantages of low noise and strong signal, and is suitable for some special scenarios in industrial production.

[0073] This embodiment also discloses a photoacoustic gas measurement method based on a T-type differential Helmholtz structure, comprising: preparing C2H2 gas of different concentrations, storing the C2H2 gas in a cavity based on the Helmholtz structure in an MTRDH photoacoustic cell; emitting a laser, and when the C2H2 gas absorbs the laser, generating an anti-phase photoacoustic signal, and combining the T-type photoacoustic cell structure to concentrate the sound pressure at the end of the resonant tube; extracting a 2f signal related to the gas concentration based on the sound pressure, and using the 2f signal to obtain gas concentration data.

[0074] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A photoacoustic gas sensing system based on a T-type differential Helmholtz structure, characterized in that: include: A laser emission module, used for emitting laser; Gas mixing module, used to prepare C2H2 gas with different concentrations; An MTRDH photoacoustic module is used to store the C2H2 gas in a cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C2H2 gas absorbs the laser, it generates an anti-phase photoacoustic signal, and combines the T-type photoacoustic cell structure to concentrate the sound pressure at the end of the resonant tube; The MTRDH photoacoustic cell includes: a first buffer cavity, a second buffer cavity is provided at a position parallel to the first buffer cavity for storing the C2H2 gas, first windows are installed at both ends of the first buffer cavity and the second buffer cavity, a high-reflectivity right-angle prism is installed outside the first window to reflect the laser back and forth, so that the C2H2 gas is absorbed by multiple optical paths, the first buffer cavity and the second buffer cavity are connected by a first double-connected tube structure, and the maximum value of the acoustic standing wave is transferred to the tube belly of the buffer cavity, the first double-connected tube includes: a first connecting tube and a second connecting tube, a first air inlet and a first air outlet are designed in the middle of the first connecting tube and the second connecting tube, which are used to divert the airflow into two different buffer tubes in equal amounts to form common-mode noise, and a first resonant tube and a second resonant tube are respectively provided in the first buffer cavity and the second buffer cavity to form a T-shaped photoacoustic cell structure to concentrate the sound pressure at the ends of the first resonant tube and the second resonant tube; The MTRDH photoacoustic cell further includes: a third buffer cavity, a fourth buffer cavity is provided at a position parallel to the third buffer cavity for storing the C2H2 gas, a second window is provided at both ends of the third buffer cavity and the fourth buffer cavity, a chopper with a reflector is provided at a preset angle outside the second window of the third buffer cavity, a reflector is provided outside the second window of the fourth buffer cavity, and the chopper with the reflector is driven to rotate to convert the laser light passing through the third buffer cavity into a modulated laser light of a target frequency, and the laser light that does not pass through the third buffer cavity is reflected by the reflector to the fourth buffer cavity to generate a phase shift. A modulated laser with the same target frequency is used, and a second double-connected tube structure is used to connect the third buffer cavity and the fourth buffer cavity, thereby transferring the maximum value of the acoustic standing wave to the belly of the buffer cavity. The second double-connected tube includes a third connecting tube and a fourth connecting tube. A second air inlet and a second air outlet are designed in the middle of the third and fourth connecting tubes to equally divide the airflow into two different buffer tubes to form common-mode noise. A third resonant tube and a fourth resonant tube are respectively provided in the third and fourth buffer cavities to form a T-shaped photoacoustic pool structure, thereby concentrating the sound pressure at the ends of the third and fourth resonant tubes. The gas detection module is used to extract a 2f signal related to the gas concentration based on the sound pressure, and obtain gas concentration data using the 2f signal.

2. The photoacoustic gas sensing system based on a T-type differential Helmholtz structure according to claim 1, characterized in that: The laser emission module includes: The signal generating unit is used to generate a low-frequency triangle wave and two high-frequency sine waves with the same frequency, modulate one high-frequency sine wave with one low-frequency triangle wave to form a driving signal, and use the other high-frequency sine wave to form a reference signal; A current and temperature control driving unit, configured to receive the driving signal and drive the laser emitting unit; The laser emitting unit is used for emitting laser.

3. The photoacoustic gas sensing system based on a T-type differential Helmholtz structure according to claim 1, characterized in that: The MTRDH photoacoustic module includes: The MTRDH photoacoustic unit stores the C2H2 gas in the cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C2H2 gas absorbs the laser, it generates an anti-phase photoacoustic signal, and combines the T-type photoacoustic cell structure to concentrate the sound pressure at the end of the resonant tube; The vacuum pump unit is used to discharge the exhaust gas generated in the MTRDH photoacoustic unit along the gas outlet.

4. The photoacoustic gas sensing system based on a T-type differential Helmholtz structure according to claim 1, characterized in that: The MTRDH photoacoustic cell further includes: a fifth buffer cavity, a sixth buffer cavity is provided at a position parallel to the fifth buffer cavity for storing the C2H2 gas, a third window is provided at both ends of the fifth buffer cavity and the sixth buffer cavity, a target chopper is provided outside the third window in a direction perpendicular to the buffer cavity, and the rotation speed of the target chopper is controlled. When the first laser passes into the fifth buffer cavity, the second laser is projected onto the target chopper, and when the second laser passes into the sixth buffer cavity, the first laser is projected onto the target chopper, and an alternating frequency is set so that the lasers are alternately passed through. The fifth buffer chamber and the sixth buffer chamber are connected by a third double-connected tube structure to transfer the maximum value of the acoustic standing wave to the tube belly of the buffer chamber. The third double-connected tube includes: a fifth connecting tube and a sixth connecting tube. A third air inlet and a third air outlet are designed in the middle of the fifth connecting tube and the sixth connecting tube to divert the airflow into two different buffer tubes in equal amounts to form common-mode noise. A fifth resonant tube and a sixth resonant tube are respectively provided in the fifth buffer chamber and the sixth buffer chamber to form a T-type photoacoustic pool structure to concentrate the sound pressure at the ends of the fifth resonant tube and the sixth resonant tube.

5. The photoacoustic gas sensing system based on a T-type differential Helmholtz structure according to claim 1, characterized in that: The gas detection module comprises: a differential microphone unit, configured to receive the sound pressure; a subtraction circuit unit, configured to calculate the sound pressure; a phase-locked amplifier unit, configured to perform frequency locking using a reference signal and extract the 2f signal based on a calculation result; A DAQ data acquisition unit, configured to acquire the 2f signal; The Labview monitoring unit is used to obtain the gas concentration data using the 2f signal.

6. A photoacoustic gas measurement method based on a T-type differential Helmholtz structure, using the photoacoustic gas sensing system based on a T-type differential Helmholtz structure according to any one of claims 1 to 5, characterized in that: include: preparing C2H2 gases of different concentrations, and storing the C2H2 gases in a cavity based on a Helmholtz structure in an MTRDH photoacoustic cell; Emitting laser light, when the C2H2 gas absorbs the laser light, it generates an anti-phase photoacoustic signal, and combines the T-type photoacoustic cell structure to concentrate the sound pressure at the end of the resonant tube; A 2f signal related to the gas concentration is extracted according to the sound pressure, and the gas concentration data is acquired using the 2f signal.

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