Photoacoustic gas sensing system based on T-type differential Helmholtz structure and measuring method

Abstract

The invention relates to a photoacoustic gas sensing system based on a T-type differential Helmholtz structure and a measuring method, and the system comprises a laser emission module which is used for emitting laser; the gas mixing module is used for preparing C2H2 gas with different concentrations; the MTRDH photoacoustic module is used for storing C2H2 gas into the cavity on the basis of a Helmholtz structure in an MTRDH photoacoustic cell, after the C2H2 gas absorbs the laser, an inverted photoacoustic signal is generated, and sound pressure is concentrated at the tail end of the resonance tube in combination with a T-shaped photoacoustic cell structure; and the gas detection module is used for extracting a 2f signal related to the gas concentration according to the sound pressure and carrying out concentration detection by utilizing the 2f signal. According to the invention, the gas detection of concentrated sound pressure distribution, improved SNR and reduced NNEA can be realized.

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 commonly used chemical raw material in industrial production, and is used to manufacture polyethylene, propylene, etc. Acetylene is also an important solvent, used for extracting and dissolving certain gases. At the same time, acetylene is a flammable and explosive gas. When it mixes with oxygen in the air within a certain range, it will form an explosive mixture. The explosion limit of acetylene is very wide, being 2.1% - 80% (volume ratio), which increases the danger after its leakage. Therefore, in industrial production, there is an urgent need to develop a highly sensitive C 2 H 2 gas detection sensor.

[0003] Commonly used gas detection methods include semiconductor type and catalytic combustion type. However, compared with other sensors, electrochemical sensors are more vulnerable to poisoning and pollution; catalytic combustion sensors can only detect combustible gases. In recent years, spectroscopic sensors have been widely used in gas concentration detection due to their advantages such as high selectivity, high sensitivity, and non-invasiveness. The PAS technology is one of the main technologies for gas detection. It is an indirect detection technology. A certain concentration of gas has a high absorbance for light of a specific wavelength, and the detection is achieved by measuring the acoustic wave signal generated by the photoacoustic effect. At the same time, photoacoustic spectroscopy has the advantages of low gas sample demand, the ability to detect multiple gas components, fast reaction time, and high selectivity.

[0004] Currently, the acoustic structures of photoacoustic spectroscopy (PAS) sensors are divided into T-type photoacoustic cells, Helmholtz-type photoacoustic cells, and so on. Methods for enhancing photoacoustic signals include optimizing the structures of the buffer cavity and the resonance cavity of the photoacoustic cell, increasing the optical power, increasing the effective optical path, and other methods. Currently, the design of photoacoustic cells focuses more on optimizing one of the structures, and there is less research on the combination of different structures.

[0005] The T-type photoacoustic cell has the characteristics of a small volume and a concentrated sound pressure distribution. By using the above methods to enhance the photoacoustic signal of the T-type photoacoustic cell, although there is a partial performance improvement, due to the large airflow noise of the T-type photoacoustic cell, it is difficult to further improve the signal-to-noise ratio (SNR) through the above methods. Compared with the T-type photoacoustic cell, the differential structure of the Helmholtz photoacoustic cell can increase the photoacoustic signal, but the sound pressure distribution at the resonance frequency is not concentrated, and the airflow noise is relatively large in the form of a single connecting pipe, and the SNR is greatly reduced in the detection of low-concentration gases, which limits the significant improvement of sensitivity.

[0006] In the design of a differential Helmholtz sensor using photoacoustic spectroscopy, the prior art proposed a photoacoustic spectroscopy sensor based on a differential Helmholtz photoacoustic cell (DHPAC) with enhanced multiple reflections. A window plate was installed on the cavity wall of one resonant cavity of the Helmholtz photoacoustic cell, and a pair of prisms were placed at both ends of the window plate to reflect the infrared light back and forth four times, making the effective optical path become 4 times the original. The 2f signal was enhanced by 3.71 times compared to the single optical path, and the MDL of this PAS sensor was 14 ppb. In the design of a T-shaped photoacoustic sensor, the prior art developed a T-shaped photoacoustic cell. Gold-plated covers were used at both ends of the cell body instead of traditional window plates, and fiber collimators were embedded in the covers to make the light beam reflect multiple times in the absorption cell to increase the equivalent absorption path of the sample gas. Under the conditions of a lock-in integration time of 1 s and a single detection time of 5 s, the SNR of the proposed method was increased by 14.6 times. However, the above designs cannot simultaneously achieve multiple enhancement of photoacoustic signals, concentrated sound pressure distribution, and suppression of common-mode noise for gas detection. Moreover, the T-shaped photoacoustic cell has a large airflow noise, and it is difficult to further improve the SNR by optimizing the structures 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 concentrated, and the airflow noise is relatively large in the form of a single connecting pipe, and the SNR is greatly reduced in low-concentration gas detection, limiting the significant improvement of sensitivity.

[0007] Optimizing the structures of the buffer cavity and resonant cavity of the photoacoustic cell can only slightly increase the photoacoustic signal and improve the quality factor, and the optimization process will cause the T-shaped photoacoustic cell to be too large; the sound pressure distribution of the Helmholtz photoacoustic cell is more dispersed, and the unique advantages of the characteristic structure photoacoustic sensor cannot be exerted. The Helmholtz photoacoustic cell proposed by the prior art uses a single connecting pipe, the sound pressure distribution is not concentrated, and the airflow noise cannot be effectively suppressed.

[0008] Currently, the sensitivity of photoacoustic spectroscopy detection can be improved by increasing the optical power. Using a fiber amplifier is one of the most common methods. Through the external amplification method, the output power of the laser can be greatly increased. However, the applicable wavelength range of the fiber amplifier is only a small part of the near-infrared wavelength range, and this method has limitations and low applicability.

[0009] Increasing the effective optical path can also improve the photoacoustic signal. The T-shaped photoacoustic cell developed by the prior art uses the method of gold-plated covers to make the light reflect multiple times in the buffer cavity, increasing the path of the gas-absorbing light. However, the in-cell contact reflection, and the light absorption by the material will bring about a solid photoacoustic effect, thus generating noise and reducing the SNR of the system, and the NNEA will increase accordingly. Therefore, the current implementation of improving the photoacoustic signal through in-cell contact reflection has poor results. Summary of the Invention

[0010] To solve the problems existing in the above-mentioned prior art, the object of the present invention is to propose an optoacoustic gas sensing system and a measurement method based on a T-shaped differential Helmholtz structure, which can double the optoacoustic signal while concentrating the sound pressure distribution at the end of the resonant cavity, and the structure of the double connecting pipes can greatly suppress the common-mode noise.

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

[0012] An optoacoustic gas sensing system based on a T-shaped differential Helmholtz structure, comprising:

[0013] A laser emission module for emitting a laser;

[0014] A gas mixing module for preparing C 2 H 2 gases of different concentrations;

[0015] An MTRDH optoacoustic module for storing the C 2 H 2 gas in the cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C 2 H 2 gas absorbs the laser, it generates out-of-phase optoacoustic signals and concentrates the sound pressure at the end of the resonant tube in combination with the T-shaped photoacoustic cell structure;

[0016] A gas detection module for extracting the 2f signal related to the gas concentration according to the sound pressure and using the 2f signal for concentration detection.

[0017] Optionally, the laser emission module includes:

[0018] A signal generation unit for generating a low-frequency triangular wave and two high-frequency sine waves with the same frequency, modulating one high-frequency sine wave with one low-frequency triangular wave to form a driving signal, and using the other high-frequency sine wave to form a reference signal;

[0019] A current and temperature control driving unit for receiving the driving signal to drive the laser emission unit;

[0020] A laser emission unit for emitting a laser.

[0021] Optionally, the MTRDH optoacoustic module includes:

[0022] An MTRDH optoacoustic unit for storing the C 2 H 2 gas in the cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C 2 H 2After the gas absorbs the laser, an in-phase photoacoustic signal is generated, and the sound pressure is concentrated at the end of the resonance tube by combining with the T-type photoacoustic cell structure;

[0023] A vacuum pump unit for exhausting the waste gas generated in the MTRDH photoacoustic unit along the air outlet.

[0024] Optionally, the MTRDH photoacoustic cell includes: a first buffer chamber, a second buffer chamber is arranged at a parallel position of the first buffer chamber for storing the C 2 H 2 gas. First window plates are installed at both ends of the first buffer chamber and the second buffer chamber. A high-reflectivity right-angle prism is installed outside the first window plate to reflect the laser back and forth, so that the C 2 H 2 gas has a multi-pass absorption. The first buffer chamber and the second buffer chamber are connected by a first double connection pipe structure to transfer the maximum value of the acoustic standing wave to the belly of the buffer chamber. The second double connection pipe includes: a first connection pipe and a second connection pipe. A first air inlet and a first air outlet are designed in the middle of the first connection pipe and the second connection pipe to equally divide the air flow into two different buffer pipes to form a common-mode noise. A first resonance pipe and a second resonance pipe are respectively arranged in the first buffer chamber and the second buffer chamber to form a T-type photoacoustic cell structure to concentrate the sound pressure at the ends of the first resonance pipe and the second resonance pipe.

[0025] Optionally, the MTRDH photoacoustic cell further includes: a third buffer chamber, a fourth buffer chamber is arranged at a parallel position of the third buffer chamber for storing the C 2 H 2 gas. Second window plates are installed at both ends of the third buffer chamber and the fourth buffer chamber. A chopper with a mirror is installed at a preset angle outside the second window plate of the third buffer chamber, and a mirror is arranged outside the second window plate of the fourth buffer chamber. The chopper with the mirror is driven to rotate to convert the laser passing through the third buffer chamber into a modulated laser of a target frequency, and the laser not passing through the third buffer chamber is reflected by the mirror to the fourth buffer chamber to generate a modulated laser of the same target frequency. The third buffer chamber and the fourth buffer chamber are connected by a second double connection pipe structure to transfer the maximum value of the acoustic standing wave to the belly of the buffer chamber. The second double connection pipe includes: a third connection pipe and a fourth connection pipe. A second air inlet and a second air outlet are designed in the middle of the third connection pipe and the fourth connection pipe to equally divide the air flow into two different buffer pipes to form a common-mode noise. A third resonance pipe and a fourth resonance pipe are respectively arranged in the third buffer chamber and the fourth buffer chamber to form a T-type photoacoustic cell structure to concentrate the sound pressure at the ends of the third resonance pipe and the fourth resonance pipe.

[0026] Optionally, the MTRDH photoacoustic cell further includes: a fifth buffer cavity, and a sixth buffer cavity is arranged at a parallel position of the fifth buffer cavity for storing the C 2 H 2 gas. Third window plates are installed at both ends of the fifth buffer cavity and the sixth buffer cavity. A target chopper is arranged in a direction perpendicular to the buffer cavity outside the second window plate. The rotation speed of the target chopper is controlled. When the first laser is introduced into the fifth buffer cavity, the second laser is projected onto the target chopper. When the second laser is introduced into the sixth buffer cavity, the first laser is projected onto the target chopper. An alternating introduction frequency is set so that the lasers are alternately introduced into the two buffer cavities. The fifth buffer cavity and the sixth buffer cavity are connected by a third double connection pipe structure to transfer the maximum value of the acoustic standing wave to the belly of the buffer cavity. The third double connection pipe includes: a fifth connection pipe and a sixth connection pipe. A third air inlet and a third air outlet are designed in the middle of the fifth connection pipe and the sixth connection pipe for equally dividing the air flow into two different buffer pipes to form common-mode noise. Fifth resonance pipes and sixth resonance pipes are respectively arranged in the fifth buffer cavity and the sixth buffer cavity to form a T-shaped photoacoustic cell structure to concentrate the sound pressure at the ends of the fifth resonance pipe and the sixth resonance pipe.

[0027] Optionally, the gas detection module includes:

[0028] A differential microphone unit for receiving the sound pressure;

[0029] A subtraction circuit unit for calculating the sound pressure;

[0030] A lock-in amplifier unit for frequency locking using a reference signal and extracting the 2f signal according to the calculation result;

[0031] A DAQ data acquisition unit for acquiring the 2f signal;

[0032] A Labview monitoring unit for performing concentration detection using the 2f signal.

[0033] To achieve the above object, the present invention also provides a photoacoustic gas measurement method based on a T-shaped differential Helmholtz structure, including:

[0034] Preparing C 2 H 2 gases with different concentrations, and storing the C 2 H 2 gases into the cavity based on the Helmholtz structure in the MTRDH photoacoustic cell;

[0035] Emitting a laser. When the C 2 H 2After the gas absorbs the laser, an inverse photoacoustic signal is generated, and the sound pressure is concentrated at the end of the resonance tube by combining with the T-type photoacoustic cell structure;

[0036] According to the sound pressure, a 2f signal related to the gas concentration is extracted, and the gas concentration data is obtained by using the 2f signal.

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

[0038] The present invention combines the advantages of the T-type photoacoustic cell and the Helmholtz photoacoustic cell, so that the sound pressure distribution is concentrated at the end of the resonance tube at the resonance frequency; the differential double connection tube structure can greatly suppress the common-mode noise and further improve the photoacoustic signal; two right-angled prisms are used to realize multiple reflections of the laser, increasing the optical path to increase the photoacoustic signal. Compared with the traditional T-type photoacoustic sensor and Helmholtz photoacoustic sensor, the sensor of the present invention has higher sensitivity and SNR index, and has 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 the T-type photoacoustic cell and the Helmholtz photoacoustic cell. On the basis of the structure of the Helmholtz photoacoustic cell, two resonance tubes are added to form a double-T structure, and a right-angled prism with a reflectivity of 94% is mounted to realize multiple reflections of the laser, reducing the airflow noise, greatly increasing the photoacoustic signal, greatly improving the SNR and detection sensitivity of the system, and further reducing the NNEA.

[0040] The present invention is based on the mechanism of increasing the signal by the inverse light of the MTRDH photoacoustic cell. Different from the traditional method of enhancing the photoacoustic signal, two beams of light with the same frequency and opposite phases are respectively introduced into the two buffer cavities of the MTRDH photoacoustic cell to generate two in-phase photoacoustic signals. After the two signals are superimposed, they are detected by a differential microphone. This method can double the photoacoustic signal. Description of the Drawings

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0042] Figure 1 It is a 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 It is a schematic diagram of the principle of the MTRDH photoacoustic cell according to an embodiment of the present invention;

[0044] Figure 3Schematic diagram of the first principle of in-phase optomechanical 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 in-phase optomechanical modulation based on the MTRDH photoacoustic cell according to an embodiment of the present invention. Detailed implementation manners

[0046] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0047] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners.

[0048] As Figure 1 shown, a photoacoustic gas sensing system based on a T-shaped differential Helmholtz structure includes: a laser emission module for emitting a laser; a gas mixing module for preparing C 2 H 2 gases with different concentrations; an MTRDH photoacoustic module for storing C 2 H 2 gases into a cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C 2 H 2 gases absorb the laser, an in-phase photoacoustic signal is generated, and the sound pressure is concentrated at the end of the resonant tube in combination with the T-shaped photoacoustic cell structure; a gas detection module for extracting a 2f signal related to the gas concentration according to the sound pressure and using the 2f signal for concentration detection.

[0049] Furthermore, the laser emission module includes: a signal generation unit for generating a low-frequency triangular wave and two high-frequency sine waves with the same frequency, modulating one high-frequency sine wave with one low-frequency triangular wave to form a driving signal, and using the other high-frequency sine wave to form a reference signal; a current and temperature control driving unit for receiving the driving signal to drive the laser emission unit; and a laser emission unit for emitting a laser.

[0050] Specifically, the sensor detection system first generates a low-frequency triangular wave and a high-frequency sine wave as the driving signal and the reference signal through a signal generator. The current and temperature control driving unit includes a current driver and a temperature controller. The laser emission unit uses a C 2 H 2 laser, and drives the C 2 H 2The laser, and the reference signal is sent to a lock-in amplifier for frequency locking. The C used 2 H 2 The output optical power of the laser is about 17 mW.

[0051] Among them, the low frequency is generally below 1 Hz. Since the main function of the low-frequency triangular wave is to sweep the frequency to obtain the second harmonic of the absorption peak, the frequency sweeping should not be too fast, otherwise it will cause overlapping crosstalk of the second harmonic. The high frequency is generally above 500 Hz. Because the resonance frequency of the photoacoustic cell is generally above 1000 Hz, the second harmonic of the high-frequency sine wave should be above 1000 Hz.

[0052] Further, the MTRDH photoacoustic module includes: an MTRDH photoacoustic unit, which stores the C 2 H 2 gas in the cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C 2 H 2 gas absorbs the laser, an in-phase photoacoustic signal is generated, and the sound pressure is concentrated at the end of the resonance tube in combination with the T-type photoacoustic cell structure; a vacuum pump unit for exhausting the waste gas generated in the MTRDH photoacoustic unit along the air 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-type photoacoustic cells.

[0053] Further, the MTRDH photoacoustic cell includes: a first buffer cavity, a second buffer cavity is arranged at a parallel position of the first buffer cavity for storing C 2 H 2 gas. First window plates 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 plate to reflect the laser back and forth, so that the C 2 H 2 gas has a multi-pass absorption. The first double connection tube structure is used to connect the first buffer cavity and the second buffer cavity to transfer the maximum value of the acoustic standing wave to the belly of the buffer cavity. The second double connection tube includes: a first connection tube and a second connection tube. A first air inlet and a first air outlet are designed in the middle of the first connection tube and the second connection tube to equally divide the air flow into two different buffer tubes to form common-mode noise. First resonance tubes and second resonance tubes 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 resonance tube and the second resonance tube.

[0054] Specifically, the MTRDH photoacoustic cell in the present invention is different from the traditional PA absorption cell. It generates two out-of-phase photoacoustic signals through the Helmholtz structure and concentrates the sound pressure at the end of the resonance tube by combining the characteristics of the T-type photoacoustic cell structure. 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 window plates with a transmittance greater than 90% are installed at both ends of the buffer cavity. Two high-reflectivity right-angle prisms with a reflectivity of 94% are installed at both ends outside the window plates. The laser reflects back and forth four times between the two right-angle prisms to achieve four-fold optical path absorption of the gas for detecting C with a concentration of 0-2000 ppm 2 H 2 . The schematic diagram of the principle of the MTRDH photoacoustic cell is as shown in Figure 2 . The MTRDH photoacoustic cell is composed of two T-type photoacoustic cells of the same size connected by two connecting tubes. A differential microphone is installed at the resonance tube of each T-type photoacoustic cell. The specially designed double-connecting tube structure can also transfer the maximum value of the acoustic standing wave to the belly of the buffer tube. Inlets and outlets are designed in the middle of the two connecting tubes respectively, and the air flow will be equally divided into two different buffer tubes to form common-mode noise. The Helmholtz differential structure can generate acoustic signals with opposite phases. The signals collected by the microphone are sent to a subtraction circuit to eliminate the common-mode noise, thus greatly improving the detection sensitivity of photoacoustic spectroscopy

[0055] The COMSOL Multiphysics software is used to model and simulate the PA unit. By simulating the gas flow velocity at the resonance point of the photoacoustic cell, the magnitude of the air flow noise is determined. The normal inflow velocity of the inlet is set to 2 m / s, the turbulence intensity IT is 0.05, and the time is set to 20 s. The air flow velocity at the resonance point of the traditional Helmholtz double-connecting tube photoacoustic cell is stable at 0.019 m / s. The air flow velocity at the resonance point of the MTRDH photoacoustic cell of the present invention is greatly reduced and stable at 1.1×10 -7 m / s, thus verifying that the designed MTRDH photoacoustic cell has the characteristic of lower air flow noise. The resonance frequency of the photoacoustic cell and the sound pressure at the resonance point are simulated, and the frequency-domain sound pressure analysis is carried out in the range of 1100-1500 Hz. The resonance frequency of the traditional double-connecting tube Helmholtz photoacoustic cell is 1361.6 Hz, and the absolute sound pressure is on the order of 10 8 . The MTRDH photoacoustic cell incorporates the structure of the T-type photoacoustic cell. Due to the change in volume, the resonance frequency slightly decreases to 1341.1 Hz, but the absolute sound pressure value increases by nearly one order of magnitude, reaching 10 9 of the order of magnitude

[0056] The performance of the MTRDH photoacoustic cell is verified through experiments. The dynamic gas distribution method of the gas mixing system is used to prepare gas samples. The prepared C with a concentration of 0-2000 ppm 2 H2 They are respectively introduced into the MTRDH photoacoustic cell, and for each concentration of C 2 H 2 continuous measurement is carried out for 50 s. When the sensor system is configured for single-ended sound pressure output, the standard deviation noise is 1.03 mV, the calculated SNR is 2602, and the corresponding lowest detection limit (MDL) is 769 ppb. When the sensor system does not add a prism, it behaves as a single-pass PA photoacoustic cell. By observing the 2f signal waveform of 2000 ppm C 2 H 2 the amplitude of the waveform is 1.76 V. When adding a prism, it behaves as a multi-pass PA photoacoustic cell, and the amplitude of the waveform is 5.6 V. The calculated standard deviation noise is 0.29 mV, and from this, the SNR is calculated to be 19310, and the corresponding MDL is 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 with double-ended output. The SNR increases by 7.4 times compared with that of single-ended output, and the detection limit is lower. It can be seen the superiority of the MTRDH photoacoustic cell used in the present invention. When detecting the system noise in the case of introducing pure nitrogen, the standard deviations of the signals measured by the two microphones are obtained as σ1 = 1.28 mV and σ2 = 1.03 mV respectively, and the standard deviation σ3 of the differential signal obtained after signal subtraction is 0.29 mV. It can be seen that the differential structure of the MTRDH photoacoustic cell designed in the present invention has strong anti-noise ability; among them, photoacoustic (PA) is that when a pulsed laser irradiates a gas sample, the gas pressure will generate periodic expansion and contraction, thereby generating an acoustic signal, and this acoustic signal generated by photoexcitation is called a photoacoustic signal.

[0058] The following is the performance comparison (including SNR, LoD, NNEA, etc.) of the MTRDH photoacoustic cell designed in Table 1 with different types of common existing photoacoustic cells. The MTRDH photoacoustic cell has its unique advantages in terms of 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 type of spectral analysis and detection technology based on the photoacoustic effect. Signal-to-noise ratio (SNR) is an index that measures the ratio of signal intensity to background noise intensity. Detection limit (LoD) is the lowest detection concentration that can be distinguished from noise when the sample is extracted, processed, and detected according to the requirements of the analysis method. Normalized noise equivalent absorption coefficient (NNEA) is an important parameter characterizing the detection sensitivity of the sensor.

[0059] Table 1

[0060]

[0061] Further, the MTRDH photoacoustic cell includes: a third buffer chamber, and a fourth buffer chamber is arranged at a parallel position of the third buffer chamber for storing C 2 H 2 gas. Second window plates are installed at both ends of the third buffer chamber and the fourth buffer chamber. A chopper with a mirror is installed at a preset angle outside the second window plate of the third buffer chamber, and a mirror is arranged outside the second window plate of the fourth buffer chamber. The chopper with the mirror is driven to rotate to convert the laser passing through the third buffer chamber into a modulated laser with a target frequency, and the laser not passing through the third buffer chamber is reflected by the mirror to the fourth buffer chamber to generate a modulated laser with the same target frequency. The second double connection pipe structure is used to connect the third buffer chamber and the fourth buffer chamber to transfer the maximum value of the acoustic standing wave to the belly of the buffer chamber. The second double connection pipe includes: a third connection pipe and a fourth connection pipe. A second air inlet and a second air outlet are designed in the middle of the third connection pipe and the fourth connection pipe to equally divide the air flow into two different buffer pipes to form common-mode noise, and a third resonance pipe and a fourth resonance pipe are respectively arranged in the third buffer chamber and the fourth buffer chamber to form a T-shaped photoacoustic cell structure to concentrate the sound pressure at the ends of the third resonance pipe and the fourth resonance pipe.

[0062] Further, the MTRDH photoacoustic cell includes: a fifth buffer chamber, and a sixth buffer chamber is arranged at a parallel position of the fifth buffer chamber for storing C 2 H 2 gas. Third window plates are installed at both ends of the fifth buffer chamber and the sixth buffer chamber. A target chopper is arranged outside the second window plate in a direction perpendicular to the buffer chamber. The rotation speed of the target chopper is controlled. When the first laser passes through the fifth buffer chamber, the second laser shines on the target chopper. When the second laser passes through the sixth buffer chamber, the first laser shines on the target chopper. An alternating passing frequency is set to make the laser alternately pass through the two buffer chambers. The third double connection pipe structure is used to connect the fifth buffer chamber and the sixth buffer chamber to transfer the maximum value of the acoustic standing wave to the belly of the buffer chamber. The third double connection pipe includes: a fifth connection pipe and a sixth connection pipe. A third air inlet and a third air outlet are designed in the middle of the fifth connection pipe and the sixth connection pipe for equally dividing the air flow into two different buffer pipes to form common-mode noise, and a fifth resonance pipe and a sixth resonance pipe are respectively arranged in the fifth buffer chamber and the sixth buffer chamber to form a T-shaped photoacoustic cell structure to concentrate the sound pressure at the ends of the fifth resonance pipe and the sixth resonance pipe.

[0063] Specifically, the MTRDH photoacoustic cell designed in the present invention can also use modulated inverse-phase light to further increase the intensity of the photoacoustic signal. Inverse-phase light refers to two beams of light with a phase difference of 180°. The MTRDH photoacoustic cell has two symmetric T-shaped photoacoustic cells, upper and lower. A window plate with a transmittance greater than 90% is installed at both ends of the buffer cavity of each T-shaped photoacoustic cell. When the laser only passes into one of the buffer cavities, photoacoustic signals with opposite phases will be generated in the upper and lower T-shaped photoacoustic cells. When the laser also passes into the other buffer cavity, that is, when it passes into both buffer cavities simultaneously, if the phases of these two laser beams are the same, the photoacoustic signals generated by these two laser beams stimulating the gas will cancel each other out; if the phases of these two laser beams are opposite, the photoacoustic signals generated by these two laser beams stimulating the gas will be superimposed on each other. Adjusting the phase of the light can be achieved through physical modulation and through optical interference techniques. Common physical modulations include electro-optic modulation and acousto-optic modulation. In the MTRDH photoacoustic cell, the inverse-phase light generated by these modulation techniques is respectively introduced into two different buffer cavities, and in-phase photoacoustic signals can be generated.

[0064] The present invention respectively introduces two methods of mechanically modulating inverse-phase light, which are introduced into the developed MTRDH photoacoustic cell to enhance the photoacoustic signal. The schematic diagrams of two methods for detecting inverse-phase light using mechanical modulation are as Figure 3 and Figure 4 shown. The first method is to install a chopper with a mirror at the laser incidence, with an angle of 45° with the window plate, and use a motor to drive the chopper to rotate. A mirror is installed on the left side of the second buffer cavity, also with an angle of 45° with the window plate. The light passing into the first buffer cavity becomes modulated laser light with a frequency of 682 Hz after passing through the chopper. The laser light that does not pass through the chopper is reflected by the mirror on the chopper into the mirror on the left side of the second buffer cavity, and then enters the second buffer cavity through the mirror, and the modulation frequency is also 682 Hz. The inverse-phase light alternately injected at 682 Hz generates in-phase photoacoustic signals in the MTRDH photoacoustic cell, and these two signals can be detected by a differential microphone after being superimposed. 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 install a chopper with a large enough size at the laser incidence, placed parallel to the window plate, as Figure 4As shown in the figure, a motor is used to drive the chopping blade to rotate. Lasers with exactly the same wavelength and power are introduced into the photoacoustic cell. The position of the chopping blade is adjusted so that when Laser 1 is introduced into the photoacoustic cell, Laser 2 shines on the chopping blade; when Laser 2 is introduced into the photoacoustic cell, Laser 1 shines on the chopping blade, thereby realizing the alternating incidence of the lasers in the two buffer cavities. The rotation speed of the chopping blade is controlled so that the alternating introduction frequency of the lasers is 682 Hz. After mechanical modulation, the out-of-phase light alternately introduced at 682 Hz can also generate in-phase photoacoustic signals in the MTRDH photoacoustic cell. After superposition, the photoacoustic signal can be increased to twice the original value, while the noise after superposition is only 1.414 times the original value, further improving the SNR of the system.

[0066] Further, the gas detection module includes: a differential microphone unit for receiving the sound pressure; a subtraction circuit unit for calculating the sound pressure; a lock-in amplifier unit for frequency locking using a reference signal and extracting the 2f signal according to the calculation result; a DAQ data acquisition unit for acquiring the 2f signal; and a Labview monitoring unit for detecting the concentration using the 2f signal.

[0067] Since the Helmholtz type structure is used in this embodiment, the phases of the sound pressure signals detected by the two MEMS microphones are opposite. The subtraction circuit mainly performs subtraction on the signals detected by the two microphones through an operational amplifier circuit to realize the subtraction of the differential signals.

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

[0069] In the sensor detection system of the present invention, a low-frequency triangular wave and a high-frequency sine wave are first generated by a signal generator as a driving signal and a reference signal, and the C 2 H 2 laser is driven through a current driver and a temperature controller, and the reference signal is sent to a lock-in amplifier for frequency locking. The C 2 H 2 laser used outputs an optical power of about 17 mW. After the laser enters the photoacoustic cell, it is reflected four times through two right-angle prisms. The sound signals are obtained by using two differential microphones. After being operated by a subtraction circuit, they are sent to a lock-in amplifier. The lock-in amplifier extracts the 2f signal related to the gas concentration and sends it to the Labview monitoring platform embedded in a personal computer (PC). Using standard C 2 H 2 gas and pure nitrogen (N2 ) Prepare C with different concentrations through a gas mixing system 2 H 2 gas. An air inlet and an air outlet are respectively arranged in the middle of the two connecting pipes to realize the dynamic measurement of the gas.

[0070] The structure of the novel sensor with double connecting pipes proposed by the present invention can shift the maximum value of the sound pressure to the position of the belly of the buffer cavity. The designed T-shaped structure can make the sound pressure distribution in the two end cavities more concentrated, concentrated at the end of the resonance pipe, and the sound signal at this point is collected by the microphone, greatly enhancing the photoacoustic signal. The newly proposed mechanism of mechanically modulating the inverse light based on the MTRDH photoacoustic cell enables the photoacoustic signal to increase exponentially without changing the original Helmholtz structure.

[0071] Under the structure of the double connecting pipes of the present invention, an air inlet and an air outlet are installed at the connecting pipes to generate symmetric airflows, thereby generating common-mode noise, while the differential structure of the Helmholtz photoacoustic cell can just eliminate this common-mode noise, solving the disadvantage of large airflow noise of the T-shaped photoacoustic cell. While suppressing the noise, the photoacoustic signal output by the MTRDH photoacoustic cell is equivalent to the superposition of two T-shaped photoacoustic cells, greatly improving the SNR of the system and further reducing the NNEA, 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 of industrial production.

[0073] This embodiment also discloses a photoacoustic gas measurement method based on a T-shaped differential Helmholtz structure, including: preparing C with different concentrations 2 H 2 gas, storing the C 2 H 2 gas into the cavity based on the Helmholtz structure in the MTRDH photoacoustic cell; emitting a laser, when the C 2 H 2 gas absorbs the laser, generating an inverse photoacoustic signal, and concentrating the sound pressure at the end of the resonance pipe in combination with the T-shaped photoacoustic cell structure; extracting the 2f signal related to the gas concentration according to the sound pressure, and using the 2f signal to obtain the gas concentration data.

[0074] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention should all fall within the protection scope determined 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; The MTRDH photoacoustic module is used to store the C2H2 gas in the cavity based on the Helmholtz structure in the MTRDH photoacoustic cell. When the C2H2 gas absorbs the laser, an anti-phase photoacoustic signal is generated, and the sound pressure is concentrated at the end of the resonance tube in combination with the T-type photoacoustic cell structure. The gas detection module is used to extract a 2f signal related to the gas concentration according to 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 comprises: 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 another high-frequency sine wave to form a reference signal; A current and temperature control driving unit, used for receiving the driving signal to 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, an anti-phase photoacoustic signal is generated, and the sound pressure is concentrated at the end of the resonance tube in combination with the T-type photoacoustic cell structure. The vacuum pump unit is used to discharge the waste 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 comprises: a first buffer cavity, a second buffer cavity is arranged at a position parallel to the first buffer cavity, and is used to store 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 absorbs 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 comprises: 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, and are used to divert the airflow into two different buffer tubes in equal amounts to form common mode noise; 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.

5. 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 comprises: a third buffer cavity, a fourth buffer cavity is arranged at a position parallel to the third buffer cavity, and is used to store the C2H2 gas; second windows are 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; the chopper with the reflector is driven to rotate, and the laser light passing through the third buffer cavity is converted into a modulated laser light of a target frequency, and the laser light not passing through the third buffer cavity is reflected to the fourth buffer cavity through the reflector to generate a phase shift. The 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, and transfers 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, and a third resonant tube and a fourth resonant tube are respectively arranged 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.

6. 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 comprises: a fifth buffer chamber, a sixth buffer chamber is arranged at a position parallel to the fifth buffer chamber, and is used to store the C2H2 gas; a third window is installed at both ends of the fifth buffer chamber and the sixth buffer chamber; a target chopper is arranged outside the second window in a direction perpendicular to the buffer chamber; the rotation speed of the target chopper is controlled, and when the first laser passes into the fifth buffer chamber, the second laser is projected onto the target chopper; when the second laser passes into the sixth buffer chamber, the first laser is projected onto the target chopper; and an alternating frequency is set so that the lasers are alternately passed. The fifth buffer chamber is inserted into two buffer chambers, 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 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 equally divert the airflow into two different buffer tubes to form common mode noise. A fifth resonant tube and a sixth resonant tube are respectively arranged 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.

7. 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, used for calculating the sound pressure; A phase-locked amplifier unit, used to perform frequency locking using a reference signal and extract the 2f signal according to a calculation result; A DAQ data acquisition unit, used for acquiring the 2f signal; The Labview monitoring unit is used to obtain the gas concentration data using the 2f signal.

8. 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 as claimed in any one of claims 1 to 7, characterized in that: include: preparing C2H2 gas of different concentrations, and storing the C2H2 gas in a cavity based on a Helmholtz structure in an MTRDH photoacoustic cell; The laser is emitted, and when the C2H2 gas absorbs the laser, an anti-phase photoacoustic signal is generated, and the sound pressure is concentrated at the end of the resonance tube in combination with a T-type photoacoustic cell structure; 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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