Photoacoustic dual-gas detection system and method

Abstract

The invention provides a photoacoustic dual-gas detection system and method. The system comprises a modulation and driving module which generates a first modulation signal and a second modulation signal; the laser outputs corresponding laser beams under the driving of the first modulation signal and the second modulation signal; the photoacoustic cell is used for reacting the to-be-detected gas with the laser beam to generate a photoacoustic signal; the microphone converts the photoacoustic signal into an electric signal, a first electric signal is obtained under the first modulation signal, and a second electric signal is obtained under the second modulation signal; the signal demodulation and processing module demodulates the electric signal; inverting the first gas concentration according to the first electric signal; and inverting the second gas concentration according to the second electric signal. According to the invention, only a simple structure of a single laser and a single photoacoustic cell is adopted, and the concentrations of the two gases are sequentially inverted by setting two modulation signals of the laser according to the difference of sound velocities of the two gases, so that the complexity and cost of the system are reduced, and the difficulty of signal decoupling is reduced.

Description

Technical Field

[0001] This invention belongs to the field of photoacoustic cell gas detection, specifically relating to a photoacoustic dual-gas detection system and method. Background Technology

[0002] High-voltage power equipment, as the "heart" of power transmission and distribution, undergoes decomposition of its insulating oil and composite insulation materials when mechanical, thermal, or electrical faults occur, producing various characteristic gases. Hydrogen, a common byproduct of overheating and low-energy partial discharge, is almost always present in the early stages of various faults. Acetylene, primarily associated with high-temperature overheating and high-energy arc discharge, is considered a crucial diagnostic signal for late-stage degradation of power equipment. Therefore, simultaneous detection of hydrogen and acetylene can enable early warning and reliable diagnosis of faults in high-voltage power equipment, ensuring the safe and stable operation of the power system.

[0003] For a long time, gas chromatography has been the preferred method for analyzing dissolved gases in oils. However, traditional gas chromatographs are both time-consuming and expensive, making it difficult to achieve in-situ online measurement of target gases. Later, researchers applied semiconductor detectors and fuel cells to gas chromatographs to improve the sensitivity and portability of the equipment. However, these methods have their own shortcomings in terms of sensitivity, selectivity, and lifespan.

[0004] Photoacoustic spectroscopy, as an indirect absorption spectroscopy technique, relies primarily on the conversion of light energy absorbed by a gas into an acoustic signal to detect trace gases. Compared to gas chromatography, it offers advantages such as non-contact detection, rapid response, and small size, and has gradually become the mainstream method for detecting dissolved gases in oil. In photoacoustic spectroscopy systems, even gas molecules with strong absorption fingerprints (such as acetylene, carbon dioxide, and nitrous oxide) produce relatively weak photoacoustic signals, typically requiring a resonant photoacoustic cell to enhance the system's detection sensitivity. Furthermore, for diatomic homonuclear molecules with extremely weak absorption fingerprints (such as hydrogen, oxygen, and nitrogen), they lack strong absorption electric dipole moments, making detection using photoacoustic spectroscopy particularly challenging.

[0005] On the one hand, the absorption spectrum of hydrogen mainly consists of extremely weak vibrational bands from electric quadrupole transitions, resulting in a very weak photoacoustic signal amplitude. Traditional amplitude-based detection methods struggle to achieve high-precision quantitative measurements. Although researchers have extended the absorption optical path using high-reflectivity mirrors to improve the detection limit of hydrogen to some extent, the installation and maintenance of these mirrors introduce uncertainties into the system. On the other hand, hydrogen alters the physical parameters of the gas within the photoacoustic spectrum, including density, sound velocity, and thermal conductivity, thereby affecting the resonant frequency and phase of the photoacoustic cell. Researchers have proposed using frequency shifting to detect hydrogen, but this ignores the phase information contained in the signal, leading to poor anti-interference capabilities and a limited response range.

[0006] Furthermore, in complex dissolved gas environments within oil, a single gas is insufficient for reliable assessment of the condition of high-voltage power equipment. When dealing with multi-component gas detection, multiple excitation sources and detectors are typically required, which not only increases system complexity and cost but also presents significant challenges in signal decoupling. Summary of the Invention

[0007] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention proposes a photoacoustic dual gas detection system and method, which reduces the complexity and cost of the system and reduces the difficulty of signal decoupling.

[0008] To achieve the above objectives, according to one aspect of the present invention, a photoacoustic dual-gas detection system is provided for detecting the concentrations of a first gas and a second gas in a gas to be tested, wherein the sound velocity of the first gas is more than twice the sound velocity of the second gas; comprising: The modulation and driving module is used to generate a modulation signal; the modulation signal includes a first modulation signal and a second modulation signal, the first modulation signal is a sinusoidal modulation signal, and the second modulation signal is a superposition modulation signal of a sinusoidal and a triangular wave; A single laser is used to output corresponding laser beams under the drive of a first modulation signal and a second modulation signal, respectively; the output wavelength of the laser is aligned with the absorption spectrum of the second gas. A single photoacoustic cell is used to generate photoacoustic signals by reacting the gas under test with the laser beam; A microphone is used to convert photoacoustic signals into electrical signals; wherein a first electrical signal is obtained under a first modulation signal and a second electrical signal is obtained under a second modulation signal. The signal demodulation and processing module is used to use the modulated signal as a reference signal to perform phase-locked amplification and demodulation of the electrical signal; to invert the first gas concentration based on the first electrical signal; and to invert the second gas concentration based on the second electrical signal.

[0009] According to the above scheme, the photoacoustic cell includes: The housing serves to provide mechanical support and isolate the body from external environmental interference. The resonant cavity, housed within the housing, is used to generate photoacoustic signals by reacting the gas under test with the laser beam. Two optical windows, connected to the housing, are used for the laser beam to enter and exit the photoacoustic cell; The air inlet and outlet are located on the housing and are used for the gas to be tested to enter and exit the photoacoustic cell.

[0010] According to the above scheme, the photoacoustic cell also includes two buffer cavities with a volume larger than the resonant cavity, located between the optical window and the resonant cavity, so that the laser beam passes through the first optical window, the first buffer cavity, the resonant cavity, the second buffer cavity, and the second optical window in sequence. The air inlet is connected to one of the buffer chambers, and the air outlet is connected to the other buffer chamber.

[0011] According to the above scheme, the two ends of the resonant cavity are transition sections used to shorten the length of the resonant cavity. The cross sections of the transition sections are all nonlinear structures, and the direction of the cross sections is the same as the transmission direction of the laser beam.

[0012] According to the above scheme, the nonlinear structure is rounded or conical; the maximum radius of the rounded corner... R = R 2 -R 1, of which R 2 is the maximum radius of the resonant cavity. R 1 represents the minimum radius of the resonant cavity; the maximum taper of the cone. ,in L The length of the cone.

[0013] According to the above scheme, the resonant cavity is connected to an acoustic wave connection channel, which is connected to a microphone.

[0014] According to the above scheme, the first gas is hydrogen and the second gas is acetylene; The length of the resonant cavity allows the photoacoustic cell to have a resonant frequency above 2kHz.

[0015] According to the above scheme, it also includes an airflow control module, which is set at the front end of the air inlet and is used to control the flow rate of the gas to be tested from the air inlet into the photoacoustic cell.

[0016] According to another aspect of the present invention, a dual-gas detection method utilizing the aforementioned photoacoustic dual-gas detection system is provided, comprising the following steps: S1. Obtaining the reference resonant frequency and reference phase: The modulation frequency of the first modulation signal is within a preset modulation frequency range; the laser outputs a corresponding laser beam under the first modulation signal, and the laser beam generates a photoacoustic signal in the photoacoustic cell without a first gas environment. The photoacoustic signal is converted into a first electrical signal by a microphone. The reference amplitude frequency response curve and the reference phase frequency response curve of the photoacoustic signal are extracted and obtained based on the resonant frequency and phase contained in the first electrical signal. The reference resonant frequency and reference phase of the system are extracted based on the reference amplitude frequency response curve and the reference phase frequency response curve. S2. Establishment of calibration relationships between the first gas concentration and phase, and between the first gas concentration and resonant frequency: The laser outputs a corresponding laser beam under the first modulation signal. The laser beam generates multiple photoacoustic signals in multiple photoacoustic cells with known first gas concentrations. The photoacoustic signals are converted into first electrical signals by a microphone. The resonant frequency and phase contained in each first electrical signal are extracted. Based on the reference resonant frequency and reference phase, calibration relationships between the first gas concentration and phase and between the first gas concentration and resonant frequency are established respectively. At this time, the modulation frequency of the first modulation signal is within a preset modulation frequency range. S3. Acquisition of the first gas concentration and the resonant frequency at the first gas concentration: The laser outputs a corresponding laser beam under the first modulation signal. The laser beam generates a photoacoustic signal in the photoacoustic cell in the gas environment to be tested. The photoacoustic signal is converted into a first electrical signal by a microphone. The resonant frequency and phase contained in the first electrical signal are extracted as the resonant frequency and phase under the first gas concentration. The first gas concentration in the gas to be tested is inverted by using the calibration relationship between the first gas concentration and the phase or between the first gas concentration and the resonant frequency. S4. Establishment of the calibration model for the second gas concentration and second harmonic peak value under the first gas concentration: The modulation frequency of the second modulation signal is set to the resonant frequency at the known first gas concentration. The laser outputs a corresponding laser beam under the second modulation signal. The laser beam generates multiple photoacoustic signals in multiple photoacoustic cells with a first gas and a second gas at the same time. The concentration of the first gas is a known fixed concentration, and the concentration of the second gas is a known multiple different concentrations. The photoacoustic signal is converted into a second electrical signal by a microphone. The second harmonic component is extracted from each second electrical signal to obtain the second harmonic peak value. A calibration model of the second gas concentration and the second harmonic peak value under a known first gas concentration is established. S5. Obtaining the concentration of the second gas: The laser outputs a corresponding laser beam under the second modulation signal. The laser beam generates a photoacoustic signal in the gas environment to be tested. The photoacoustic signal is converted into a second electrical signal by a microphone. The second harmonic component in the second electrical signal is extracted to obtain the second harmonic peak value in the second harmonic component. Based on the calibration model of the second gas concentration and the second harmonic peak value under the known first gas concentration obtained in S4, the second gas concentration is inverted.

[0017] Following the method described above, in S3, When the resonant frequency contained in the first electrical signal is higher than or equal to the preset resonant frequency, the calibration relationship between the first gas concentration and the resonant frequency is selected to invert the first gas concentration; when the resonant frequency contained in the first electrical signal is lower than the preset resonant frequency, the calibration relationship between the first gas concentration and the phase is selected to invert the first gas concentration. The preset resonant frequency is the resonant frequency corresponding to the first gas at a preset concentration.

[0018] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects: 1. Using only a single laser and a single photoacoustic cell, based on the difference in sound velocity between the two gases, by setting two modulation signals of the laser, the concentration of the first gas with a higher sound velocity is first inverted using the sinusoidal modulation signal, and then the concentration of the second gas with a lower sound velocity is inverted by extracting the second harmonic peak using the superimposed sinusoidal and triangular wave modulation signal. This reduces the complexity and cost of the system, and also reduces the difficulty of signal decoupling.

[0019] 2. By setting the resonant cavity of the photoacoustic cell with a nonlinear structure, the equivalent length can be reduced, the resonant frequency can be increased, and the sensitivity of the first gas concentration detection can be improved. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a system structure provided in an embodiment of the present invention.

[0021] Figure 2 This is a schematic diagram of a photoacoustic cell structure provided in an embodiment of the present invention.

[0022] Figure 3 This is a comparison chart of the sensitivity of linear and nonlinear resonant cavities provided in an embodiment of the present invention.

[0023] Figure 4 This is a schematic diagram of the frequency response curve under high hydrogen concentration provided in an embodiment of the present invention.

[0024] Figure 5 This is a schematic diagram of the signal phase change under low hydrogen concentration provided in an embodiment of the present invention.

[0025] Figure 6 This is a flowchart of the method provided in an embodiment of the present invention.

[0026] In the picture: 1-Laser, 2-Modulation and driving module, 3-Collimator, 4-Photoacoustic cell, 5-Microphone, 6-Signal demodulation and processing module, 7-Airflow control module; 4-1-Shell, 4-2-First optical window, 4-3-First buffer cavity, 4-4-Second optical window, 4-5-Second buffer cavity, 4-6-Resonant cavity, 4-7-Acoustic wave connection channel, 4-8-Air inlet, 4-9-Air outlet. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0028] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0029] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified. In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0030] like Figure 1As shown, this embodiment provides a photoacoustic dual-gas detection system for detecting the concentrations of a first gas and a second gas in a test gas. The first gas and the second gas have the following characteristics: the speed of sound of the first gas is more than twice that of the speed of sound of the second gas, that is, the molar mass of the first gas is smaller and the molar mass of the second gas is larger, resulting in a significant difference in their speeds of sound. For example, the speed of sound of the second gas is 400 m / s, and the speed of sound of the first gas is 800 m / s.

[0031] This system includes a modulation and driving module 2, a single laser 1, a single photoacoustic cell 4, a single microphone 5, and a signal demodulation and processing module 6.

[0032] The modulation and drive module 2 is used to generate a modulation signal. The modulation signal includes a first modulation signal and a second modulation signal; the first modulation signal is a sinusoidal modulation signal, and the second modulation signal is a superposition modulation signal of a sine wave and a triangular wave. The modulation and drive module 2 also uses the generated modulation signal as a reference signal for the signal demodulation module 6 for phase-locked amplification processing.

[0033] Laser 1 is used to output corresponding laser beams under the drive of a first modulation signal and a second modulation signal, respectively. The output wavelength of laser 1 is aligned with the absorption spectrum of the second gas, thereby enabling a photoacoustic effect.

[0034] The photoacoustic cell 4 is used to generate photoacoustic signals by reacting the gas to be tested with the laser beam.

[0035] Microphone 5 is used to convert photoacoustic signals into electrical signals; wherein a first electrical signal is obtained under a first modulation signal and a second electrical signal is obtained under a second modulation signal.

[0036] The signal demodulation and processing module 6 is used to use the modulated signal as a reference signal to perform phase-locked amplification and demodulation of the electrical signal; to invert the concentration of the first gas based on the first electrical signal; and to extract the second electrical signal to invert the concentration of the second gas.

[0037] The modulation signal generated by the modulation and drive module 2 is transmitted to the laser 1 to drive its operation and achieve signal modulation. There are two types of modulation signals, which are rapidly switched using time-division multiplexing: one is a sinusoidal signal, and the other is a modulation signal that superimposes a sinusoidal signal and a triangular wave signal. The modulated laser beam is focused by the collimator 3 and enters the photoacoustic cell 4 from one side. After propagating along its central axis, it exits the photoacoustic cell 4 from the other side. Inside the photoacoustic cell 4, the laser beam reacts with the gas being tested, generating a photoacoustic signal. The microphone 5 converts the photoacoustic signal into an electrical signal and feeds it back to the signal demodulation and processing module 6. The first or second modulation signal generated by the modulation and drive module 2 also serves as a reference signal for the signal demodulation and processing module 6 to achieve phase-locked amplification.

[0038] Taking hydrogen as the first gas and acetylene as the second gas as an example, this invention introduces the principle of simultaneously detecting the concentrations of two gases.

[0039] Photoacoustic spectroscopy is essentially a spectroscopic analysis method based on the absorption of electromagnetic waves by matter, mainly involving two key processes: light absorption and sound excitation. The light absorption process follows Beer-Lambert's law, meaning that the degree of light absorption by a sample is closely related to its concentration and the absorption optical path length. When acetylene gas in a photoacoustic cell is irradiated with modulated monochromatic light, the absorbed light energy is converted into heat energy through non-radiative transitions, thus forming a sound pressure wave within the cell. Therefore, the higher the concentration of acetylene gas, the stronger the generated sound wave signal. Hydrogen gas, due to its extremely small molecular weight, extremely high sound velocity, and extremely high thermal conductivity, affects the excitation and transmission of sound waves within the photoacoustic cell.

[0040] The formula for calculating the resonant frequency of the photoacoustic cell is:

[0041] in, υ m The speed of sound of the mixed component gas. L eff The equivalent length of the resonant cavity; R and T These are the gas constant and absolute temperature of the gas mixture, respectively. m =Σx i C pi / Σx i C vi The specific heat ratio of the mixed gas is given by . x i It represents i The proportion of gas components in the gas mixture C p and C v These refer to the specific heat capacity of a gas under constant pressure and constant volume, respectively. M m = Σx i M i denoted as , where is the molar mass of the gas mixture.

[0042] As shown in the above formula, the resonant frequency of the photoacoustic cell is directly proportional to the speed of sound and inversely proportional to the equivalent length. Therefore, the higher the hydrogen concentration, the greater the speed of sound in the cell, thus increasing the resonant frequency of the system. At this point, the frequency shift Δ... f With hydrogen concentration CH The relationship between them can be represented as

[0043] in, υ b The background gas velocity is denoted as .

[0044] However, when the hydrogen concentration is low, the system's frequency shift is small. Furthermore, the presence of system background noise and external environmental noise makes extracting hydrogen concentration from the frequency shift unreliable. In the low-concentration range, the phase is more sensitive to changes in hydrogen concentration. When hydrogen is added to the gas mixture, due to the small molecular weight and high speed of sound of hydrogen, the speed of sound in the mixture increases. This means that the propagation time of sound waves in the gas is shortened, thus causing an advance in the phase of sound wave propagation. (Phase of photoacoustic signal) The expression is

[0045] Where ω is the modulation angular frequency, l Let be the distance along the direction of sound wave propagation. Therefore, the phase of the photoacoustic signal is proportional to the speed of sound. At this point, the phase shift... With hydrogen concentration C H The relationship between them can be represented as

[0046] Based on the above principles, this invention provides a dual-gas detection method utilizing the aforementioned photoacoustic dual-gas detection system, such as... Figure 1 and Figure 6 As shown, it includes the following steps: S1. Obtaining the reference resonant frequency and reference phase: Under the first modulation signal generated by the modulation and driving module 2, the laser beam output by the laser 1 is corresponding. In the absence of the first gas, the laser beam and the gas react in the photoacoustic cell 4 to generate a photoacoustic signal. The signal is converted by the microphone 5 to obtain the first electrical signal at this time. The signal demodulation and processing module 6 extracts and obtains the reference amplitude frequency response curve and reference phase frequency response curve of the photoacoustic signal under this environment based on the resonant frequency and phase contained in the first electrical signal, and extracts the reference resonant frequency and reference phase of the system.

[0047] At this point, the first modulation signal is a sinusoidal modulation signal, and the modulation frequency is within a preset modulation frequency range. The frequency sweep is performed within this preset modulation frequency range. The preset modulation frequency range is determined based on the dynamic range of the first gas. The detection of any physical quantity has a range; for example, the detection range for hydrogen is 0%-4%. The range of resonant frequency variation is calculated using this range, thus setting the preset frequency scanning range.

[0048] The output wavelength of the laser 1 is aligned with the absorption spectrum of the second gas, thereby enabling a photoacoustic effect to occur during subsequent detection.

[0049] S2. Establishment of calibration relationships between the first gas concentration and phase, and between the first gas concentration and resonant frequency: Since the presence of the first gas causes a shift in the resonant frequency and phase, the laser 1 outputs a corresponding laser beam under the first modulation signal generated by the modulation and driving module 2. In multiple gas environments with known first gas concentrations, the laser beam and gas react in the photoacoustic cell 4 to generate photoacoustic signals. The signal demodulation and processing module 6 extracts the resonant frequency and phase contained in the first electrical signal generated in each gas environment with known first gas concentration, and establishes calibration relationships between the first gas concentration and phase, and between the first gas concentration and resonant frequency, based on the reference resonant frequency and reference phase of the system obtained in S1.

[0050] Specifically, when the concentration of the first gas is higher than or equal to a concentration threshold, the concentration of the first gas can be tracked using the resonant frequency; however, when the concentration of the first gas is lower than the concentration threshold, the concentration of the first gas cannot be tracked using the resonant frequency, but it can be tracked using the phase. The concentration threshold can be obtained during the calibration process.

[0051] Therefore, it is necessary to establish calibration relationships between the first gas concentration and phase and resonant frequency respectively. When detecting the first gas concentration in the future, the corresponding calibration relationship can be selected to retrieve a more accurate first gas concentration value based on the level of the first gas concentration.

[0052] For a single gas, calibration only needs to be performed once. Subsequent detection processes can directly use the calibrated results without requiring recalibration.

[0053] S3. Acquisition of the first gas concentration and the resonant frequency at the first gas concentration: Continuing under the first modulation signal generated by the modulation and driving module 2, the laser 1 outputs a corresponding laser beam. In the environment of the gas to be tested, the laser beam and the gas to be tested react in the photoacoustic cell 4 to generate a photoacoustic signal. The photoacoustic signal is converted into a first electrical signal by a microphone. The resonant frequency and phase contained in the first electrical signal are extracted as the resonant frequency and phase under the first gas concentration. Using the calibration relationship between the first gas concentration and the phase or the calibration relationship between the first gas concentration and the resonant frequency, the first gas concentration in the gas to be tested is inverted.

[0054] When the resonant frequency contained in the first electrical signal is higher than or equal to the preset resonant frequency, the calibration relationship between the first gas concentration and the resonant frequency is selected to invert the first gas concentration; when the resonant frequency contained in the first electrical signal is lower than the preset resonant frequency, the calibration relationship between the first gas concentration and the phase is selected to invert the first gas concentration.

[0055] The preset resonant frequency is the resonant frequency corresponding to the first gas at a preset concentration. Since the frequency shift is large when the first gas concentration is low, a calibration relationship between the first gas concentration and the resonant frequency is used; conversely, when the first gas concentration is low again, the frequency shift is small, requiring a conversion to a calibration relationship between the first gas concentration and the phase. The preset concentration is the turning point at which the selected calibration relationship needs to be changed, and the preset resonant frequency is the resonant frequency corresponding to the first gas concentration at that turning point. Frequency shift refers to the deviation between the resonant frequency of the first electrical signal and the system's reference resonant frequency.

[0056] S4. Establishment of the calibration model for the second gas concentration and second harmonic peak value under the first gas concentration: The modulation frequency of the second modulation signal is set to the resonant frequency under the known first gas concentration.

[0057] The laser outputs a corresponding laser beam under the second modulation signal. The laser beam generates multiple photoacoustic signals in multiple photoacoustic cells with a first gas and a second gas in a gas environment. The concentration of the first gas is a known fixed concentration, and the concentration of the second gas is a known multiple different concentrations.

[0058] The photoacoustic signal is converted into a second electrical signal by a microphone. The second harmonic component is extracted from each second electrical signal to obtain the second harmonic peak value. A calibration model of the second gas concentration and the second harmonic peak value under a known first gas concentration is established.

[0059] The following example illustrates the specific steps for building a model: 1) Given the concentration of the first gas... Based on the calibration relationship between the first gas concentration and the resonant frequency, the first gas concentration is obtained as follows: The resonant frequency of the gas is used as the modulation frequency of the second modulation signal, and the gas environment simultaneously has a concentration of... 1) Given a first gas and a second gas with a concentration of b, obtain the second harmonic peak value under this state; 2) Still set the concentration of the first gas to be... The resonant frequency of the gas is used as the modulation frequency of the second modulation signal, and the gas environment simultaneously has a concentration of... The first gas and its concentration are ( The second gas is used to obtain the second harmonic peak value under this state; ...; finally, the known concentration of the first gas is obtained. The second gas concentration and second harmonic peak value calibration model.

[0060] Change the known first gas concentration to ( Repeat the above steps to obtain the known concentration of the first gas as ( The calibration model for the second gas concentration and second harmonic peak value under ( ).

[0061] Finally, calibration models for the second gas concentration and second harmonic peak value under multiple known first gas concentrations are obtained. These models are retained after the first calibration, eliminating the need for step S4 in subsequent detection processes.

[0062] S5. Obtaining the concentration of the second gas: The laser outputs a corresponding laser beam under the second modulation signal. The laser beam generates a photoacoustic signal in the gas environment under test. The photoacoustic signal is converted into a second electrical signal by a microphone. The second harmonic component is extracted from the second electrical signal, and the second harmonic peak value is obtained. Based on the calibration model of the second gas concentration and the second harmonic peak value under the known first gas concentration obtained in S4, the second gas concentration is inverted. At this time, the second modulation signal is based on the sinusoidal modulation signal with a superimposed half-period signal of a triangular wave, and the modulation frequency is the resonant frequency under the first gas concentration obtained in S3.

[0063] As can be seen from the above method, after calibration, the present invention only requires two detections, S3 and S5, to obtain the concentrations of the two gases in the gas to be tested.

[0064] like Figure 2 As shown, in this embodiment, the photoacoustic cell includes a housing 4-1, a resonant cavity 4-6, an optical window, an air inlet 4-8, and an air outlet 4-9.

[0065] The housing 4-1 serves for mechanical support and isolation from external environmental interference. The material of the housing 4-1 is selected from metallic materials or polymeric materials, wherein the metallic materials include at least one of brass, stainless steel, aluminum alloy, titanium alloy, and alloy combinations thereof, and the polymeric materials include at least one of photosensitive resin, polytetrafluoroethylene, and modified composite materials thereof.

[0066] The first optical window 4-2 and the second optical window 4-4 are connected to the housing 4-1 and are used for the laser beam to enter and leave the resonant cavity. The material can be CaF2.

[0067] The resonant cavity 4-6 is located in the housing 4-1 and is used to generate photoacoustic signals by reacting the gas under test with the laser beam.

[0068] Continue as Figure 2As shown, in order to stabilize airflow and eliminate external noise, in some embodiments, the photoacoustic cell also includes two buffer cavities with a volume larger than the resonant cavity, making the gas flow rate smoother. For example... Figure 2 The first buffer cavity 4-3 and the second buffer cavity 4-5 shown are located between the optical window and the resonant cavity, allowing the laser beam to pass sequentially through the first optical window 4-2, the first buffer cavity 4-3, the resonant cavity 4-6, the second buffer cavity 4-5, and the second optical window 4-4. The air inlet 4-8 communicates with one of the buffer cavities (e.g., the first buffer cavity 4-3), and the air outlet 4-9 communicates with the other buffer cavity (e.g., the second buffer cavity 4-5). The maximum radius of the resonant cavity is equal to the radii of the two buffer cavities.

[0069] More preferably, in some embodiments, the two ends of the resonant cavity 4-6 are transition sections for shortening the length of the resonant cavity. The cross-sections of these transition sections are all non-linear structures, and the direction of the cross-sections is the same as the propagation direction of the laser beam. The non-linear structure has rounded corners (corresponding to...). Figure 2 (the ① section in the middle) or the cone (corresponding to) Figure 2 (Section ② in the diagram); Maximum radius of the fillet. R = R 2 -R 1, of which R 2 is the maximum radius of the resonant cavity. R 1 represents the minimum radius of the resonant cavity; the maximum taper of the cone. ,in L The length is the diameter of the cone. In this embodiment, the maximum radius of the resonant cavity is the same as the radius of the buffer cavity.

[0070] This embodiment uses nonlinear resonant cavities with rounded or conical cross-sections at both ends, which allows for a smoother transition, lower acoustic impedance, and a shorter equivalent length, thereby improving frequency resolution and enhancing the sensitivity of detecting the first gas with a higher sound velocity.

[0071] In this embodiment, the first gas is hydrogen, and the second gas is acetylene; the length of the resonant cavity 4-6 is between 15 and 80 mm to ensure that the resonant frequency of the photoacoustic cell is above 2 kHz. A higher resonant frequency results in less low-frequency noise interference from the environment, a larger Q value, and thus greater frequency resolution, which is beneficial for the detection of the first gas. Taking hydrogen as an example, the hydrogen detection sensitivity... S H (That is, the frequency shift caused by a unit hydrogen concentration) can be expressed as:

[0072] in, This is the resonant frequency offset. This represents the change in hydrogen concentration. The change in the speed of sound L effThis is the equivalent length of the photoacoustic cell.

[0073] When the change in sound speed caused by the change in hydrogen concentration is constant, the equivalent length of the resonant cavity can be shortened. L eff This can improve the detection sensitivity of hydrogen. Figure 3 The graph shows a comparison of the sensitivity of linear and nonlinear resonant cavities provided in this embodiment of the invention. For a 20 mm linear resonant cavity, its hydrogen sensitivity is 42.2 Hz / %. When a nonlinear resonant cavity is used to shorten the equivalent length to 15 mm, the hydrogen sensitivity increases to 56.3 Hz / %.

[0074] In some embodiments, the housing 4-1 is also provided with an air inlet 4-8 and an air outlet 4-9 for gas exchange, that is, the gas to be tested enters the resonant cavity through the air inlet 4-8 and leaves the resonant cavity through the air outlet 4-9.

[0075] Still as Figure 2 As shown, in some embodiments, the resonant cavity 4-6 is connected to an acoustic wave connection channel 4-7, which is connected to a microphone for transmitting acoustic waves from the resonant cavity 4-6 to the microphone.

[0076] Continue as Figure 1 As shown, in some embodiments, the system further includes an airflow control module 7, which is located at the front end of the air inlet and is used to assist gas exchange, so that the gas to be tested can enter the photoacoustic cell better from the air inlet to complete the gas exchange, so that the concentration of the gas to be tested in the resonant cavity 4-6 is the same as that outside.

[0077] Figure 4 This is a schematic diagram of the frequency response curve under high hydrogen concentration provided in an embodiment of the present invention. As the hydrogen concentration increases, the speed of sound in the mixed gas also increases. Since the resonant frequency of the photoacoustic cell is proportional to the speed of sound, this causes the resonant frequency of the system to increase.

[0078] Figure 5 This is a schematic diagram of the signal phase change under low hydrogen concentration provided by an embodiment of the present invention. In the low concentration range, when hydrogen is added to the mixed gas, the sound speed of the mixed gas will increase due to the small molecular weight and high sound speed of hydrogen. This means that the propagation time of the sound wave in the gas is shortened, thereby causing the sound wave propagation phase to increase.

[0079] This invention's method is applicable to fault diagnosis of high-voltage power equipment. By employing the system of this invention, a location is set at which characteristic gases may be generated in the high-voltage power equipment. The first gas is hydrogen, and the second gas is acetylene. When the detected hydrogen concentration exceeds a certain preset value, it indicates that the high-voltage power equipment has experienced overheating and / or low-energy partial discharge; when the detected acetylene concentration exceeds a certain preset value, it indicates that the high-voltage power equipment has experienced high-temperature overheating and / or high-energy arc discharge.

[0080] This invention achieves simultaneous detection of hydrogen and acetylene, characteristic gases in dissolved gases, using a photoacoustic spectroscopy system based on a single laser and a microphone. It features lower cost, a simpler system architecture, and higher detection efficiency, resulting in better overall integration. The compact photoacoustic cell proposed in this invention has a shorter equivalent length compared to the traditional H-type photoacoustic cell, thus providing greater hydrogen detection sensitivity. Simultaneously, the photoacoustic hydrogen detection method combining resonant frequency and phase proposed in this invention has a wider response range and stronger anti-interference capability. Finally, by utilizing frequency tracking and wavelength modulation-second harmonic technology, accurate simultaneous measurement of dual gas concentrations is achieved, which can be widely applied to fault early warning and condition diagnosis of high-voltage power equipment.

[0081] The method of the present invention can also be applied to other similar environments, as long as the difference in sound velocity between the first gas and the second gas is significant.

[0082] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0083] It should be noted that, depending on the implementation needs, the various steps / components described in this application can be broken down into more steps / components, or two or more steps / components or parts of the operation of steps / components can be combined into new steps / components to achieve the purpose of this invention.

[0084] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A photoacoustic dual-gas detection system for detecting the concentrations of a first gas and a second gas in a gas to be tested, wherein the speed of sound of the first gas is more than twice the speed of sound of the second gas; characterized in that: include: The modulation and driving module is used to generate a modulation signal; the modulation signal includes a first modulation signal and a second modulation signal, the first modulation signal is a sinusoidal modulation signal, and the second modulation signal is a superposition modulation signal of a sinusoidal and a triangular wave; A single laser is used to output corresponding laser beams under the drive of a first modulation signal and a second modulation signal, respectively; the output wavelength of the laser is aligned with the absorption spectrum of the second gas. A single photoacoustic cell is used to generate photoacoustic signals by reacting the gas under test with the laser beam; A microphone is used to convert photoacoustic signals into electrical signals; wherein a first electrical signal is obtained under a first modulation signal and a second electrical signal is obtained under a second modulation signal. The signal demodulation and processing module is used to use the modulated signal as a reference signal to perform phase-locked amplification and demodulation of the electrical signal; to invert the first gas concentration based on the first electrical signal; and to invert the second gas concentration based on the second electrical signal.

2. The photoacoustic dual-gas detection system according to claim 1, characterized in that: The photoacoustic pool includes: The housing serves to provide mechanical support and isolate the body from external environmental interference. The resonant cavity, housed within the housing, is used to generate photoacoustic signals by reacting the gas under test with the laser beam. Two optical windows, connected to the housing, are used for the laser beam to enter and exit the photoacoustic cell; The air inlet and outlet are located on the housing and are used for the gas to be tested to enter and exit the photoacoustic cell.

3. The photoacoustic dual-gas detection system according to claim 2, characterized in that: The photoacoustic cell also includes two buffer cavities with volumes larger than the resonant cavity, located between the optical window and the resonant cavity, so that the laser beam passes through the first optical window, the first buffer cavity, the resonant cavity, the second buffer cavity, and the second optical window in sequence; The air inlet is connected to one of the buffer chambers, and the air outlet is connected to the other buffer chamber.

4. The photoacoustic dual-gas detection system according to claim 2, characterized in that: The two ends of the resonant cavity are transition sections used to shorten the length of the resonant cavity. The cross-sections of the transition sections are all nonlinear structures, and the direction of the cross-sections is the same as the transmission direction of the laser beam.

5. The photoacoustic dual-gas detection system according to claim 4, characterized in that: The nonlinear structure is rounded or conical; the maximum radius of the rounded corner... R = R 2 -R 1, of which R 2 is the maximum radius of the resonant cavity. R 1 represents the minimum radius of the resonant cavity; the maximum taper of the cone. ,in L The length of the cone.

6. The photoacoustic dual-gas detection system according to claim 2, characterized in that: The resonant cavity is connected to an acoustic wave connection channel, which is connected to a microphone.

7. The photoacoustic dual-gas detection system according to claim 2, characterized in that: The first gas is hydrogen, and the second gas is acetylene; The length of the resonant cavity allows the photoacoustic cell to have a resonant frequency above 2kHz.

8. The photoacoustic dual-gas detection system according to claim 2, characterized in that: It also includes an airflow control module, which is located at the front end of the air inlet and is used to control the flow rate of the gas to be tested as it enters the photoacoustic cell from the air inlet.

9. A dual-gas detection method implemented using the photoacoustic dual-gas detection system according to any one of claims 1-7, characterized in that: Includes the following steps: S1. Obtaining the reference resonant frequency and reference phase: The modulation frequency of the first modulation signal is within a preset modulation frequency range; the laser outputs a corresponding laser beam under the first modulation signal, and the laser beam generates a photoacoustic signal in the photoacoustic cell without a first gas environment. The photoacoustic signal is converted into a first electrical signal by a microphone. The reference amplitude frequency response curve and the reference phase frequency response curve of the photoacoustic signal are extracted and obtained based on the resonant frequency and phase contained in the first electrical signal. The reference resonant frequency and reference phase of the system are extracted based on the reference amplitude frequency response curve and the reference phase frequency response curve. S2. Establishment of calibration relationships between the first gas concentration and phase, and between the first gas concentration and resonant frequency: The laser outputs a corresponding laser beam under the first modulation signal. The laser beam generates multiple photoacoustic signals in multiple photoacoustic cells with gas environments of known first gas concentrations. The photoacoustic signals are converted into first electrical signals by a microphone. The resonant frequency and phase contained in each first electrical signal are extracted. Based on the reference resonant frequency and reference phase, calibration relationships between the first gas concentration and phase and between the first gas concentration and resonant frequency are established respectively. At this time, the modulation frequency of the first modulation signal is within a preset modulation frequency range. S3. Acquisition of the first gas concentration and the resonant frequency at the first gas concentration: The laser outputs a corresponding laser beam under the first modulation signal. The laser beam generates a photoacoustic signal in the photoacoustic cell in the gas environment to be tested. The photoacoustic signal is converted into a first electrical signal by a microphone. The resonant frequency and phase contained in the first electrical signal are extracted as the resonant frequency and phase under the first gas concentration. The first gas concentration in the gas to be tested is inverted by using the calibration relationship between the first gas concentration and the phase or between the first gas concentration and the resonant frequency. S4. Establishment of the calibration model for the second gas concentration and second harmonic peak value under the first gas concentration: The modulation frequency of the second modulation signal is set to the resonant frequency under the known first gas concentration; The laser outputs a corresponding laser beam under the second modulation signal. The laser beam generates multiple photoacoustic signals in multiple photoacoustic cells with a first gas and a second gas in a gas environment. The concentration of the first gas is a known fixed concentration, and the concentration of the second gas is a known multiple different concentrations. The photoacoustic signal is converted into a second electrical signal by a microphone. The second harmonic component is extracted from each second electrical signal to obtain the second harmonic peak value. A calibration model of the second gas concentration and the second harmonic peak value under a known first gas concentration is established. S5. Obtaining the concentration of the second gas: The laser outputs a corresponding laser beam under the second modulation signal. The laser beam generates a photoacoustic signal in the gas environment to be tested. The photoacoustic signal is converted into a second electrical signal by a microphone. The second harmonic component in the second electrical signal is extracted to obtain the second harmonic peak value in the second harmonic component. Based on the calibration model of the second gas concentration and the second harmonic peak value under the known first gas concentration obtained in S4, the second gas concentration is inverted.

10. The dual-gas detection method according to claim 9, characterized in that: In S3, When the resonant frequency contained in the first electrical signal is higher than or equal to the preset resonant frequency, the calibration relationship between the first gas concentration and the resonant frequency is selected to invert the first gas concentration; when the resonant frequency contained in the first electrical signal is lower than the preset resonant frequency, the calibration relationship between the first gas concentration and the phase is selected to invert the first gas concentration. The preset resonant frequency is the resonant frequency corresponding to the first gas at a preset concentration.

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