A gas photoacoustic spectroscopy detection system with temperature compensation and a detection method thereof

Abstract

The application discloses a kind of gas photoacoustic spectroscopy detection systems with temperature compensation and its detection method, belong to gas detection technical field.The detection system provided by the application includes data processing and acquisition module, double-frequency resonant photoacoustic cell, laser and function generator and the like structure.Two resonant cavities are in the double-frequency resonant photoacoustic cell, two kinds of modulation frequency are generated using function generator to carry out double-frequency modulation to laser, and modulation frequency is set on the two sides of the resonant frequency of two resonant cavities simultaneously.When temperature changes, the resonant frequency drift of two resonant cavities causes photoacoustic signal to present opposite change trend on the two sides of non-resonant frequency, and the drift caused by temperature is offset by superimposing two photoacoustic signals, solve the problem that the photoacoustic signal of traditional photoacoustic cell fluctuates with temperature, significantly improve the stability and signal-to-noise ratio of detection system, greatly expand the application range of photoacoustic spectroscopy technology in the field of gas detection technology under extreme temperature conditions.

Description

Technical Field

[0001] This invention belongs to the field of gas detection technology, specifically relating to a gas photoacoustic spectroscopy detection system with temperature compensation and its detection method. Background Technology

[0002] Photoacoustic spectroscopy, based on the photoacoustic effect, inverts gas concentration by detecting the acoustic signal generated after gas molecules absorb modulated light. Resonant photoacoustic cells can significantly improve detection sensitivity due to their resonant amplification effect, but their performance is significantly affected by temperature: temperature changes cause drift in sound velocity and resonant frequency, leading to attenuation of the photoacoustic signal.

[0003] In existing technologies, temperature compensation schemes are mainly divided into two categories: active temperature control and mechanical tuning. Active temperature control (such as CN102169085B, which uses cooling water circulation and an electric heating element) can achieve a wide temperature range of -40℃ to 90℃, but the system is complex, slow in response, energy-intensive, and the acoustic signal is easily affected by noise in the temperature control system. Mechanical tuning (such as CN111380805B, which adjusts the cavity length via piezoelectric ceramics) has a faster response, but suffers from mechanical wear and insufficient long-term stability. Furthermore, the single-resonant-cavity design cannot eliminate the inherent effects of temperature drift. Therefore, there is an urgent need for a device solution that combines high stability, fast response, and a compact structure. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a temperature-compensated gas photoacoustic spectroscopy detection system and method, suitable for high-precision gas concentration detection. This invention employs a dual-frequency resonant photoacoustic cell design, where the laser operates at two modulation frequencies, located on either side of the corresponding resonant cavity frequency. When the temperature changes, the resonant frequencies of the two cavities drift, causing the photoacoustic signals to exhibit opposite trends on either side of the non-resonant frequency. By superimposing the two photoacoustic signals, the temperature-induced drift is offset, achieving automatic compensation for the photoacoustic cell signal drift. This significantly improves the system's stability and signal-to-noise ratio under temperature-changing environments, thus solving the problem of decreased gas concentration detection capability caused by the drift of a single resonant frequency in conventional photoacoustic cells.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] The first aspect of the present invention is to provide a gas photoacoustic spectroscopy detection system with temperature compensation, comprising:

[0007] The data processing and acquisition module is used to record and analyze the amplitude and phase of the photoacoustic signal; it includes a first lock-in amplifier, a second lock-in amplifier, and a data acquisition card connected to the electrical signal output terminals of the first and second lock-in amplifiers; the data acquisition card is connected to a host computer.

[0008] The dual-frequency resonant photoacoustic cell includes a buffer cavity one, a first resonant cavity, a buffer cavity two, a second resonant cavity, and a buffer cavity three, which are coaxial and connected in sequence. Buffer cavity one and buffer cavity three are respectively provided with an air inlet and an air outlet. The first resonant cavity and the second resonant cavity have different lengths, corresponding to different resonant frequencies. The walls of the dual-frequency resonant photoacoustic cell are respectively provided with a first sound wave transmission channel and a second sound wave transmission channel at the top middle position of the first resonant cavity and the second resonant cavity, which are perpendicularly connected to them. The first sound wave transmission channel is connected to a first sound wave signal collection mechanism, and the second sound wave transmission channel is connected to a second sound wave signal collection mechanism. The signal output terminals of the first sound wave signal collection mechanism and the second sound wave signal collection mechanism are respectively connected to the electrical signal input terminals of the first lock-in amplifier and the second lock-in amplifier.

[0009] A laser is used to provide laser light for detection; the wavelength of the laser light is matched with the absorption spectrum of the gas being measured.

[0010] The function generator has its first electrical signal output terminal connected to the signal input terminal of the laser, used to simultaneously perform dual-frequency modulation on the laser beam; the second electrical signal output terminal of the function generator is connected to the reference signal input terminals of the first lock-in amplifier and the second lock-in amplifier.

[0011] A laser power meter is used to monitor the power of a laser.

[0012] As a preferred technical solution, the first and second resonant cavities are cylindrical in shape, forming a first-order cylindrical resonance; and the length of the first resonant cavity is shorter than the length of the second resonant cavity. More preferably, the length ratio of the first and second resonant cavities is ≤3:4, the purpose of which is to ensure that the resonant frequencies of the two resonant cavities have a significant difference and that their frequency response curves do not intersect, thus ensuring that the frequency responses of the two resonant cavities do not interfere with each other, and that the photoacoustic signals do not interfere with each other.

[0013] As a preferred technical solution, buffer cavities one, two, and three are of the same size; a laser entrance window is installed at the front end of buffer cavity one, and a laser exit window is installed at the rear end of buffer cavity three. Specifically, both the laser entrance window and the laser exit window are anti-reflection windows, located on the outer sides of buffer cavity one and buffer cavity three, respectively. More preferably, the length ratio of buffer cavity one to the length of the first resonant cavity is 1:3; the ratio of the longitudinal section diameter of buffer cavity one to the longitudinal section diameter of the first resonant cavity is 4:1; and the material of the buffer cavities is the same as that of the resonant cavities.

[0014] As a preferred technical solution, the first acoustic wave transmission channel and the first acoustic wave transmission channel have the same structure, which is a conduit or an acoustic hole; the first acoustic wave signal collection mechanism and the first acoustic wave signal collection mechanism have the same structure, which are used to detect photoacoustic signals, including but not limited to a microphone or a microphone.

[0015] A second aspect of the present invention is to provide a gas photoacoustic spectroscopy detection method with temperature compensation, which is performed using the gas photoacoustic spectroscopy detection system as described in the first aspect above, and includes the following steps:

[0016] The initial frequency response curves of the first and second resonant cavities in the dual-frequency resonant photoacoustic cell were measured to determine the initial resonant frequencies of the first and second resonant cavities, which were denoted as _____. f 1 and f 2;

[0017] Two modulation frequencies are generated using a function generator to perform dual-frequency modulation on the laser. The modulation frequencies are set to the left and right sides of the initial resonant frequencies of the two resonant cavities, respectively, and are set as F1= f 1±Δ f and F2= f 2±Δ f , where Δ f For frequency offset, Δ f At the same time, it is smaller than the full width at half maximum (FWHM) of the frequency response curve of the first resonant cavity and the full width at half maximum (FWHM) of the frequency response curve of the second resonant cavity, ensuring that the superimposed photoacoustic signal is always in the linear response range.

[0018] Two photoacoustic signals are detected simultaneously in the first and second resonant cavities. When temperature changes cause the resonant frequency to drift, the two photoacoustic signals show opposite trends on both sides of the non-resonant frequency. The two photoacoustic signals are superimposed by the data processing and acquisition module to cancel out the signal drift caused by temperature.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The gas photoacoustic spectroscopy detection system provided by this invention is based on a dual-frequency resonant photoacoustic cell structure. By designing a first resonant cavity and a second resonant cavity, the photoacoustic cell has two resonant frequencies, greatly expanding its application range. Furthermore, the design of three buffer cavities not only broadens the application scenarios of the photoacoustic cell but also increases the system's sampling volume and improves the sampling flow rate.

[0021] This invention is based on a dual-frequency resonant photoacoustic cell structure design. It utilizes a function generator to produce two modulation frequencies for dual-frequency modulation of a laser, with these modulation frequencies set on either side of the resonant frequency of the two resonant cavities. When the temperature changes, the resonant frequencies of the two cavities drift, causing the photoacoustic signal to exhibit opposite trends on either side of the non-resonant frequency. By superimposing the two photoacoustic signals, the temperature-induced drift is offset, achieving automatic temperature compensation. This avoids photoacoustic signal drift caused by temperature fluctuations, which is beneficial for improving the quality factor and signal-to-noise ratio of the resonant cavity. It solves the problem of photoacoustic signal fluctuations with temperature in traditional photoacoustic cells, significantly improving the stability of the detection system and greatly expanding the application scope of photoacoustic spectroscopy in gas detection technology under extreme temperature conditions. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the gas photoacoustic spectroscopy detection system with temperature compensation provided by the present invention.

[0023] Figure 2 This is a schematic diagram illustrating the principle of temperature compensation in this invention;

[0024] Figure 3 The frequency scanning results in the dual-frequency resonant photoacoustic cell during actual measurement using the system of this invention;

[0025] Figure 4 This is a graph showing the temperature correction results.

[0026] Figure reference numerals: 1-Function generator; 2-Laser; 3-Dual-frequency resonant photoacoustic cell; 301-Buffer cavity one; 302-Buffer cavity two; 303-Buffer cavity three; 311-First resonant cavity; 312-Second resonant cavity; 321-First acoustic wave transmission channel; 322-Second acoustic wave transmission channel; 331-Air inlet; 332-Air outlet; 341-Laser entrance window; 342-Laser exit window; 401-First acoustic wave signal collection mechanism; 402-Second acoustic wave signal collection mechanism; 501-First lock-in amplifier; 502-Second lock-in amplifier; 6-Laser power meter; 7-Data acquisition card; 8-Host computer. Detailed Implementation

[0027] The present invention will be further described below with reference to embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments are not intended to limit the present invention. In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail.

[0028] Example 1

[0029] refer to Figure 1A temperature-compensated gas photoacoustic spectroscopy detection system includes a data processing and acquisition module, a dual-frequency resonant photoacoustic cell 3, a laser 2, a function generator 1, and a laser power meter 6, wherein:

[0030] The data processing and acquisition module includes a first lock-in amplifier 501, a second lock-in amplifier 502, and a data acquisition card 7 connected to the electrical signal output terminals of the first lock-in amplifier 501 and the second lock-in amplifier 502; the data acquisition card 7 is connected to the host computer 8.

[0031] The dual-frequency resonant photoacoustic cell 3 includes a buffer cavity 301, a first resonant cavity 311, a second resonant cavity 302, a second resonant cavity 312, and a third buffer cavity 303, which are coaxial and connected in sequence. The first buffer cavity 301 and the third buffer cavity 303 are respectively provided with an air inlet 331 and an air outlet 332. The first resonant cavity 311 and the second resonant cavity 312 have different lengths, which correspond to different resonant frequencies. A laser entrance window 341 is installed at the front end of the first buffer cavity 301, and a laser exit window 342 is installed at the rear end of the third buffer cavity 303. Specifically, the laser entrance window 341 and the laser exit window 342 are both anti-reflection windows, which are located on the outside of the first buffer cavity 301 and the third buffer cavity 303, respectively. The walls of the dual-frequency resonant photoacoustic cell 3 are respectively provided with a first acoustic wave transmission channel 321 and a second acoustic wave transmission channel 322 at the top middle position of the first resonant cavity 311 and the second resonant cavity 312, which are vertically connected to them. The first acoustic wave transmission channel 321 is connected to a first acoustic wave signal collection mechanism 401, and the second acoustic wave transmission channel 322 is connected to a second acoustic wave signal collection mechanism 402. The signal output terminals of the first acoustic wave signal collection mechanism 401 and the second acoustic wave signal collection mechanism 402 are respectively connected to the electrical signal input terminals of the first lock-in amplifier 501 and the second lock-in amplifier 502.

[0032] In a preferred embodiment, the first resonant cavity 311 and the second resonant cavity 312 are cylindrical in shape, forming a first-order cylindrical resonance. The length of the first resonant cavity 311 is shorter than that of the second resonant cavity 312. Preferably, the length ratio of the first resonant cavity 311 to the second resonant cavity 312 is ≤3:4, so that the resonant frequencies of the two resonant cavities have a significant difference and their frequency response curves do not intersect. Specifically, the length of the first resonant cavity 311 is L1 = 60 mm, and its cross-sectional diameter is D1 = 10 mm; the length of the second resonant cavity 312 is L2 = 80 mm, and its cross-sectional diameter is D2 = 10 mm.

[0033] In a preferred embodiment, buffer cavity one 301, buffer cavity two 302, and buffer cavity three 303 have the same dimensions. The length ratio of buffer cavity one 301 to the length of the first resonant cavity 311 is 1:3; the ratio of the longitudinal cross-sectional diameter of buffer cavity one 301 to the longitudinal cross-sectional diameter of the first resonant cavity 311 is 4:1. More specifically, each buffer cavity has a length of 20mm and a cross-sectional diameter of 40mm.

[0034] In a preferred embodiment, the first acoustic wave transmission channel 321 and the first acoustic wave transmission channel 321 have the same structure, which is a conduit or an acoustic hole; the first acoustic wave signal collection mechanism 401 and the first acoustic wave signal collection mechanism 401 have the same structure, which are used to detect photoacoustic signals, including but not limited to a microphone or a microphone.

[0035] Example 2

[0036] This embodiment provides a gas photoacoustic spectroscopy detection method with temperature compensation, which is performed using the gas photoacoustic spectroscopy detection system as described in Embodiment 1 above, and includes the following steps:

[0037] The initial frequency response curves of the first and second resonant cavities in the dual-frequency resonant photoacoustic cell were measured to determine the initial resonant frequencies of the first and second resonant cavities, which were denoted as _____. f 1 and f 2;

[0038] Two modulation frequencies are generated using a function generator to perform dual-frequency modulation on the laser. The modulation frequencies are set to the left and right sides of the initial resonant frequencies of the two resonant cavities, respectively, and are set as F1= f 1±Δ f and F2= f 2±Δ f , where Δ f For frequency offset, Δ f At the same time, it is smaller than the full width at half maximum (FWHM) of the frequency response curve of the first resonant cavity and the full width at half maximum (FWHM) of the frequency response curve of the second resonant cavity.

[0039] Two photoacoustic signals are detected simultaneously in the first and second resonant cavities. When temperature changes cause the resonant frequency to drift, the two photoacoustic signals exhibit opposite trends on both sides of the non-resonant frequency. The two photoacoustic signals are superimposed by the data processing and acquisition module to cancel out the signal drift caused by temperature. The average value of the two photoacoustic signals is the photoacoustic signal after real-time temperature compensation.

[0040] Combination Figure 2 The working principle of the detection system regarding temperature compensation in this invention will be explained as follows:

[0041] For a cylindrical resonant cavity, the formula for the resonant frequency is as shown in equation (1):

[0042] (1)

[0043] in, L eff For the equivalent cavity length, c Since the speed of sound is denoted as , it can be seen that the resonant frequency of the photoacoustic cell is affected by the speed of sound.

[0044] The formula for the speed of sound is equation (2):

[0045] (2)

[0046] In the formula γ It is the ratio of isobaric heat capacity to isochoric heat capacity, and it is a function of temperature. M The relative molar mass is... T For temperature, R The gas constant is denoted by a molar gas constant. The resonant frequency of the photoacoustic cell is affected by structural and environmental parameters. For example, the operating frequency of the photoacoustic cell will drift due to the influence of ambient temperature, and the photoacoustic signal will fluctuate accordingly. Acoustic resonant cavities have resonant frequencies. The frequency response curve of an ideal acoustic resonant cavity approximately follows a Lorenz linear distribution, meaning the photoacoustic signal is maximum at the resonant frequency and gradually decreases towards the sides. Therefore, by setting the modulation frequency of the laser on both sides of the resonant frequency, the photoacoustic signals generated by the two modulation frequencies exhibit opposite changes with temperature. Superimposing these two oppositely changing photoacoustic signals eliminates the influence of temperature variations.

[0047] like Figure 2 As shown in Figure a, the resonant frequencies of the first and second resonant cavities calibrated in the dual-frequency resonant photoacoustic cell are respectively... f 1 and f 2. The laser is controlled by a function generator at a frequency F1= f 1±Δ f and F2= f 2±Δ f Dual-frequency modulation is performed at the location, where Δ f For frequency offset, Δ f Simultaneously, the half-width at half maximum (FWHM) of the frequency response curve of the first resonant cavity is smaller than that of the full width at half maximum (FWHM) of the frequency response curve of the second resonant cavity, ensuring that the superimposed photoacoustic signal always remains within the linear response range; two photoacoustic signals (S1 and S2) are detected simultaneously in the first and second resonant cavities; when temperature changes cause the resonant frequency to drift, the two photoacoustic signals (S1 and S2) exhibit opposite trends on both sides of the non-resonant frequency. The data acquisition card 7 and the host computer 8 receive and output the two photoacoustic signals (S1 and S2). T and S2 T When the temperature changes (e.g.) Figure 2 The temperature in Figure b increases or as shown in Figure b. Figure 2As the temperature decreases (as shown in Figure c), since the two modulation frequencies are located on opposite sides of the resonant frequency, the two photoacoustic signals exhibit opposite changes. Therefore, the sum of the amplitudes of the photoacoustic signals from the first and second resonant cavities remains consistent with the sum of the amplitudes of the two photoacoustic signals before the temperature change, i.e., S1 + S2 = S1. T +S2 T This invention solves the problem of temperature-induced drift in traditional photoacoustic cells by superimposing two photoacoustic signals to offset the drift, significantly improving the stability and signal-to-noise ratio of the detection system and greatly expanding the application scope of photoacoustic spectroscopy in gas detection technology under extreme temperature conditions.

[0048] Actual measurements were performed using the gas photoacoustic spectroscopy detection system described in Example 1 above. High-concentration aerosol was introduced into the system, and the frequency distribution of the dual-frequency resonant photoacoustic cell was observed. The results are shown below. Figure 3 , Figure 3 The photoacoustic signals and their fitted signals in the first and second resonant cavities are shown. It can be determined that the resonant frequency of the first resonant cavity is... f 1 = 2667.7 Hz, the resonant frequency of the second resonant cavity. f 2 = 2098.5 Hz. In the experiment, Δ f Set to 300Hz, the resonant frequencies of the two cavities are respectively set to... f The left side of 1 (2667.7Hz) at F1 (2367.7Hz) and f At F2 (2398.5Hz), to the right of 2 (2098.5Hz).

[0049] In actual measurements, when the temperature rises from 15°C to 25°C, the photoacoustic signal in the first resonant cavity increases from the original S1 = 0.47V to S1. T =0.474V, the photoacoustic signal of the second resonant cavity decreased from the original S2=0.22V to S2 T =0.216V, the measurement result is as follows Figure 4 As shown, although the photoacoustic signals in the two resonant cavities drift, their sum remains stable (S1 + S2 = S1). T +S2 T This demonstrates that the system provided by the present invention effectively eliminates photoacoustic signal drift caused by temperature fluctuations.

[0050] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A gas photoacoustic spectroscopy detection system with temperature compensation, characterized in that, include: The data processing and acquisition module includes a first lock-in amplifier, a second lock-in amplifier, a data acquisition card, and a host computer; The dual-frequency resonant photoacoustic cell includes a first buffer cavity, a second buffer cavity, a third buffer cavity, and a fourth buffer cavity, all coaxially connected in sequence. The first and third buffer cavities each have an air inlet and an air outlet, respectively. The first and second resonant cavities have different lengths. A first acoustic wave transmission channel and a second acoustic wave transmission channel are connected to the top center of the first and second resonant cavities, respectively. The first acoustic wave transmission channel is connected to a first acoustic wave signal collection mechanism, and the second acoustic wave transmission channel is connected to a second acoustic wave signal collection mechanism. The signal output terminals of the first and second acoustic wave signal collection mechanisms are connected to the electrical signal input terminals of the first and second lock-in amplifiers, respectively. Laser, used to provide laser light for detection; The function generator has its first electrical signal output terminal connected to the signal input terminal of the laser; and its second electrical signal output terminal connected to the reference signal input terminals of the first and second lock-in amplifiers. A laser power meter is used to monitor the power of a laser.

2. The gas photoacoustic spectroscopy detection system with temperature compensation according to claim 1, characterized in that, The first and second resonant cavities are cylindrical in shape.

3. The gas photoacoustic spectroscopy detection system with temperature compensation according to claim 1, characterized in that, The length of the first resonant cavity is shorter than the length of the second resonant cavity.

4. The gas photoacoustic spectroscopy detection system with temperature compensation according to claim 3, characterized in that, The length ratio of the first resonant cavity to the second resonant cavity is ≤3:

4.

5. The gas photoacoustic spectroscopy detection system with temperature compensation according to any one of claims 1 to 4, characterized in that, The buffer chambers 1, 2, and 3 are of the same size; a laser entrance window is installed at the front end of the buffer chamber 1, and a laser exit window is installed at the rear end of the buffer chamber 3.

6. The gas photoacoustic spectroscopy detection system with temperature compensation according to claim 5, characterized in that, Both the laser entrance window and the laser exit window are anti-reflective windows, located on the outside of buffer cavity one and buffer cavity three, respectively.

7. The gas photoacoustic spectroscopy detection system with temperature compensation according to claim 5, characterized in that, The length ratio of the buffer cavity one to the length of the first resonant cavity is 1:3; the ratio of the longitudinal section diameter of the buffer cavity one to the longitudinal section diameter of the first resonant cavity is 4:

1.

8. The gas photoacoustic spectroscopy detection system with temperature compensation according to claim 1, characterized in that, The first acoustic wave transmission channel and the first acoustic wave transmission channel have the same structure, which is a conduit or an acoustic hole.

9. The gas photoacoustic spectroscopy detection system with temperature compensation according to claim 1, characterized in that, The first sound wave signal collection mechanism and the first sound wave signal collection mechanism have the same structure, which is a microphone or a microphone.

10. A gas photoacoustic spectroscopy detection method with temperature compensation, characterized in that, It is performed using the gas photoacoustic spectroscopy detection system as described in any one of claims 1 to 9, and includes the following steps: The initial frequency response curves of the first and second resonant cavities in the dual-frequency resonant photoacoustic cell were measured to determine the initial resonant frequencies of the first and second resonant cavities, which were denoted as _____. f 1 and f 2; Two modulation frequencies are generated using a function generator to perform dual-frequency modulation on the laser. The modulation frequencies are set to the left and right sides of the initial resonant frequencies of the two resonant cavities, respectively, and are set as F1= f 1±Δ f and F2= f 2±Δ f , where Δ f For frequency offset, Δ f At the same time, it is smaller than the full width at half maximum (FWHM) of the frequency response curve of the first resonant cavity and the full width at half maximum (FWHM) of the frequency response curve of the second resonant cavity. Two photoacoustic signals are detected simultaneously in the first and second resonant cavities. When temperature changes cause the resonant frequency to drift, the two photoacoustic signals show opposite trends on both sides of the non-resonant frequency. The two photoacoustic signals are superimposed by the data processing and acquisition module to cancel the signal drift caused by temperature.

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