An extended-range resonant photoacoustic cell for acetylene detection and its detection method

Abstract

This application relates to an extended-range resonant photoacoustic cell for acetylene detection and its detection method, belonging to the technical field of partial discharge in high-voltage insulation transformers. Aiming at the problem of poor sensitivity of photoacoustic spectroscopy detection devices, an extended-range resonant photoacoustic cell for acetylene detection is provided, including: a first buffer chamber, a second buffer chamber, a laser, four reflectors, a photodetector and a controller arranged in a housing. The resonant gas chamber is arranged between the first buffer chamber and the second buffer chamber to connect the first buffer chamber and the second buffer chamber; the laser is located on one side of the first buffer chamber; the four reflectors are symmetrically arranged in pairs in the first buffer chamber and the second buffer chamber respectively; the photodetector is used to detect optical signals; the microphone is used for acoustic signals; the controller is used to process the optical signals and acoustic signals to obtain the detection results of characteristic gases. The four reflectors can not only increase the absorbance, but also realize continuous monitoring and improve the detection sensitivity.

Description

Technical Field

[0001] The present invention relates to the technical field of partial discharge in high-voltage insulating transformers, and particularly relates to an extended-resonance photoacoustic cell for acetylene detection and a detection method thereof. Background Art

[0002] The insulating oil inside large oil-immersed power transformers will gradually age and decompose into a small amount of hydrocarbon gases, such as acetylene (C2H2), methane (CH4), and carbon monoxide (CO), under high temperature and partial discharge conditions. By detecting the volume fractions of these gases, potential faults of the transformer can be analyzed and judged. Currently, gas chromatography is mainly used for detection, which has disadvantages such as consuming carrier gas, complex gas paths, and unstable long-term operation, bringing many inconveniences to operation and maintenance. Photoacoustic spectroscopy is a spectroscopic technique based on the photoacoustic effect discovered by A.G. Bell in 1880. When a light source in the molecular absorption band irradiates a sample, the sample molecules absorb light energy and transition to the excited state. The molecules in the excited state return to the ground state through collision relaxation, and at the same time, the absorbed light energy is converted into the internal energy of the molecules, resulting in a local temperature increase of the molecules. When the laser irradiating the sample molecules is modulated, the local temperature of the molecules changes periodically, thereby generating a periodic pressure change, that is, sound waves. By recording the relationship between the sound signal and the laser wavelength with a sound sensor such as a microphone, the photoacoustic spectroscopy signal is obtained. It has advantages such as high sensitivity, strong stability, and fast response speed. Since 2000, Kelman Company in the UK has applied photoacoustic spectroscopy technology to the detection of gases and trace water in insulating oil and developed a portable on-line monitoring system.

[0003] In the existing photoacoustic spectroscopy detection technology, in order to reduce the noise of the detection results, a double-chamber differential photoacoustic cell is selected, that is, it has two sets of resonance cavities and buffer cavities with the same shape and size, and their noises are the same. The vibrations of the two cavities are in opposite phases. Through the differential operation of the signals of the two microphones, since only one mirror is set in the photoacoustic cell to increase the optical path, while obtaining a detection result with a high signal-to-noise ratio, it restricts the improvement of the gas absorbance in the photoacoustic cell chamber in photoacoustic spectroscopy detection, and limits the detection sensitivity. Summary of the Invention

[0004] Aiming at the problems existing in the prior art, the present invention provides an extended-resonance photoacoustic cell for acetylene detection and a detection method thereof. By setting 4 mirrors to realize the circulation of the laser signal in the photoacoustic cell and increase the optical path of the laser signal, not only can the absorbance be increased, but also the continuous monitoring of the acetylene gas in the photoacoustic cell can be realized, thereby increasing the absorbance of the characteristic gas and realizing the improvement of the detection sensitivity of the photoacoustic spectroscopy of the characteristic gas.

[0005] An extended-resonance photoacoustic cell for acetylene detection provided by the present invention includes:

[0006] Housing;

[0007] A first buffer chamber is provided inside the housing. An air inlet window is provided at the top of the first buffer chamber, and an optical window is provided on the side, so that the characteristic gas can enter the first buffer chamber from the air inlet window;

[0008] A second buffer chamber is provided inside the housing. The second buffer chamber is provided with an exhaust window, so that the characteristic gas can be discharged from the second buffer chamber through the exhaust window;

[0009] A resonance gas chamber is provided between the first buffer chamber and the second buffer chamber to connect the first buffer chamber and the second buffer chamber;

[0010] A laser, which is located on one side of the first buffer chamber and outside the optical window, and provides laser light to generate a photoacoustic spectroscopy effect with the characteristic gas in the resonance gas chamber;

[0011] Four reflectors, which are symmetrically arranged in pairs in the first buffer chamber and the second buffer chamber respectively, so that the laser light emitted by the laser passes through the reflection of the four reflectors, and then the optical path of the laser signal is doubled;

[0012] A photodetector for detecting the optical signal of the photoacoustic spectroscopy effect;

[0013] A microphone, which is located at the top of the resonance gas chamber and is used to detect the acoustic signal of the photoacoustic spectroscopy effect;

[0014] A controller, which is respectively connected to the laser, the photodetector, and the microphone, is used to filter and amplify the electrical signal of the photodetector, perform negative feedback regulation on the laser to ensure stable light source output, and at the same time process the detection signal obtained by the microphone to obtain the detection result of the characteristic gas.

[0015] When the characteristic gas sequentially enters the first buffer chamber, the resonance gas chamber, and the second buffer chamber from the air inlet window, the laser light emitted by the laser enters the first buffer chamber from the optical window, passes through the reflection of the four reflectors, doubles the optical path of the laser signal, and generates a photoacoustic effect with the characteristic gas in the resonance gas chamber.

[0016] If the laser continuously emits laser signals, it will change the temperature conditions of the detection environment. Therefore, the laser operates at a set time interval to obtain discrete detection results at specific time intervals and cannot obtain continuous detection results. Four reflectors can enable the laser signal to circulate in the photoacoustic cell. Such a setting can not only increase the absorbance but also achieve continuous monitoring of acetylene gas in the photoacoustic cell. By setting four reflectors, the optical path of the laser signal is increased, thereby increasing the absorbance of the characteristic gas and improving the sensitivity of the photoacoustic spectroscopy detection of the characteristic gas.

[0017] Further, the first buffer chamber and the second buffer chamber are symmetrically arranged on both sides of the resonant chamber. The two buffer chambers should adopt a symmetrical layout. The symmetrical arrangement helps to obtain the maximum sound pressure signal at the characteristic frequency, thereby improving the sensitivity of the photoacoustic spectroscopy detection.

[0018] Further, there are two resonant chambers, and the two resonant chambers are arranged vertically to form an upper resonant chamber and a lower resonant chamber.

[0019] Further, the microphone is installed on the center line outside the resonant chamber and is horizontal with the inner surface of the upper resonant chamber.

[0020] A detection method provided by the present invention uses the above-mentioned extended resonant photoacoustic cell for acetylene detection, and includes the following steps:

[0021] Step 1, control the characteristic gas (acetylene gas) to enter the first buffer chamber;

[0022] Step 2, emit a laser signal and adjust the laser signal so that it can be absorbed by the characteristic gas (i.e., the light beam is consistent with the absorption spectral line of acetylene gas molecules);

[0023] Step 3, collect the acoustic signal formed after the characteristic gas absorbs the laser signal and is converted;

[0024] Step 4, demodulate the acoustic signal through a lock-in amplifier to obtain the photoacoustic spectroscopy detection result of the characteristic gas.

[0025] The acoustic signal converted after the gas molecules in the photoacoustic cell absorb light energy is collected by a microphone installed at the center of the resonant cavity flush with the cavity wall, and the sensitivity of the microphone is 50 mV / Pa.

[0026] Further, Step 1 specifically includes:

[0027] Step 1.1, adjust the angle of the reflector so that the laser emitted by the light source returns along the resonant chamber, and at the same time avoid the light hitting the inner wall to reduce the noise introduced by absorption;

[0028] Step 1.2: First, fill the photoacoustic cell with high-purity N2, then fill it with the characteristic gas (acetylene gas with a volume fraction of 0.01%), and adjust the rate; adjust the application pressure of the extended resonance photoacoustic cell for acetylene detection to be under atmospheric pressure conditions, and control the temperature at room temperature (25°C).

[0029] Further, the specific process of Step 2 is as follows:

[0030] Step 2.1: Modulate the current injected into the laser by superimposing the triangular wave output by the function generator and the sine wave output by the lock-in amplifier to obtain a modulation frequency, so that the output light source wavelength falls within the absorption peak range of the characteristic gas (acetylene absorption peak), and take the average value of multiple measurements of the photoacoustic signal; the role of controlling the modulation frequency is to enable the emitted light source to be better absorbed by acetylene gas and improve the detection sensitivity; modulate the light source frequency before the light source emits.

[0031] Step 2.2: Change the operating temperature of the laser to adjust the resonance frequency, and use the resonance spectroscopy method to measure the actual resonance frequency of the photoacoustic cell in real time, realizing the negative feedback adjustment of the resonance frequency, so that the operating frequency can be accurately adjusted to the standard frequency of the laser in real time. The resonance frequency increases with the increase of temperature and decreases with the decrease of temperature. The temperature changed is the operating temperature of the laser.

[0032] Further, in Step 4, demodulate the acoustic wave signal through the lock-in amplifier. The specific process includes:

[0033] Step 4.1: Provide a reference signal through the lock-in amplifier, whose frequency is very close to the expected frequency of the acoustic wave signal and has a fixed phase relationship; after this reference signal is mixed with the obtained acoustic wave signal, sum and difference frequency signals are generated through the mixer, and then the high-frequency components are filtered out through the low-pass filter, and the low-frequency difference frequency signal is retained, where the difference frequency signal contains the amplitude and phase information of the acoustic wave signal; in this process, the real part (X) and the imaginary part (Y) are extracted separately, and they correspond to the amplitude and phase information of the acoustic wave signal respectively.

[0034] Step 4.2: Determine the amplitude and phase information of the acoustic wave signal by measuring the amplitude and phase of the mixed frequency signal through the lock-in amplifier;

[0035] Step 4.3: Further amplify the generated difference frequency signal through the amplifier to enhance the intensity of the detection signal and make it suitable for subsequent processing.

[0036] Step 4.4, the amplified difference frequency signal is converted into a digital signal by a data acquisition card. The signal processed by the lock-in amplifier is input into the data acquisition card (DAQ). The data acquisition card converts the analog signal into a digital signal so that it can be read and processed by a computer. The data acquisition card realizes the conversion from analog signal to digital signal through an analog-to-digital converter (ADC), and has a certain resolution and sampling rate to provide data accuracy and data points.

[0037] Step 4.5, the photoacoustic spectroscopy detection result is obtained after processing. To extract useful information and conduct final data analysis.

[0038] The digital signal is stored in the memory of the data acquisition card and read, processed and analyzed by a computer. On the computer, various software tools and algorithms can be used to further process the signal, such as filtering, Fourier transform, spectrum analysis, etc., to extract useful information and conduct final data analysis. The processed signal data can be used to display the amplitude, phase or other related parameters of the acoustic wave signal, so as to obtain the photoacoustic spectroscopy detection result.

[0039] Since in the photoacoustic spectroscopy detection system, the acoustic signal generated due to the photoacoustic effect in the resonant gas chamber is usually very weak and may be submerged by noise, a high-sensitivity microphone is used to collect the photoacoustic signal; after processing, it is ensured that the weak acoustic wave signal generated from the photoacoustic cell can be effectively detected and analyzed to achieve high-precision detection of gas components.

[0040] As can be seen from the above technical solutions, the present invention has the following advantages:

[0041] (1) Solved the technical problem that in order to reduce the noise of the detection result, a double-chamber differential photoacoustic cell was selected. Since only one mirror was set in the photoacoustic cell to increase the optical path, while obtaining a detection result with a high signal-to-noise ratio, it restricted the improvement of the gas absorbance in the photoacoustic cell of the photoacoustic spectroscopy detection, and limited the detection sensitivity.

[0042] (2) By setting a mirror in the resonant gas chamber, the optical path of the laser signal is increased, thereby improving the absorbance of the characteristic gas. This design utilizes the principle of Beer-Lambert law, making the detection sensitivity significantly improved, which is particularly important for the precise detection of characteristic gases such as acetylene.

[0043] (3) Through the design of the left second buffer chamber, the acoustic background noise and the noise influence caused by gas flow are effectively reduced. This structural design helps to improve the signal-to-noise ratio of the system, ensuring that while obtaining high sensitivity, high-quality detection results can also be obtained.

[0044] (4) The controller filters and amplifies the electrical signals transmitted by the photodetector, and uses the output end to perform negative feedback regulation on the laser to ensure stable output of the light source. This design helps to maintain the stability of the light source during the photoacoustic spectroscopy detection process, and further improves the accuracy and reliability of the detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] 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 for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0046] Figure 1 It is a schematic structural diagram of an extended-range resonant photoacoustic cell for acetylene detection;

[0047] Figure 2 It is a detection curve graph of acetylene gas in Specific Embodiment 1 of the present invention;

[0048] Figure 3 It is a detection curve graph of a photoacoustic cell with low sensitivity;

[0049] Figure 4 It is a detection curve graph of the extended-range resonant photoacoustic cell for acetylene detection of the present application;

[0050] Among them, 1 - outer shell; 2 - laser; 3 - optical window; 4 - first buffer cavity; 5 - intake window; 6 - resonant gas chamber; 6A - upper resonant gas chamber; 6B - lower resonant gas chamber; 7 - microphone; 8 - second buffer cavity; 9 - exhaust window; 10 - photodetector; 11 - controller; 12 - mirror. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0051] In order to make the invention purpose, features, and advantages of the present invention more obvious and understandable, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the embodiments described below are only some embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0052] Embodiment 1

[0053] As Figure 1 shown, an extended-range resonant photoacoustic cell for acetylene detection provided by the present invention includes:

[0054] Outer shell 1;

[0055] The first buffer chamber 4 is arranged inside the housing 1. An air inlet window 5 is provided at the top of the first buffer chamber 4, and an optical window 3 is provided on the side, so that the characteristic gas can enter the first buffer chamber 4 from the air inlet window 5.

[0056] The second buffer chamber 8 is arranged inside the housing 1. An exhaust window 9 is provided in the second buffer chamber 8, so that the characteristic gas can be discharged from the second buffer chamber 8 through the exhaust window 9.

[0057] The resonant gas chamber 6 is arranged between the first buffer chamber 4 and the second buffer chamber 8 to connect the first buffer chamber 4 and the second buffer chamber 8.

[0058] The laser 2 is located on one side of the first buffer chamber 4 and outside the optical window 3. By providing laser light, a photoacoustic spectroscopy effect occurs with the characteristic gas in the resonant gas chamber 6.

[0059] There are four reflecting mirrors 12, which are symmetrically arranged in pairs in the first buffer chamber 4 and the second buffer chamber 8 respectively, so that the laser light emitted by the laser 2 is reflected by the four reflecting mirrors 12, and then the optical path of the laser signal is doubled. Specifically, two reflecting mirrors are arranged opposite to each other in the first buffer chamber 4, and two reflecting mirrors are arranged opposite to each other in the second buffer chamber 8. The reflecting mirrors 12 in the first buffer chamber 4 and the reflecting mirrors 12 in the second buffer chamber 8 are symmetrically arranged along the center line of the housing. The included angle between the 4 reflecting mirrors 12 and the horizontal plane is 30 - 60°, and the included angles between the 4 reflecting mirrors 12 and the horizontal plane are adjusted respectively as needed during use.

[0060] The photodetector 10 is used to detect the optical signal of the photoacoustic spectroscopy effect.

[0061] The microphone 7 is located at the top of the resonant gas chamber 6 and is used to detect the acoustic signal of the photoacoustic spectroscopy effect.

[0062] The controller 11 is respectively connected to the laser 2, the photodetector 10, and the microphone 7. It is used to filter and amplify the electrical signal of the photodetector 10, and use the output end to perform negative feedback adjustment on the laser 2 to ensure stable light source output. At the same time, it processes the detection signal obtained by the microphone 7 to obtain the detection result of the characteristic gas.

[0063] When the characteristic gas sequentially enters the first buffer chamber 4, the resonant gas chamber 6, and the second buffer chamber 8 from the air inlet window 5, the laser light emitted by the laser 2 enters the first buffer chamber 4 from the optical window 3, is reflected by the four reflecting mirrors 12, the optical path of the laser signal is doubled, and a photoacoustic effect is generated with the characteristic gas in the resonant gas chamber 6.

[0064] For the outer shell 1, a photoacoustic cell is made of a material with a larger wave impedance. The quality factor and the photoacoustic signal obtained under the same excitation are larger, such as pure iron, brass, and red copper with the top three wave impedances. Brass is preferably used as the outer shell material of the photoacoustic cell. Since its thermal conductivity is good enough, heat caused by laser heating of the gas in the cell can be exported through the boundary, so that the temperature uniformity of the gas in the cavity is good enough, and thus a larger quality factor is obtained.

[0065] For the laser 2, it is a tunable laser that combines an ordinary laser with a certain tuning mechanism so that the wavelength of the laser output can be adjusted within a certain range.

[0066] The optical window 3 is arranged in the first buffer cavity 4, so that the light source emitted by the laser can enter the gas chamber, generate a photoacoustic effect with the characteristic gas in the resonant gas chamber, and realize the photoacoustic spectroscopy detection of the characteristic gas;

[0067] For the optical window 3, the optical window is set according to the beam diameter. In order to ensure the reliability of the photoacoustic spectroscopy detection of the characteristic gas, the diameter of the set optical window should not exceed 90% of the beam diameter. Such a design can ensure that there is not too much loss when the beam passes through the optical window, and at the same time avoid the beam being clipped in the aperture of the system, resulting in energy waste and possible damage.

[0068] For the first buffer cavity 4, an air inlet window is arranged at the upper end, and an optical window is arranged on the left side. The characteristic gas enters the resonant gas chamber through the first buffer cavity, and the laser signal is incident on the resonant gas chamber through the optical window;

[0069] The first buffer cavity and the second buffer cavity are symmetrically arranged on both sides of the resonant gas chamber. The two buffer cavities should adopt a symmetrical layout. The symmetrical arrangement helps to obtain the maximum sound pressure signal at the characteristic frequency, thereby improving the sensitivity of the photoacoustic spectroscopy detection.

[0070] Preferably, the first buffer cavity 4 and the second buffer cavity 8 have a size of 70 mm in length and 60 mm in width. The buffer cavities of this size can effectively reduce the acoustic background noise and improve the signal-to-noise ratio of the system. The design of using a rounded corner connection between the buffer cavity and the resonant cavity can reduce the noise influence caused by gas flow, thereby increasing the amplitude of the sound pressure signal and improving the detection sensitivity of the system.

[0071] The resonant gas chamber 6 is connected to the first buffer cavity 4 and the second buffer cavity 8. The characteristic gas fills the first buffer cavity 4, the resonant gas chamber 6, and the second buffer cavity 8 from the air inlet window 5 at the upper end of the first buffer cavity 4. The laser emitted by the laser 2 is incident into the resonant gas chamber 6 through the optical window 3 arranged on the first buffer cavity 4. After being reflected by four reflectors 12, the optical path of the laser signal is doubled, and the laser signal generates a detectable photoacoustic effect with the characteristic gas in the resonant gas chamber 6.

[0072] There are two resonant gas chambers 6, and the two resonant gas chambers are arranged vertically to form an upper resonant gas chamber 6A and a lower resonant gas chamber 6B. Two resonant cavities with exactly the same shape and size have the same noise. During the process of reflecting the incident laser signal by the mirror, the optical path of the laser signal is doubled and the photoacoustic signal is also doubled, improving the signal-to-noise ratio.

[0073] The microphone 7 is installed at the center outside the resonant gas chamber and is horizontal with the inner surface of the resonant gas chamber to collect the acoustic signal generated inside the resonant gas chamber due to the photoacoustic effect.

[0074] The upper end of the second buffer chamber 8 is provided with an exhaust window 9. The second buffer chamber 8 can reduce the acoustic background noise. The first buffer chamber 4 and the second buffer chamber 8 are connected by a rounded corner connection method by the upper resonant gas chamber 6A and the lower resonant gas chamber 6B to reduce the noise impact caused by gas flow, thereby increasing the amplitude of the sound pressure signal and improving the detection sensitivity of the system.

[0075] The second buffer chamber 8 and the first buffer chamber 4 are exactly the same in size and shape.

[0076] The photodetector 10 is installed on the right side of the second buffer chamber 8, and the photosensitive surface is aligned with the light output end of the photoacoustic cell. The photodetector 10 converts the received optical signal into an electrical signal and sends it to the controller 11 for data processing.

[0077] The controller 11 is connected to the laser 2, the microphone 7, and the photodetector 10. It filters and amplifies the electrical signal received by the photodetector 10, and uses the output end to perform negative feedback regulation on the laser 2 to ensure stable light source output; it processes the detection signal transmitted by the microphone 7 to obtain the detection result of the characteristic gas.

[0078] Principle description:

[0079] Based on the principle of the Beer–Lambert law: When a beam of monochromatic light irradiates the surface of an absorption medium, after passing through a certain thickness of the medium, since the medium absorbs a part of the light energy, the intensity of the transmitted light will decrease. The greater the concentration of the absorption medium and the greater the thickness of the medium, the more significant the decrease in light intensity. The relationship is:

[0080]

[0081] Among them:

[0082] A represents the absorbance of acetylene gas in the photoacoustic cell;

[0083] represents the intensity of the incident optical signal of the laser;

[0084] Indicates the intensity of the transmitted light of acetylene gas in the photoacoustic cell;

[0085] K represents the absorption coefficient of acetylene in the photoacoustic cell for the laser signal;

[0086] l represents the optical path of the laser signal in the photoacoustic cell, usually in cm;

[0087] c represents the concentration of acetylene gas in the photoacoustic cell, and the unit can be g / L or mol / L.

[0088] According to the above formula, the physical meaning of Beer-Lambert's law is that when a parallel monochromatic laser signal vertically passes through the acetylene gas in the photoacoustic cell, the absorbance A of the acetylene gas in the photoacoustic cell is proportional to the concentration c of the acetylene gas and the optical path l of the laser signal in the photoacoustic cell.

[0089] In order to improve the sensitivity of photoacoustic spectroscopy for analyzing characteristic gases, reflectors are set at four relative positions in the buffer cavity. The installation angle of the reflectors meets the requirement of reflecting the laser signal to the next reflector. According to Beer-Lambert's law, the optical path of the laser signal is increased by adding reflectors, thereby increasing the absorbance of the characteristic gas and realizing the improvement of the sensitivity of photoacoustic spectroscopy detection of characteristic gases.

[0090] Acetylene gas with a certain concentration is filled into the first buffer cavity 4 through the intake window 5. Under the action of air pressure, the filled acetylene gas gradually fills the resonant gas chamber 6 and the second buffer cavity 8. At this time, the exhaust window 9 is in a closed state. While filling acetylene gas through the intake window 5, the controller 11 adjusts the parameters of the laser 2 to emit a laser signal that conforms to the parameters of the resonant gas chamber and the absorption wavelength of acetylene gas. The laser 2 emits a laser signal, which enters the gas chamber through the optical window 3 and enters the upper resonant gas chamber 6A after passing through the first reflector 12 near the laser 2. The laser signal passing through the upper resonant gas chamber 6A is transmitted to the second reflector 12 located in the upper part of the second buffer cavity 8. The laser signal reflected by the second reflector 12 is transmitted to the third reflector 12 located in the lower part of the second buffer cavity 8. The laser signal reflected by the third reflector 12 is transmitted into the lower resonant gas chamber 6B. The laser signal passing through the lower resonant gas chamber 6B is transmitted to the fourth reflector 12 located in the upper part of the first buffer cavity 4. The laser signal reflected by the fourth reflector 12 is transmitted to the first reflector 12, and the laser signal circulates between the four reflectors 12. During this process, since the wavelength of the laser signal conforms to the absorption wavelength of acetylene gas, the laser signal generates a photoacoustic signal with acetylene gas, which is picked up by the microphone 7 and converted into an electrical signal and transmitted to the controller 11. The controller 11 processes and analyzes the signal transmitted by the microphone 7 and presents it as Figure 2It is shown to the user in the following manner. During the continuous detection of acetylene gas, the photodetector 10 converts the received optical signal into an electrical signal and sends it to the controller 11 for data processing. The controller 11 filters and amplifies the received electrical signal, and uses the output terminal to perform negative feedback adjustment on the laser 2 to ensure stable light source output. During the detection of acetylene gas, according to the Beer-Lambert law, the optical path of the laser signal is increased by adding a mirror, thereby increasing the absorbance of the characteristic gas and realizing the improvement of the sensitivity of the photoacoustic spectroscopy detection of the characteristic gas.

[0091] Such as Figure 3 And Figure 4 , comparing the test curves generated by the photoacoustic cell with low sensitivity and the photoacoustic cell of this embodiment under the same conditions, it can be seen that the sensitivity has increased by 2.5 times. Comparison conditions: Acetylene gas with a volume fraction of 0.01% is filled into the photoacoustic cell at a rate of 200 mL / min, the application pressure of the photoacoustic cell is controlled under atmospheric pressure conditions, and the temperature is controlled at 25 °C.

[0092] Embodiment 2

[0093] A detection method, using the extended resonance photoacoustic cell for acetylene detection in Embodiment 1, includes the following steps:

[0094] Step 1, controlling the characteristic gas (acetylene gas) to enter the first buffer chamber 4 at a specific rate (such as 200 mL / min) through a mass flowmeter; the specific content of Step 1 includes:

[0095] Step 1.1, adjusting the angle of the mirror 12 so that the laser emitted by the light source returns along the resonance gas chamber 6, while avoiding the light hitting the inner wall to reduce the noise introduced by absorption;

[0096] Step 1.2, first filling the photoacoustic cell with high-purity N2 for 5 minutes, then filling the characteristic gas (acetylene gas with a volume fraction of 0.01%), and adjusting the rate; adjusting the application pressure of the extended resonance photoacoustic cell for acetylene detection to be controlled under atmospheric pressure conditions, and the temperature to be controlled at room temperature (25 °C).

[0097] Step 2, turning on the laser 2 to emit a laser signal, and adjusting the laser signal through the laser controller 11 and the function generator so that it can be absorbed by the characteristic gas (that is, the light beam is consistent with the absorption spectral line of acetylene gas molecules); the specific process of Step 2:

[0098] Step 2.1, modulate the current of the injection laser 2 by superimposing the triangular wave output by the function generator and the sine wave output by the lock-in amplifier to obtain a modulation frequency, so that the output light source wavelength falls within the absorption peak (acetylene absorption peak) range of the characteristic gas, and measure the photoacoustic signal multiple times and take the average value; the role of controlling the modulation frequency is to enable the emitted light source to be better absorbed by acetylene gas and improve the detection sensitivity; modulate the light source frequency before the light source emits.

[0099] Step 2.2, change the operating temperature of the laser 2 to adjust the resonance frequency, and use the resonance acoustic spectroscopy method to measure the actual resonance frequency of the photoacoustic cell in real time, realizing the negative feedback adjustment of the resonance frequency, so that the operating frequency can be accurately adjusted to the standard frequency of the laser 2 in real time.

[0100] Step 3, collect the acoustic wave signal formed after the characteristic gas absorbs the laser signal and is converted.

[0101] Step 4, demodulate the acoustic wave signal through the lock-in amplifier, and then collect the information by the acquisition card to obtain the detection result of the characteristic gas photoacoustic spectrum. The specific process includes:

[0102] Step 4.1, provide a reference signal through the lock-in amplifier, whose frequency is very close to the expected frequency of the acoustic wave signal and has a fixed phase relationship; after this reference signal is mixed with the obtained acoustic wave signal, sum and difference frequency signals are generated through the mixer, and then the high-frequency components are filtered out through the low-pass filter, and the low-frequency difference frequency signal is retained, where the difference frequency signal contains the amplitude and phase information of the acoustic wave signal.

[0103] Step 4.2, determine the amplitude and phase information of the acoustic wave signal by measuring the amplitude and phase of the mixed frequency signal through the lock-in amplifier; in this process, the real part (X) and the imaginary part (Y) are extracted respectively, and they correspond to the amplitude and phase information of the acoustic wave signal respectively.

[0104] Step 4.3, further amplify the generated difference frequency signal through the amplifier to enhance the intensity of the detection signal and make it suitable for subsequent processing.

[0105] Step 4.4, convert the amplified difference frequency signal into a digital signal through the data acquisition card, and the signal processed by the lock-in amplifier will be input into the data acquisition card (DAQ). The data acquisition card converts the analog signal into a digital signal so that it can be read and processed by the computer. The data acquisition card realizes the conversion from analog signal to digital signal through the analog-to-digital converter (ADC), and has a certain resolution and sampling rate to provide data accuracy and data points.

[0106] Step 4.5, obtain the amplitude and phase of the acoustic wave signal, that is, the detection result of the photoacoustic spectrum, after being processed by the algorithm in the controller 11. To extract useful information and conduct final data analysis.

[0107] Among them, the algorithm for the controller to process the data collected by the data acquisition card is as follows:

[0108] S1: According to the spectral characteristics of acetylene gas under a nitrogen background, simulate and generate spectral data containing the characteristic absorption peaks of acetylene, and add simulated noise to simulate the data in the actual measurement process.

[0109] S2: Perform baseline correction on the simulated or actually collected spectral data to eliminate the baseline drift caused by instrument or environmental factors, and ensure that the characteristic peaks of acetylene gas are more obvious.

[0110] S3: Adopt the Savitzky-Golay filtering noise reduction technique to smooth the data through polynomial fitting while retaining the shape of the acetylene characteristic peaks, and complete the processing of the spectral data.

[0111] S4: Normalize the processed spectral data to standardize its numerical range to eliminate the range differences under different measurement conditions and facilitate subsequent quantitative analysis.

[0112] S5: Extract the characteristic absorption peaks of acetylene gas from the normalized spectral data, and these characteristic peaks are associated with the concentration of acetylene gas.

[0113] S6: According to the extracted characteristic absorption peaks, establish a quantitative relationship model between the concentration of acetylene gas and the absorption peak intensity to achieve accurate determination of the concentration of acetylene gas in oil.

[0114] S7: Output the concentration result of acetylene gas and compare it with a standard sample with a known concentration for verification to ensure the accuracy and reliability of the analysis result.

[0115] S8: Display the spectral data before and after processing and the analysis result of the acetylene gas concentration in the form of a chart to facilitate the observation and analysis of the detection effect of acetylene gas under a nitrogen background. The acoustic wave signal converted after the gas molecules in the photoacoustic cell absorb light energy is collected by a microphone installed flush with the cavity wall at the center of the resonant cavity, and the sensitivity of the microphone is 50 mV / Pa.

[0116] In several embodiments provided in the present application, it should be understood that the disclosed devices and systems can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of the units is only a logical function division, and there may be other division methods in actual implementation. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point, the displayed or discussed coupling or direct coupling or communication connection between each other can be through some interfaces, and the indirect coupling or communication connection of the devices or units can be in electrical, mechanical or other forms.

[0117] As described above, the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. An extended-range resonant photoacoustic cell for acetylene detection, characterized in that: It includes: shell; A first buffer chamber is arranged in the shell, and an air inlet window is provided at the top of the first buffer chamber and an optical window is provided at the side thereof, so that characteristic gas can enter the first buffer chamber through the air inlet window; A second buffer chamber is disposed in the housing, and the second buffer chamber is provided with an exhaust window so that the characteristic gas can be discharged from the second buffer chamber through the exhaust window; A resonant air chamber is arranged between the first buffer chamber and the second buffer chamber to connect the first buffer chamber and the second buffer chamber; there are two resonant air chambers, and the two resonant air chambers are arranged up and down to form an upper resonant air chamber and a lower resonant air chamber; A laser, the laser is located at one side of the first buffer cavity and outside the optical window, and provides laser light to generate a photoacoustic spectroscopy effect with a characteristic gas in the resonant gas chamber; There are four reflectors, which are symmetrically arranged in pairs in the first buffer cavity and the second buffer cavity, so that the laser emitted by the laser is reflected by the four reflectors, thereby doubling the optical path of the laser signal; during use, the angles between the four reflectors and the horizontal plane are adjusted according to needs; the four reflectors can realize the circulation of the laser signal in the photoacoustic cell; by setting up four reflectors, the optical path of the laser signal is increased, thereby increasing the absorbance of the characteristic gas, and realizing the improvement of the sensitivity of the photoacoustic spectrum detection of the characteristic gas; A photodetector, used for detecting the optical signal of the photoacoustic spectroscopy effect, and for converting the optical signal into an electrical signal; A microphone, located on the top of the resonant air chamber, for detecting the acoustic signal of the photoacoustic spectroscopy effect; The controller is connected to the laser, photodetector and microphone respectively, and is used to filter and amplify the electrical signal generated by the photodetector, and to perform negative feedback regulation on the laser to ensure the stability of the light source output. At the same time, it processes the detection signal obtained by the microphone to obtain the detection result of the characteristic gas.

2. The extended-range resonant photoacoustic cell for acetylene detection according to claim 1, characterized in that: The first buffer cavity and the second buffer cavity are symmetrically arranged on both sides of the resonant air chamber.

3. The extended-range resonant photoacoustic cell for acetylene detection according to claim 1, characterized in that: The microphone is installed on the center line of the outer side of the resonance air chamber and is level with the inner surface of the upper resonance air chamber.

4. A detection method, using the extended-range resonant photoacoustic cell for acetylene detection according to any one of claims 1 to 3, comprising the following steps: Step 1, controlling the characteristic gas to enter the extended-range resonant photoacoustic cell for acetylene detection; specifically comprising: Step 1.1, adjust the angle of the reflector so that the laser emitted by the light source returns along the resonant gas chamber, while preventing the light from hitting the inner wall to reduce the noise introduced by absorption; Step 1.2, the photoacoustic cell is first filled with high-purity N2, and then filled with characteristic gas, and the rate is adjusted; the application pressure of the extended-range resonant photoacoustic cell for acetylene detection is adjusted to be controlled at normal pressure conditions, and the temperature is controlled at normal temperature; Step 2, emitting a laser signal, and adjusting the laser signal so that it can be absorbed by the characteristic gas; specifically comprising: Step 2.1, modulate the current injected into the laser by superimposing the triangular wave output by the function generator and the sine wave output by the phase-locked amplifier to obtain the modulation frequency, so that the wavelength of the output light source falls within the absorption peak range of the characteristic gas, and measure the photoacoustic signal multiple times to obtain the average value; Step 2.2, changing the operating temperature of the laser to adjust the resonance frequency, using the resonance acoustic spectrum method to measure the actual resonance frequency of the photoacoustic cell in real time, and realizing negative feedback regulation of the resonance frequency; Step 3, collecting the acoustic wave signal formed after the characteristic gas absorbs the laser signal and converts it; Step 4: Process the acoustic wave signal to obtain characteristic gas photoacoustic spectrum detection results.

5. The detection method according to claim 4, characterized in that: In step 4, the acoustic wave signal is demodulated by a lock-in amplifier, and the specific process includes: Step 4.1, a reference signal is provided through a phase-locked amplifier, and after the reference signal is mixed with the obtained sound wave signal, a sum frequency and a difference frequency signal are generated through a mixer, and then a high-frequency component is filtered out through a low-pass filter, and a low-frequency difference frequency signal is retained, wherein the difference frequency signal contains the amplitude and phase information of the sound wave signal; Step 4.2, measuring the amplitude and phase of the mixing signal by a lock-in amplifier to determine the amplitude and phase information of the acoustic wave signal; Step 4.3, further amplifying the generated difference frequency signal through an amplifier; Step 4.4, converting the amplified difference frequency signal into a digital signal; Step 4.5, after processing, the amplitude and phase of the sound wave signal are obtained.

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