CN107389597B - High-sensitivity gas detection device and method - Google Patents
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- SFZCNBIFKDRMGX-UHFFFAOYSA-N sulfur hexafluoride Chemical compound FS(F)(F)(F)(F)F SFZCNBIFKDRMGX-UHFFFAOYSA-N 0.000 abstract description 4
- 229960000909 sulfur hexafluoride Drugs 0.000 abstract description 3
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- RWSOTUBLDIXVET-UHFFFAOYSA-N Dihydrogen sulfide Chemical compound S RWSOTUBLDIXVET-UHFFFAOYSA-N 0.000 description 9
- 229910000037 hydrogen sulfide Inorganic materials 0.000 description 9
- WUKWITHWXAAZEY-UHFFFAOYSA-L calcium difluoride Chemical compound [F-].[F-].[Ca+2] WUKWITHWXAAZEY-UHFFFAOYSA-L 0.000 description 5
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Abstract
The invention particularly relates to a high-sensitivity decomposed gas detection device and method, and mainly solves the problem of the traditional SF (sulfur hexafluoride)6The method for detecting the decomposed substances has the technical problems of easy influence of the environment, poor stability, low detection sensitivity and cross influence among different gases. The invention effectively reduces the noise of the sensing system by skillfully designing the photoacoustic cell with the differential structure. In addition, synchronous detection of multiple gases is realized in a single photoacoustic cell through a multiplexing time division multiplexing technology, and SF can be accurately measured6The concentration of the decomposed gas is low in cost, high in detection sensitivity and good in selectivity compared with a sensing system adopting a wide-spectrum infrared light source, and can meet the requirement of SF in the conventional power system6Actual detection requirements of the decomposition products.
Description
Technical Field
The invention belongs to the technical field of gas detection devices and methods, and particularly relates to a high-sensitivity gas detection device and method.
Background
In domestic and foreign electric power systems, sulfur hexafluoride (SF)6) The gas has excellent gas insulation and strong arc extinguishing performance, and is generally used as an insulating gas in high-voltage equipment such as a gas insulated switchgear (GCB), a gas insulated metal enclosed switchgear (GIS), a Gas Insulated Transformer (GIT), and a Gas Insulated Line (GIL). Pure SF6Is a colorless, tasteless and nontoxic gas, and has stable chemical properties below 300 ℃. However, in the long-term operation of high-voltage equipment, due to the manufacturing process problems of uneven silver plating, shedding or oxide layer formation and the like, local corona, spark or arc discharge can be generated at the line joint of the high-voltage equipment, and finally local overheating faults of the equipment are caused, and SF (sulfur hexafluoride) is caused6The gas is decomposed. SF6Decomposition of the component and traces of water vapor (H) in the plant2O), oxygen (O)2) The impurities are subjected to a series of complex chemical reactions to generate H2S、SO2、SF4、CO、CF4And SOF2Decomposition products, etc., which corrode the insulation material, accelerate the aging of the insulation, and form more seriousOverheating failures may even eventually lead to equipment breakdown.
For SF6The detection of the concentration of the gas decomposition products is one of the important methods for diagnosing the fault of the high-voltage equipment, and can be used for judging the fault type of the insulating equipment. Conventional SF6The methods for detecting the decomposition products mainly include gas chromatography, detection tube method, electrochemical sensing method and the like, but the methods are easily influenced by the environment, have poor stability and low detection sensitivity, and have cross influence among different gases. In recent years, gas detection technology based on infrared optics has been applied to trace gas detection due to its advantages of good selectivity, long service life, high sensitivity, and the like. But existing optical-based SF6The technology for detecting the decomposed substances still has the defects, for example, in the patent application No. 201010295554.0, the gas production bag is required to be used for gas production, the gas concentration cannot be monitored in real time, and the device is large in size, needs manual operation of reading of an oscilloscope and is inconvenient for on-site automatic detection; in both patent applications (CN201210379772.1) and (CN201210216101.3), SF is detected by using a broad spectrum mid-infrared light source6The nature of the decomposition products is that different gases to be detected have respective characteristic absorption spectra in the infrared spectrum region and are used for distinguishing and detecting the types and the concentrations of the gases to be detected. Trace amount of SF6Characteristic absorption spectra of gases in the infrared band without overlap with their decomposition gases, but in high-pressure installations, SF6The gas acts as an insulating gas and is typically present in a concentration greater than 99%. Experiments prove that the high-concentration SF6The background gas has obvious and continuous absorption lines in the middle infrared band, so that trace SF is detected6When decomposing gas, the gas is subjected to high concentration of SF6The influence of gas absorption causes poor detection sensitivity and the inability to accurately detect SF in real time6Decompose the gas and thus cannot meet the practical application requirements.
Disclosure of Invention
The invention aims to solve the problem of the traditional SF6The method for detecting the decomposed substances has the technical problems of easy environmental influence, poor stability, low detection sensitivity and cross influence among different gases, and provides high-sensitivity SF6Decomposed gas detection deviceProvided are a method and a device.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows:
high-sensitivity SF6The decomposed gas detection device comprises a photoacoustic cell, a differential amplifier, a phase-locked amplifier, a computer system, a first function generator, a second function generator, an adder, a laser driving plate, a near-infrared excitation light source, an optical fiber amplifier and a beam collimator, wherein the signal output ends of the first function generator and the second function generator are respectively connected with two signal input ends of the adder, the output end of the adder is connected with a current driving port of the laser driving plate, the current output end of the laser driving plate is connected with the current input end of the near-infrared excitation light source, laser output by the near-infrared excitation light source is connected to a light beam input port of the optical fiber amplifier through an optical fiber, a light beam output port of the optical fiber amplifier is connected to the beam collimator through the optical fiber, a light beam output by the beam collimator passes through one resonant cavity of the photoacoustic cell, and two microphone output signals, the output signal of the differential amplifier is sent to the signal input end of the phase-locked amplifier, the synchronous output end of the first function generator is connected with the synchronous input end of the phase-locked amplifier, and the signal output end of the phase-locked amplifier is connected with the signal input end of the computer system.
The photoacoustic cell comprises a shell, two electret condenser microphones, two optical glass windows and two fixing frames, two ports of the shell are respectively provided with a gas buffer chamber, the inner cavity of the shell is horizontally provided with two parallel photoacoustic resonant cavities, and two ports of the two photoacoustic resonant cavities are communicated with the gas buffer chambers arranged at two ends of the shell, an air inlet is arranged on the side wall of the left end of the shell and is communicated with an air buffer chamber arranged at the left end of the shell, the side wall of the right end of the shell is provided with an air outlet which is communicated with an air buffer chamber arranged at the right end of the shell, two small microphone holes are arranged in parallel in the middle of the side wall of the shell and are respectively communicated with the two photoacoustic resonant cavities, the two electret capacitor microphones are respectively arranged on the two small microphone holes, and the two optical glass windows are respectively fixed at two ports of the shell through two fixing frames.
Each resonant cavity length may be 50mm, 60mm, 70mm, 80mm, 90mm, 100mm, 110mm, 120 mm; the diameters of the resonant cavities can be 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm and 12 mm; the diameter of the small hole can be 1mm, 1.5mm or 2 mm; the thickness of the buffer chamber may be 5mm, 10mm or 15 mm.
The two microphone small holes are adjacently arranged, and the diameter of each microphone small hole is 1mm-2 mm.
Sealing rings are arranged between the fixing frame and the optical glass window and between the optical glass window and the port of the shell.
The symmetrical double-resonant cavity structure has strong inhibition capability on common-mode noise, and can effectively eliminate air flow noise, window noise and external electromagnetic interference noise. The two gas buffer chambers make the resonant cavity an open cavity-open cavity structure, in which the sound wave in the cavity can form a primary longitudinal standing wave mode, two nodes, i.e. the positions with the minimum sound pressure, are positioned at two ends of the resonant cavity, and antinodes, i.e. the positions with the maximum sound pressure, are positioned at the axial center of the resonant cavity, so that the microphone can be assembled at the axial center of the resonant cavity. The microphones are arranged in parallel from the top end of the resonant cavity, so that the two microphones are close to each other, the length of a signal line is shortened, and the probability of external noise entering is reduced. The air inlet and the air outlet are arranged at the top end of the buffer air chamber, so that the interference of air flow noise to the resonant cavity can be reduced.
The material of the optical glass window can be calcium fluoride which is transparent to infrared rays, and K9 glass which is coated with an antireflection film can also be selected. The sensitivity for the electret capacitor microphone air separation is larger than-30 dB and smaller than-20 dB.
Utilizing the high-sensitivity SF6The method for detecting the gas by the decomposed gas detection device comprises the following steps:
(a) the first function generator outputs sine wave modulation signals, the frequency of the sine signals is half of the resonant frequency of the photoacoustic cell, the voltage peak value is the optimal modulation depth of the near-infrared excitation light source, the sine wave modulation signals are used for modulating laser wavelength, the second function generator outputs slope scanning signals, the center of the signal voltage corresponds to the central absorption line of the gas to be measured, the two paths of voltage signals are added through the adder and then input to the laser driving plate to drive the near-infrared excitation light source, and the driving temperature of the near-infrared excitation light source is kept unchanged; (b) the laser modulated by the wavelength is input into a light beam input port of an optical fiber amplifier of a near-infrared wave band through an optical fiber jumper, and the output power of the laser reaches 1-15 watts; (c) the high-power laser output from the beam output end of the optical fiber amplifier passes through the beam collimator to output parallel beams, and the parallel beams are input into a photoacoustic resonant cavity in the photoacoustic cell and used for exciting gas to generate photoacoustic signals; (d) the modulated photoacoustic signal is detected by a corresponding high-sensitivity electret condenser microphone and converted into a current signal; (e) weak two paths of current signals are differentially amplified by a differential amplifier and then input to a signal input end of a phase-locked amplifier, a synchronous signal of a first function generator is input to a synchronous input end of the phase-locked amplifier, the integration time of the phase-locked amplifier is set to be 1S, the filtering slope is 12dB/oct, the corresponding detection bandwidth is 0.25Hz, and finally the amplitude S of the photoacoustic signal is obtained after demodulation of the phase-locked amplifier and is transmitted to a computer system; (f) the amplitude S of the photoacoustic signal is in a proportional relationship with the gas concentration D, and can be recorded as D ═ a × S, wherein a is a proportionality coefficient, the photoacoustic signal is obtained by introducing known concentrations of different gases in advance, and the computer system can calculate the concentration of the gas to be measured by the formula D ═ a × S and the obtained amplitude S of the photoacoustic signal.
High-sensitivity SF6The decomposed gas detection device comprises a photoacoustic cell, a differential amplifier, a phase-locked amplifier, a computer system, a first function generator, a second function generator, an adder, n laser drive plates, n near-infrared excitation light sources with different wavelengths, an optical fiber amplifier, a beam collimator and n optical switches, wherein the signal output ends of the first function generator and the second function generator are respectively connected with two signal input ends of the adder, the output end of the adder is respectively connected with the current drive ports of the n laser drive plates, the current output ends of the n laser drive plates are respectively connected with the current input ends of the n near-infrared excitation light sources with different wavelengths, lasers output by the n near-infrared excitation light sources with different wavelengths are respectively connected with n input ports of the n optical switches through optical fibers, and the output port of the optical switch is connected with the optical fiber amplifierThe optical fiber amplifier comprises a light beam input port of an amplifier, a light beam output port of an optical fiber amplifier is connected to a light beam collimator through an optical fiber, light beams output by the light beam collimator pass through a resonant cavity of a photoacoustic cell, output signals of two microphones of the photoacoustic cell are connected with two inputs of a differential amplifier, output signals of the differential amplifier are sent to a signal input end of a phase-locked amplifier, a synchronous output end of a first function generator is connected with a synchronous input end of the phase-locked amplifier, and a signal output end of the phase-locked amplifier is connected with a signal input end of a computer.
The photoacoustic cell comprises a shell, two electret condenser microphones, two optical glass windows and two fixing frames, two ports of the shell are respectively provided with a gas buffer chamber, the inner cavity of the shell is horizontally provided with two parallel photoacoustic resonant cavities, and two ports of the two photoacoustic resonant cavities are communicated with the gas buffer chambers arranged at two ends of the shell, an air inlet is arranged on the side wall of the left end of the shell and is communicated with an air buffer chamber arranged at the left end of the shell, the side wall of the right end of the shell is provided with an air outlet which is communicated with an air buffer chamber arranged at the right end of the shell, two small microphone holes are arranged in parallel in the middle of the side wall of the shell and are respectively communicated with the two photoacoustic resonant cavities, the two electret capacitor microphones are respectively arranged on the two small microphone holes, and the two optical glass windows are respectively fixed at two ports of the shell through two fixing frames.
Each resonant cavity length may be 50mm, 60mm, 70mm, 80mm, 90mm, 100mm, 110mm, 120 mm; the diameters of the resonant cavities can be 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm and 12 mm; the diameter of the small hole can be 1mm, 1.5mm or 2 mm; the thickness of the buffer chamber may be 5mm, 10mm or 15 mm.
The two microphone small holes are adjacently arranged, and the diameter of each microphone small hole is 1mm-2 mm.
Sealing rings are arranged between the fixing frame and the optical glass window and between the optical glass window and the port of the shell.
The symmetrical double-resonant cavity structure has strong inhibition capability on common-mode noise, and can effectively eliminate air flow noise, window noise and external electromagnetic interference noise. The two gas buffer chambers make the resonant cavity an open cavity-open cavity structure, in which the sound wave in the cavity can form a primary longitudinal standing wave mode, two nodes, i.e. the positions with the minimum sound pressure, are positioned at two ends of the resonant cavity, and antinodes, i.e. the positions with the maximum sound pressure, are positioned at the axial center of the resonant cavity, so that the microphone can be assembled at the axial center of the resonant cavity. The microphones are arranged in parallel from the top end of the resonant cavity, so that the two microphones are close to each other, the length of a signal line is shortened, and the probability of external noise entering is reduced. The air inlet and the air outlet are arranged at the top end of the buffer air chamber, so that the interference of air flow noise to the resonant cavity can be reduced.
The material of the optical glass window can be calcium fluoride which is transparent to infrared rays, and K9 glass which is coated with an antireflection film can also be selected. The sensitivity for the electret capacitor microphone air separation is larger than-30 dB and smaller than-20 dB.
Utilizing the high-sensitivity SF6The method for detecting the gas by the decomposed gas detection device comprises the following steps:
(a) the first function generator outputs sine wave modulation signals, the frequency of the sine signals is half of the resonant frequency of the photoacoustic cell, the voltage peak value is the optimal modulation depth of the n near-infrared excitation light sources with different wavelengths, the sine wave modulation signals are used for modulating laser wavelength, the second function generator outputs slope scanning signals, the center of the signal voltage corresponds to the central absorption line of the gas to be measured, the two paths of voltage signals are added through an adder and then input to n laser driving plates to drive the n near-infrared excitation light sources with different wavelengths, and the driving temperature of the n near-infrared excitation light sources with different wavelengths is kept unchanged; (b) the n wavelength-modulated lasers are connected to the input ends of the n paths of optical switches, and a computer system controls which path is output; (c) laser beams output from the n paths of optical switches are input into a beam input port of an optical fiber amplifier of a near infrared band through optical fibers, and the output power of the laser is enabled to reach 1-15 watts; (d) after passing through a beam collimator, the high-power laser outputs a parallel beam, and is input into a photoacoustic resonant cavity in a photoacoustic cell to excite gas to generate photoacoustic signals; (e) the modulated photoacoustic signal is detected by a corresponding high-sensitivity electret condenser microphone and converted into a current signal; (f) weak two routesAfter being differentially amplified by a differential amplifier, current signals are input to a phase-locked amplifier for synchronous demodulation, synchronous signals of a first function generator are input to a synchronous input end of the phase-locked amplifier, the integration time of the phase-locked amplifier is set to be 1S, the filtering slope is 12dB/oct, the corresponding detection bandwidth is 0.25Hz, and finally the amplitude S of photoacoustic signals is obtained after demodulation of the phase-locked amplifiernAnd transmitting to the computer system; (f) amplitude S of photoacoustic signalnAnd gas concentration DnProportional relation, can be recorded as Dn＝an×SnWherein a isnIs proportional coefficient obtained by introducing known concentrations of different gases in advance, and the computer system is calculated by formula Dn＝an×SnAnd the amplitude S of the photoacoustic signal obtainednThe concentration of the gas to be detected can be calculated; (h) near-infrared excitation light sources with different wavelengths are switched through n paths of optical switches, so that different gas types can be detected in turn.
The design theory of the invention is as follows:
SF6the sensitivity of the decomposed gas detection device is determined by the signal-to-noise ratio (S/N, S is the signal amplitude, N is the noise). To obtain a higher sensitivity, the signal amplitude can be increased on the one hand and the noise can be reduced on the other hand. The invention reduces noise by skillfully designing the structure of the photoacoustic cell and adopts various ways to improve the photoacoustic signal amplitude of the sensing system. The specific structure of the photoacoustic cell and the way to boost the photoacoustic signal are illustrated as follows:
in order to reduce noise, the design of the photoacoustic cell adopts a dual-acoustic resonant cavity structure. A highly sensitive electret condenser microphone is respectively arranged at the longitudinal center of each photoacoustic resonant cavity and used for detecting photoacoustic energy accumulated in the photoacoustic resonant cavity and converting the photoacoustic energy into a current signal. Two microphones are compactly arranged on the top of each photoacoustic resonant cavity, so that the transmission distance of current signals is shortened, and weak current signals can be prevented from being submerged in electromagnetic noise. Experiments prove that the adjacently arranged microphone structure can effectively reduce the noise of the photoacoustic cell to 0.12 muV. Compared with a microphone structure which is not adjacently arranged, the detection sensitivity of the gas detection device is improved by 14 times under the conditions of the same concentration of the gas to be detected, an excitation light source and the like. The two sides of the photoacoustic resonant cavity are respectively provided with a gas buffer chamber, so that the photoacoustic resonant cavity is used as an open cavity structure, and the influence of air flow noise can be effectively reduced. One gas buffer chamber is connected with the gas inlet, and the other gas buffer chamber is connected with the gas outlet. One side of each gas buffer chamber is provided with an optical window to form a closed cavity, and the light source is ensured to enter the photoacoustic resonant cavity to generate photoacoustic signals and output the photoacoustic signals from the other end.
The resonance photoacoustic cell adopting the differential configuration can effectively avoid the interference of airflow noise, window noise and peripheral environment electromagnetic noise, and greatly improve the detection signal-to-noise ratio of the photoacoustic detection module. Due to the two identical optical resonant cavities, the microphones, the gas buffer chamber and the optical window, the structure of the photoacoustic cell is ensured to be bilaterally symmetrical, so that when no excitation light source generates photoacoustic signals, the signals collected by each microphone are identical. After the collected current signals of the two microphones are differentially amplified, theoretically, the signal current output of the photoacoustic cell is zero at the moment.
Furthermore, the photoacoustic cell adopts a resonance structure, and the sound pressure mode in the resonant cavity is a primary longitudinal standing wave mode, so that in order to enable the photoacoustic signal to form resonance with the resonance frequency of the resonant cavity, the accumulated photoacoustic energy is maximized, and the excitation light source needs to be modulated (>1kHz), so that the influence of 1/f noise can be reduced.
The photoacoustic signal amplitude S of the gas detection apparatus can be expressed as:
S＝CαP0(1)
where C is the photoacoustic cell constant, α is the absorption coefficient of the gas molecule to be measured, which is proportional to the gas concentration and the absorption line intensity of the gas to be measured, P0Is the output power of the laser light source. Therefore, three approaches are available for improving the photoacoustic signal of gas sensing systems: (1) selecting a stronger gas absorption line; (2) selecting a higher power excitation light source for use; (3) designing and manufacturing a photoacoustic cell with a larger constant C.
At present, the number of the current day,due to the wavelength limitation of the light source, the infrared absorption spectrum of the gas molecules is generally used for measuring the gas concentration in the following two wavelength regions: the near infrared region (0.78-2 μm) and the mid-infrared region (2-25 μm). The fundamental frequency vibration absorption of gas molecules in the mid-infrared region is two to three orders of magnitude stronger than the overtone vibration absorption in the near-infrared region, so the mid-infrared region is the optimal gas detection band. But due to SF6The gas has obvious and continuous absorption lines in the middle infrared, and the measurement of other gases is interfered, so that the gas cannot be used; in addition, the laser in the middle infrared wavelength region has high cost, low power and high requirement on the operating environment, and is not beneficial to industrial application and popularization. But SF when the wavelength is less than 3.5 microns6Substantially without absorption, if possible for SF6The measurement of the decomposition product is placed in the near infrared region below 3.5 microns, and the problem of carrier gas SF can be solved well6The measurement interference problem of (2) and its low price can greatly reduce the cost of the sensing system. However, the gas absorption coefficient in the near infrared wavelength region is generally lower than that in the mid infrared band by several orders of magnitude, and detection in the near infrared band means poor sensitivity and low measurement accuracy. In the present invention, the path (1) for increasing the signal amplitude is not available, and only the paths (2) and (3) can be used.
With the development of optical fiber communication technology in recent years, a low-cost optical fiber amplifier in a communication waveband can pump the output energy of near infrared laser to 1-20 watts or even higher. According to the formula (1), the power is increased, so that the defect of low absorption coefficient in a near infrared wavelength region can be made up, and the strength of the photoacoustic signal is improved. The photoacoustic cell constant C describes the detection capability of the photoacoustic cell, and can be represented by:
where F is the resonant frequency, Q is the quality factor, V is the resonant cavity volume, L is the resonant cavity length, F is the spatial overlap of the laser beam and the standing acoustic mode, and σ is the specific heat capacity of the background gas. The magnitude of the C value is mainly influenced by the Q factor, the cross-sectional area (V/L) of the resonance chamber, and the carrier gas species (σ).
The Q-factor of a photoacoustic resonance cavity is defined as the ratio of the stored acoustic energy to the dissipated acoustic energy over a period. The larger the Q value, the larger the photoacoustic sensing block constant C, and thus the larger the signal of the sensing system. The Q value of the photoacoustic detection module can be calculated by the following equation:
wherein R is the inner diameter of the resonant cavity, dvAnd dhRespectively, the thickness of the viscous boundary layer and the thickness of the thermal boundary layer, the gas viscosity mu and the gas density rho0Gas thermal conductivity к, molar mass M and gas specific heat capacity cpThickness d of viscous boundary layervAnd thickness d of thermal boundary layerhThe expression is as follows:
for the same photoacoustic cell, through theoretical calculation and experimental verification, for the photoacoustic cell in N2Low Q photoacoustic cell as background gas, at SF6The background gas can be automatically converted into a high-Q photoacoustic cell, so that photoacoustic signals are increased, and the detection signal-to-noise ratio is improved.
Thus, although the present invention uses the non-interference near infrared wavelength region with weak absorption line below 3.5 microns for SF6The measurement of the decomposition products is carried out, but the photoacoustic signal amplitude in the near infrared region is effectively improved by increasing the laser power and improving the photoacoustic cell constant.
It is worth noting that according to equation (1), the laser power is proportional to the signal amplitude, with the higher the power, the higher the amplitude. In practice, however, the power of the laser is not as high as possible. With the increase of the output power of the laser, on one hand, a saturation effect occurs, the amplitude and the power of a detected gas signal are not in a linear relation, on the other hand, the quality of a laser beam begins to be deteriorated, and part of stray light can hit the wall of the photoacoustic cell cavity, so that the noise of the system is increased. FIG. 6 shows the signal amplitude of CO at different powers, and the nonlinear relationship gradually appears as the power increases, and the experimental result shows that the optimal power range is 1-15W. As the power increases, the beam diameter becomes larger and lower noise levels are not achieved, requiring the use of larger diameter photoacoustic resonators, with diameters ranging from 5-12mm for beam masses of 1-15W.
To ensure that a single primary longitudinal standing wave mode is generated, the length of the photoacoustic resonance cavity needs to be at least 10 times greater than its diameter, and thus the length of the photoacoustic resonance cavity is set to 50-120mm, which corresponds to a resonance frequency of 550Hz-1.3 kHz. The size of the small hole of the microphone connected with the photoacoustic resonant cavity cannot be too small, otherwise, the small hole cannot effectively transmit pressure to the microphone, and cannot be too large, otherwise, a primary longitudinal standing wave mode in the cavity is destroyed, and experiments prove that the diameter of 1-2mm is the optimal diameter.
By adopting the technical scheme, compared with the background technology, the invention has the beneficial effects that: the noise of the sensing system is effectively reduced by skillfully designing the photoacoustic cell with the differential structure. A near infrared light source in the communication wavelength region is used as an excitation light source, and an optical fiber amplifier is used to pump the output power of the laser to the order of 1-15 watts. Under the condition of ensuring the cost, the method for improving the exciting light power makes up the disadvantage that the absorption coefficient of the gas to be detected in a near infrared wavelength region is lower than that in a middle infrared wavelength region, and is beneficial to industrial application and popularization. In addition, the invention adopts theoretical simulation and experimental verification respectively aiming at the situation that the high-power light source brings saturation effect and the quality of light beam is poor, so as to obtain the optimal size of the photoacoustic resonant cavity and improve the detection sensitivity of the sensor; aiming at the application background of the photoacoustic resonant cavity in different background gases, the Q value of the photoacoustic resonant cavity is theoretically and experimentally optimized, the photoacoustic resonant cavity with high Q factor is obtained, and the detection limit of the sensing system is further improved. By multiplexing time-division multiplexing techniques on sheetsSynchronous detection of multiple paths of gases is realized in each photoacoustic cell. By using the SF according to the invention6Decomposer module, detection device and method, high-power excitation light source and background gas induced high-Q photoacoustic detection module, SF capable of being accurately measured6The concentration of the decomposed gas is low in cost, high in detection sensitivity and good in selectivity compared with a sensing system adopting a wide-spectrum infrared light source, and can meet the requirement of SF in the conventional power system6Actual detection requirements of the decomposition products.
Drawings
FIG. 1 shows a single-component SF according to the invention6A schematic structural diagram of a decomposition gas detection device;
FIG. 2 shows multicomponent SF according to the invention6A schematic structural diagram of a decomposition gas detection device;
FIG. 3 is an external view of a photoacoustic cell of the present invention;
FIGS. 4 and 5 are exploded views of photoacoustic cells of the present invention;
FIG. 6 shows SF according to the present invention6The photoacoustic signal amplitude of the decomposed gas detection device under different laser powers;
FIG. 7 shows SF according to the invention6Detection device for decomposed gas for trace amount H2S, the histogram is H corresponding to the wavelength region2S gas molecular line is strong, H to be measured2The concentration of S gas was 25 ppm.
Detailed Description
Example 1
As shown in FIG. 1, a high-sensitivity single-component SF in this example6Decomposed gas detection device, including optoacoustic cell 1, difference amplifier 2, lock-in amplifier 3, computer system 4, first function generator 5, second function generator 6, adder 7, laser drive plate 8, hydrogen sulfide laser 9, optical fiber amplifier 10 and beam collimator 11, two signal input part of adder 7 are connected respectively to first function generator 5 and 6 signal output part of second function generator, and the current drive mouth of laser drive plate 8 is connected to the output of adder 7, and the current input part of hydrogen sulfide laser 9 is connected to the current output of laser drive plate 8, and the laser of hydrogen sulfide laser 9 output is connected to light through optic fibreThe optical fiber amplifier comprises a light beam input port of an optical fiber amplifier 10, a light beam output port of the optical fiber amplifier 10 is connected to an optical beam collimator 11 through an optical fiber, a light beam output by the optical beam collimator 11 passes through a resonant cavity of an optical-acoustic cell 1, two microphone output signals of the optical-acoustic cell 1 are connected with two paths of inputs of a differential amplifier 2, an output signal of the differential amplifier 2 is sent to a signal input end of a phase-locked amplifier 3, a synchronous output end of a first function generator 5 is connected with a synchronous input end of the phase-locked amplifier 3, and a signal output end of the phase-locked amplifier 3 is connected with a.
As shown in fig. 3-5, the photoacoustic cell 1 includes a housing 101, two electret condenser microphones 102 with a sensitivity of-28 dB, two optical glass windows 103 made of calcium fluoride, and two fixing frames 104, two gas buffer chambers 105 are respectively disposed at two ports of the housing 101, the thickness of the buffer chambers is 15mm, two photoacoustic resonant cavities 106 parallel to each other are horizontally disposed in an inner cavity of the housing 101, the resonant cavities have a length of 9mm and a diameter of 6mm, and two ports of the two photoacoustic resonant cavities 106 are communicated with the gas buffer chambers 105 disposed at two ends of the housing 101, an air inlet 107 is disposed on a side wall at a left end of the housing 101, the air inlet 107 is communicated with the gas buffer chamber 105 disposed at a left end of the housing 101, an air outlet 108 is disposed on a side wall at a right end of the housing 101, and the air outlet 108 is communicated with the gas buffer chamber 105 disposed at a right end of the housing 101. Two microphone small holes 109 are arranged in parallel and adjacently in the middle of the side wall of the shell 101, the diameter of each small hole is 2mm, the two microphone small holes 109 are respectively communicated with the two photoacoustic resonant cavities 106, the two electret condenser microphones 102 are respectively arranged on the two microphone small holes 109, the two calcium fluoride optical glass windows 103 are respectively fixed at two ports of the shell 101 through two fixing frames 104, and sealing rings 110 are further arranged between the fixing frames 104 and the optical glass windows 103 and between the optical glass windows 103 and the ports of the shell 101.
Utilize high sensitive monocomponent SF6The method for detecting the gas by the decomposed gas detection device comprises the following steps:
(a) the first function generator 5 outputs sine wave modulation signals, the frequency of the sine wave modulation signals is half of the resonant frequency of the photoacoustic cell 1, namely 896Hz, the peak value of the voltage is the optimal modulation depth of the hydrogen sulfide laser 9, namely 25mV, the signals are used for modulating the laser wavelength, the second function generator 6 outputs slope scanning signals, the central voltage of the signal voltage is 1.2V and corresponds to the central absorption line of the gas to be measured, the two voltage signals are added through the adder 7 and then input to the laser driving plate 8 to drive the hydrogen sulfide laser 9, and the driving temperature of the hydrogen sulfide laser 9 is kept unchanged; (b) the laser modulated by the wavelength is input into a light beam input port of the optical fiber amplifier 10 of the near-infrared band through an optical fiber jumper, and the output power of the laser reaches 1.5 watts; (c) the high-power laser output from the beam output end of the optical fiber amplifier 10 passes through the beam collimator 11, outputs a parallel beam, and is input into one photoacoustic resonant cavity in the photoacoustic cell 1 to excite the gas to generate a photoacoustic signal; (d) the modulated photoacoustic signal is detected by a corresponding high-sensitivity electret condenser microphone and converted into a current signal; (e) weak two paths of current signals are differentially amplified by a differential amplifier 2 and then input to a signal input end of a phase-locked amplifier 3, a synchronous signal of a first function generator 5 is input to a synchronous input end of the phase-locked amplifier 3, the integration time of the phase-locked amplifier 3 is set to be 1S, the filtering slope is 12dB/oct, the corresponding detection bandwidth is 0.25Hz, and finally the amplitude S of the photoacoustic signal is obtained after demodulation of the phase-locked amplifier 3 and is transmitted to a computer system 4; (f) the amplitude S of the photoacoustic signal is in a proportional relationship with the gas concentration D, and may be written as D ═ a × S, where a is a proportionality coefficient, and is obtained by introducing known concentrations of different gases in advance, and the computer system 4 can calculate the concentration of the measured gas by using the formula D ═ a × S and the obtained amplitude S of the photoacoustic signal.
FIG. 7 shows SF according to the invention6Detection device for decomposed gas for trace amount H2S, the histogram is H corresponding to the wavelength region2The molecular line of S gas is strong. The waveform diagram is H2Second harmonic signal of S gas, H to be measured2The concentration of the S gas was 25ppm, and the proportionality coefficient a was 0.104 ppm/. mu.V by obtaining the maximum value of the second harmonic 240. mu.V.
Example 2
As shown in fig. 2, the present inventionHigh sensitivity multicomponent SF in the examples6The decomposed gas detection device comprises a photoacoustic cell 1, a differential amplifier 2, a lock-in amplifier 3, a computer system 4, a first function generator 5, a second function generator 6, an adder 7, two laser driving plates 8, two near-infrared excitation light sources 9 with different wavelengths, an optical fiber amplifier 10, a beam collimator 11 and two paths of optical switches 12, wherein the signal output ends of the first function generator 5 and the second function generator 6 are respectively connected with the two signal input ends of the adder 7, the output end of the adder 7 is respectively connected with the current driving ports of the two laser driving plates 8, the current output ends of the two laser driving plates 8 are respectively connected with the current input ends of the two near-infrared excitation light sources 9 with different wavelengths, the lasers output by the two near-infrared excitation light sources 9 with different wavelengths are respectively connected with the two paths of input ports of the optical switches 12 through optical fibers, an output port of the optical switch 12 is connected with a light beam input port of the optical fiber amplifier 10, a light beam output port of the optical fiber amplifier 10 is connected to the light beam collimator 11 through an optical fiber, a light beam output by the light beam collimator 11 passes through a resonant cavity of the photoacoustic cell 1, two microphone output signals of the photoacoustic cell 1 are connected with two inputs of the differential amplifier 2), an output signal of the differential amplifier 2 is sent to a signal input end of the phase-locked amplifier 3, a synchronous output end of the first function generator 5 is connected with a synchronous input end of the phase-locked amplifier 3, and a signal output end of the phase-locked amplifier 3 is connected with a signal input end of the computer.
As shown in fig. 3-5, the photoacoustic cell 1 includes a housing 101, two electret condenser microphones 102 with a sensitivity of-28 dB, two optical glass windows 103 made of calcium fluoride, and two fixing frames 104, two gas buffer chambers 105 with a thickness of 15mm are respectively disposed at two ports of the housing 101, two photoacoustic resonant cavities 106 parallel to each other are horizontally disposed in an inner cavity of the housing 101, the length of the resonant cavities is 90mm, the diameter of the resonant cavities is 6mm, two ports of the two resonant cavities 106 are communicated with the gas buffer chambers 105 disposed at two ends of the housing 101, an air inlet 107 is disposed on a side wall at a left end of the housing 101, the air inlet 107 is communicated with the gas buffer chamber 105 disposed at the left end of the housing 101, an air outlet 108 is disposed on a side wall at a right end of the housing 101, and the air outlet 108 is communicated with the gas buffer chamber 105 disposed at the right end of the housing 101. Two microphone small holes 109 are arranged in parallel and adjacently in the middle of the side wall of the shell 101, the two microphone small holes 109 are respectively communicated with the two photoacoustic resonant cavities 106, the two electret condenser microphones 102 are respectively arranged on the two microphone small holes 109, the two optical glass windows 103 are respectively fixed at two ports of the shell 101 through two fixing frames 104, and sealing rings 110 are further arranged between the fixing frames 104 and the optical glass window 10 and between the optical glass window 103 and the ports of the shell 101.
Using the high-sensitivity multicomponent SF6The method for detecting the gas by the decomposed gas detection device comprises the following steps:
(a) the first function generator 5 outputs sine wave modulation signals, the frequency of the sine signals is 896Hz, the peak value of the voltage is the optimal modulation depth of the near-infrared excitation light sources 9 with two different wavelengths, namely 25mV, the signals are used for modulating laser wavelength, the second function generator 6 outputs slope scanning signals, the central voltage of the signal voltage is 1.2V and corresponds to the central absorption line of the gas to be measured, the two paths of voltage signals are added through an adder 7 and then input to two laser driving plates 8 to drive the near-infrared excitation light sources 9 with the two different wavelengths, and the driving temperature of the near-infrared excitation light sources 9 with the two different wavelengths is kept unchanged; (b) the two wavelength modulated lasers are connected to the input ends of two optical switches 12, and the computer system 4 controls which one of the two wavelength modulated lasers is output; (c) laser beams output from the two optical switches 12 are input into a beam input port of the optical fiber amplifier 10 in a near-infrared band through optical fibers, and the output power of the laser is enabled to reach 1.5 watts; (d) after passing through the beam collimator 11, the high-power laser outputs a parallel beam, and is input into a photoacoustic resonant cavity in the photoacoustic cell 1 to excite the gas to generate a photoacoustic signal; (e) the modulated photoacoustic signal is detected by a corresponding high-sensitivity electret condenser microphone and converted into a current signal; (f) weak two-way current signals are differentially amplified by a differential amplifier 2, then are input into a phase-locked amplifier 3 for synchronous demodulation, synchronous signals of a first function generator 5 are input into a synchronous input end of the phase-locked amplifier 3, the integration time of the phase-locked amplifier 3 is set to be 1s, the filtering slope is 12dB/oct, and the phase-locked amplifier 3 is in pairThe detection bandwidth is 0.25Hz, and the amplitude S of the photoacoustic signal is obtained after demodulation by the phase-locked amplifier 31、S2And transmitted to the computer system 4; (f) amplitude S of photoacoustic signal1、S2And gas concentration D1、D2Proportional relation, can be recorded as D1＝a1×S1And D2＝a2×S2Wherein a is1And a2Is a proportionality coefficient obtained by introducing known concentrations of different gases in advance, and the computer system 4 is calculated by formula D1＝a1×S1And D2＝a2×S2And the amplitude S of the photoacoustic signal obtained1And S2The concentration of the gas to be detected can be calculated; (h) the 2-path optical switch 12 is used for switching the near-infrared excitation light sources 9 with different wavelengths, so that the alternate detection of two gas types is realized.
Claims (8)
1. A high-sensitivity gas detection device is characterized in that: the device comprises a photoacoustic cell (1), a differential amplifier (2), a lock-in amplifier (3), a computer system (4), a first function generator (5), a second function generator (6), an adder (7), a laser driving board (8), a near-infrared excitation light source (9), an optical fiber amplifier (10) and a beam collimator (11), wherein the signal output ends of the first function generator (5) and the second function generator (6) are respectively connected with two signal input ends of the adder (7), the output end of the adder (7) is connected with a current driving port of the laser driving board (8), the current output end of the laser driving board (8) is connected with the current input end of the near-infrared excitation light source (9), laser output by the near-infrared excitation light source (9) is connected to a beam input port of the optical fiber amplifier (10) through an optical fiber, and the beam output port of the optical fiber amplifier (10) is connected to the beam collimator (11), the light beam output by the light beam collimator (11) passes through a resonant cavity of the photoacoustic cell (1), the output signals of two microphones of the photoacoustic cell (1) are connected with two inputs of a differential amplifier (2), the output signal of the differential amplifier (2) is sent to the signal input end of a phase-locked amplifier (3), the synchronous output end of a first function generator (5) is connected with the synchronous input end of the phase-locked amplifier (3), and the signal output end of the phase-locked amplifier (3) is connected with the signal input end of a computer system (4);
the photoacoustic cell (1) comprises a shell (101), two electret condenser microphones (102), two optical glass windows (103) and two fixed frames (104), two gas buffer chambers (105) are respectively arranged at two ports of the shell (101), two parallel photoacoustic resonant cavities (106) are horizontally arranged in an inner cavity of the shell (101), two ports of the two photoacoustic resonant cavities (106) are communicated with the gas buffer chambers (105) arranged at two ends of the shell (101), an air inlet hole (107) is arranged on the side wall at the left end of the shell (101), the air inlet hole (107) is communicated with the gas buffer chamber (105) arranged at the left end of the shell (101), an air outlet hole (108) is arranged on the side wall at the right end of the shell (101), the air outlet hole (108) is communicated with the gas buffer chamber (105) arranged at the right end of the shell (101), two microphone small holes (109) are arranged in parallel in the middle of the side wall of the shell (101), two microphone small holes (109) are respectively communicated with two photoacoustic resonant cavities (106), two electret capacitor microphones (102) are respectively arranged on the two microphone small holes (109), and two optical glass windows (103) are respectively fixed at two ports of the shell (101) through two fixing frames (104);
the length of each resonant cavity is 50mm, 60mm, 70mm, 80mm, 90mm, 100mm, 110mm or 120 mm; the resonant cavity diameter is 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm or 12 mm; the diameter of the small hole is 1mm, 1.5mm or 2 mm; the thickness of the buffer chamber may be 5mm, 10mm or 15 mm.
2. A highly sensitive gas detection device according to claim 1, wherein: the two microphone small holes (109) are arranged adjacently, and the diameter of each microphone small hole is 1mm-2 mm.
3. A highly sensitive gas detection device according to claim 2, wherein: and sealing rings (110) are also arranged between the fixing frame (104) and the optical glass window (103) and between the optical glass window (103) and the port of the shell (101).
4. A method for gas detection using the highly sensitive gas detection device according to any one of claims 1 to 3, wherein: the method comprises the following steps:
(a) the gas detection device comprises a first function generator (5), a second function generator (6), a laser driving board (8), a near-infrared excitation light source (9), a first function generator (5), a second function generator (6), a second function generator (9), a third function generator (9), a fourth function generator (9), a fifth function generator (2), a sixth function generator (9), a fifth function generator (9), a sixth function generator (7), a fifth function generator (9), a sixth function generator (9; (b) the laser modulated by the wavelength is input into a light beam input port of an optical fiber amplifier (10) of a near-infrared wave band through an optical fiber jumper, and the output power of the laser reaches 1-15 watts; (c) the high-power laser output from the beam output end of the optical fiber amplifier (10) passes through a beam collimator (11), outputs a parallel beam, and is input into a photoacoustic resonant cavity in the photoacoustic cell (1) to be used for exciting gas to generate photoacoustic signals; (d) the modulated photoacoustic signal is detected by a corresponding high-sensitivity electret condenser microphone and converted into a current signal; (e) weak two paths of current signals are differentially amplified by a differential amplifier (2) and then input to a signal input end of a phase-locked amplifier (3), a synchronous signal of a first function generator (5) is input to a synchronous input end of the phase-locked amplifier (3), the integration time of the phase-locked amplifier (3) is set to be 1S, the filtering slope is 12dB/oct, the corresponding detection bandwidth is 0.25Hz, and finally the amplitude S of the photoacoustic signal is obtained after the phase-locked amplifier (3) is demodulated and transmitted to a computer system (4); (f) the amplitude S of the photoacoustic signal is in a proportional relation with the gas concentration D, and can be recorded as D ═ a × S, wherein a is a proportionality coefficient, the photoacoustic signal is obtained by introducing known different gas concentrations in advance, and the computer system (4) can calculate the concentration of the measured gas by the formula D ═ a × S and the obtained amplitude S of the photoacoustic signal.
5. A high-sensitivity gas detection device is characterized in that: the optical fiber laser comprises a photoacoustic cell (1), a differential amplifier (2), a phase-locked amplifier (3), a computer system (4), a first function generator (5), a second function generator (6), an adder (7), n laser drive plates (8), n near-infrared excitation light sources (9) with different wavelengths, an optical fiber amplifier (10), a beam collimator (11) and an n-way optical switch (12), wherein the signal output ends of the first function generator (5) and the second function generator (6) are respectively connected with two signal input ends of the adder (7), the output end of the adder (7) is respectively connected with a current drive port of the n laser drive plates (8), the current output ends of the n laser drive plates (8) are respectively connected with the current input ends of the n near-infrared excitation light sources (9) with different wavelengths, and lasers output by the n near-infrared excitation light sources (9) with different wavelengths are respectively connected with the n-way input ends of the n-way optical switch (12) through optical fibers The output port of the optical switch (12) is connected with the light beam input port of the optical fiber amplifier (10), the light beam output port of the optical fiber amplifier (10) is connected to the light beam collimator (11) through an optical fiber, the light beam output by the light beam collimator (11) passes through a resonant cavity of the photoacoustic cell (1), the output signals of two microphones of the photoacoustic cell (1) are connected with two inputs of the differential amplifier (2), the output signal of the differential amplifier (2) is sent to the signal input end of the phase-locked amplifier (3), the synchronous output end of the first function generator (5) is connected with the synchronous input end of the phase-locked amplifier (3), and the signal output end of the phase-locked amplifier (3) is connected with the signal input end of the computer system 4;
the photoacoustic cell (1) comprises a shell (101), two electret condenser microphones (102), two optical glass windows (103) and two fixed frames (104), two gas buffer chambers (105) are respectively arranged at two ports of the shell (101), two parallel photoacoustic resonant cavities (106) are horizontally arranged in an inner cavity of the shell (101), two ports of the two photoacoustic resonant cavities (106) are communicated with the gas buffer chambers (105) arranged at two ends of the shell (101), an air inlet hole (107) is arranged on the side wall at the left end of the shell (101), the air inlet hole (107) is communicated with the gas buffer chamber (105) arranged at the left end of the shell (101), an air outlet hole (108) is arranged on the side wall at the right end of the shell (101), the air outlet hole (108) is communicated with the gas buffer chamber (105) arranged at the right end of the shell (101), two microphone small holes (109) are arranged in parallel in the middle of the side wall of the shell (101), two microphone small holes (109) are respectively communicated with two photoacoustic resonant cavities (106), two electret capacitor microphones (102) are respectively arranged on the two microphone small holes (109), and two optical glass windows (103) are respectively fixed at two ports of the shell (101) through two fixing frames (104);
the length of each resonant cavity is 50mm, 60mm, 70mm, 80mm, 90mm, 100mm, 110mm or 120 mm; the resonant cavity diameter is 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm or 12 mm; the diameter of the small hole is 1mm, 1.5mm or 2 mm; the thickness of the buffer chamber may be 5mm, 10mm or 15 mm.
6. The highly sensitive gas detecting device according to claim 5, wherein: the two microphone small holes (109) are arranged adjacently, and the diameter of each microphone small hole is 1mm-2 mm.
7. The highly sensitive gas detecting device according to claim 6, wherein: and a sealing ring (110) is also arranged between the fixing frame (104) and the port of the shell (101).
8. A method for gas detection using the high sensitivity gas detection device according to any one of claims 5 to 7, wherein: the method comprises the following steps:
(a) the first function generator (5) outputs sine wave modulation signals, the frequency of the sine wave modulation signals is half of the resonant frequency of the photoacoustic cell (1), the peak value of the voltage is the optimal modulation depth of n near-infrared excitation light sources (9) with different wavelengths, the signals are used for modulating laser wavelength, the second function generator (6) outputs slope scanning signals, the center of the signal voltage corresponds to the central absorption line of the gas to be measured, the two paths of voltage signals are added through an adder (7) and then input to n laser driving plates (8) to drive the n near-infrared excitation light sources (9) with different wavelengths, and the driving temperature of the n near-infrared excitation light sources (9) with different wavelengths is kept unchanged; (b) the n wavelength-modulated lasers are connected to the input ends of n paths of optical switches (12), and a computer system (4) controls which path is output; (c) laser beams output from the n paths of optical switches (12) are input into a beam input port of an optical fiber amplifier (10) in a near infrared band through optical fibers, and the output power of the laser reaches 1-15 watts; (d) the high-power laser passes through a beam collimator (11) and outputs a parallel beamAnd after being input into a photoacoustic resonant cavity in the photoacoustic cell (1), the photoacoustic resonant cavity is used for exciting gas to generate photoacoustic signals; (e) the modulated photoacoustic signal is detected by a corresponding high-sensitivity electret condenser microphone and converted into a current signal; (f) weak two-path current signals are differentially amplified by a differential amplifier (2), then are input into a phase-locked amplifier (3) for synchronous demodulation, synchronous signals of a first function generator (5) are input into a synchronous input end of the phase-locked amplifier (3), the integration time of the phase-locked amplifier (3) is set to be 1S, the filtering slope is 12dB/oct, the corresponding detection bandwidth is 0.25Hz, and finally the amplitude S of the photoacoustic signal is obtained after demodulation of the phase-locked amplifier (3)nAnd transmitted to the computer system (4); (f) amplitude S of photoacoustic signalnAnd gas concentration DnProportional relation, can be recorded as Dn＝an×SnWherein a isnIs proportional coefficient obtained by introducing known concentrations of different gases in advance, and the computer system (4) is based on formula Dn＝an×SnAnd the amplitude S of the photoacoustic signal obtainednThe concentration of the gas to be detected can be calculated; (h) near-infrared excitation light sources (9) with different wavelengths are switched through n paths of optical switches (12), so that different gas types can be detected in turn.
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