Method for selecting wavelength of light source of mixed gas distributed sensing system with overlapping absorption spectrum and detecting gas concentration

Abstract

The application relates to a mixed gas distributed sensing system light source wavelength selection and gas concentration detection method, and relates to the field of distributed gas detection. The method comprises the following steps: according to the types of each to-be-detected gas and the corresponding absorption spectrum data, determining an optimal detection wavelength through an algorithm; measuring the absorption coefficients of each to-be-detected gas at each detection wavelength; pulse-modulating a plurality of selected lasers with different wavelengths, and combining time division multiplexing technology to obtain light intensity information corresponding to a plurality of gas detection points; according to the obtained data, calculating the concentrations of each component gas in the mixed gas at different detection points according to the method, and realizing multi-point distributed detection of multiple gases. The method has the capability of multi-point distributed detection of the concentrations of each component of the mixed gas with overlapping absorption spectra, provides a new solution for multi-point distributed detection of multi-component mixed gas, and has the advantages of low cost, no power supply required for the detection point, safety and reliability and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of distributed gas detection, in particular to a light source wavelength selection and gas concentration detection method for a mixed gas distributed sensing system with overlapping absorption spectra. BACKGROUND

[0002] Tunable diode laser absorption spectroscopy (TDLAS) is a detection technology for measuring trace gas concentration by gas absorption information of specific wavelength laser. Compared with traditional electrochemical detection method, TDLAS has the advantages of high sensitivity, good selectivity, no consumables in the sensor, low maintenance cost, fast response speed, low power consumption, and can be used for multi-component gas detection, etc. It has broad application prospects in coal, gas and petrochemical industries.

[0003] For multi-component gases with non-overlapping absorption spectra, the concentrations of each gas in the mixture can be measured by monitoring the absorption at each corresponding wavelength, as the wavelengths corresponding to the characteristic absorption peaks of each gas do not overlap. However, for mixed gases with overlapping absorption spectra, it is not possible to obtain the concentrations of each component in the mixture by monitoring the absorption at a single wavelength, as two or more gases may absorb at the same wavelength. This raises questions such as how to determine the optimal operating wavelength and how to quantitatively evaluate the selection of wavelengths. On the other hand, current TDLAS-based gas sensing schemes typically only allow for single-point or limited-point gas monitoring, and cannot achieve multi-point distributed sensing of multi-component gases with overlapping absorption spectra. For example, Wang et al. proposed a method for detecting the concentrations of each component in a mixed gas with overlapping absorption spectra based on TDLAS. By establishing a relationship model between the absorption spectrum "weight" and the concentrations of each component in the gas mixture, single-point detection of mixed gases with overlapping absorption spectra was achieved (Wang Y, Wei YB, Zhang TT, Hu J, Zhao WS, Li YF, Wang ZW, Liu TY. Gas concentration monitoring method, device and system: CN106525742A [P]. 20191001). However, this single-laser scheme requires the laser wavelength to be scanned over a large range, which is demanding in terms of device performance and control system requirements. More importantly, if the absorption spectra of different gases in the mixture do not differ significantly within the scannable wavelength range, the detection performance may be poor. In addition, this scheme can only detect a single point of gas. Dong Xiaopeng's group at Xiamen University proposed a multi-point gas detection scheme based on time division multiplexing technology (Dong XP, Yang GH. A cascaded multi-point gas detection method with branch gas chambers: CN114965289B [P]. 20240823). However, this scheme does not address the measurement of mixed gases with overlapping absorption spectra or related wavelength selection issues. For gases with non-overlapping absorption spectra and multiple distinct peaks, Zhang et al. proposed using wavelength division multiplexing technology for multi-point and multi-gas measurement (Zhang H, Lu Y, Duan L, Zhao Z, Shi W, Yao J. Intracavity absorption multiplexed sensor network based on dense wavelength division multiplexing filter. Optics express. 2014 Oct 6;22(20):24545-50.).There are also reports of using optical switches and multiple detectors to separate different detection point gas signals, while modulating multiple lasers at different frequencies to distinguish different components of the mixed gas, achieving mixed gas measurement of methane and water vapor (Wang Z, Chang J, Yu H, Tian C, Zhang H, Zhang X, Tang L, Zhang Q, Feng Y. Multi-component and multi-point trace gas sensing in wavelength modulation spectroscopy based on wavelength stabilization. Photonic Sensors. 2019 Dec; 9:376-87), but this scheme is also not suitable for multi-point detection of mixed gases with overlapping absorption spectra. SUMMARY

[0004] The purpose of the present application is to solve the problem of multi-point, distributed detection of mixed gases with overlapping absorption spectra. The content includes the selection and quantitative evaluation of multiple laser wavelengths, the optical path design of a multi-point, distributed gas detection system based on time division multiplexing, and the concentration calculation and determination method of each component of the mixed gas. The specific technical scheme and detection steps are as follows:

[0005] The present application provides a distributed sensing system for mixed gases with overlapping absorption spectra, which includes a pulse driving module, a laser, a gas chamber, a combiner, a coupler, a delay fiber, a photoelectric detection and data acquisition module, and a host computer.

[0006] The pulse driving module is used to drive the laser to generate stable pulse signals.

[0007] The laser serves as a light source. The laser can be set to multiple, each laser generating light of a specific wavelength. The wavelength is selected according to the absorption spectrum characteristics of the gas to be measured.

[0008] The gas chamber is used to contain the gas to be measured. Light propagates in the gas chamber and can reflect the light signal. The number of gas chambers corresponds to the number of detection points.

[0009] The combiner is used to combine the light source signals required for gas sensing onto one optical fiber for transmission, to simplify the system structure.

[0010] The coupler is used to transmit the light source signal to the main road, and also to transmit the signal light reflected from the gas chamber to the photoelectric detection and data acquisition module. The splitting ratio of the coupler can be set according to the actual situation to optimize the signal transmission efficiency.

[0011] The delay optical fiber is used for distinguishing signals of different lasers, preventing signals from overlapping in time domain, and forming obvious distinction in time domain by transmitting signals corresponding to each laser through delay optical fibers with different lengths;

[0012] The photoelectric detection and data acquisition module is used for converting received signals into electrical signals and collecting the signals, transmitting the signals to the host computer for processing, improving the signal-to-noise ratio by superposition and averaging of collected time domain signals, and improving the detection limit of the system on the gas concentration;

[0013] The host computer is used for processing and analyzing collected data, and calculating the concentration of each component gas; the pulse signals reflected by each gas chamber to the photoelectric detection and data acquisition module are distinguished in time domain by the delay optical fiber, and the relative change of the received reflected signals is calculated by the host computer, so that the gas to be measured can be calibrated, and then the detection of the mixed gas concentration can be performed according to the calibrated coefficient.

[0014] Further, the laser, the gas chamber, the coupler and the delay optical fiber can be provided as a plurality of, the number of the gas chamber, the delay optical fiber and the coupler corresponds to the number of detection points; a plurality of gas chambers, delay optical fibers and couplers are required for a plurality of gas detection points, and each coupler is connected with a gas chamber and a delay optical fiber.

[0015] Further, the coupler is provided with a plurality of couplers, and the first coupler has a splitting ratio of 50:50.

[0016] The present application provides a light source wavelength selection and gas concentration detection method, which adopts a mixed gas distributed sensing system with overlapping absorption spectrum, and the specific method comprises the following steps:

[0017] 1) According to the type and absorption spectrum characteristics of the gas to be measured, the absorption spectrum interval of the gas to be measured is determined, especially the part that may overlap; an absorption coefficient matrix of the gas to be measured is established, and a plurality of optimal detection wavelengths are determined according to the absorption spectrum data of the gas to be measured;

[0018] 2) At the selected optimal detection wavelength, the absorption coefficient of each kind of gas to be measured at each detection wavelength is measured respectively;

[0019] 3) A pulse driving module is used to drive a plurality of lasers to generate pulse signals of each optimal detection wavelength, a combiner combines the signals from each laser to an optical fiber for transmission; different delay optical fibers are used to ensure that the signals from each laser do not overlap in time domain; based on time division multiplexing technology, the light intensity information of different wavelengths of each different detection point is obtained from the received signals;

[0020] 4) The reflected signals of each gas chamber are transmitted to the photoelectric detection and data acquisition module through the first coupler, the photoelectric detection and data acquisition module converts them into electric signals for collection and transmission to the host computer for processing to improve the signal-to-noise ratio; the host computer calculates the concentration of each component gas in the mixed gas to be measured at different detection points using the measured absorption coefficient and collected signal intensity parameters, and realizes distributed detection of mixed gas with overlapping absorption spectra.

[0021] In step 1), the method for determining a plurality of optimal detection wavelengths specifically comprises the following steps:

[0022] 1.1) Determine the type of gas to be measured: determine which types of gas are contained in the mixed gas.

[0023] 1.2) Determine the absorption spectral interval: obtain the characteristic absorption peak and absorption spectral interval of each gas to be measured according to the relevant database (such as HITRAN), especially the part that may overlap.

[0024] 1.3) Establish the absorption coefficient matrix: within the determined absorption spectral interval, establish the absorption coefficient matrix of the gas to be measured for each gas at each possible detection wavelength , the element in the coefficient matrix is the absorption cross section of the th gas at wavelength , in the absorption spectral interval , which is obtained from the HITRAN database; assuming there are types of gases to be measured and detection wavelengths, the absorption coefficient matrix of the gas to be measured is represented as:

[0025] ;

[0026] 1.4) Calculate the condition number and select the optimal detection wavelength: traverse all possible wavelength combinations and calculate the condition number of the absorption coefficient matrix. The wavelength combination with the smallest condition number is the optimal detection wavelength. Specifically: traverse each item in within the absorption spectral interval to calculate the condition number of the absorption coefficient matrix, so that the wavelength combination corresponding to the smallest condition number is the optimal wavelength for mixed gas detection.

[0027] In step 3), the light intensity information of different wavelengths at each different detection point is obtained, assuming that the types of gases to be measured are , denoted as gas 1, 2, 3… ; at least wavelength lasers are required, with wavelengths denoted as , have several laser for several gas to be tested, for a certain detection point, there are pulse signals of different wavelengths, the signal intensity of different wavelengths is recorded as , when detecting mixed gas, the measured value of the signal intensity of different wavelengths is recorded as , the relative change of light intensity of different wavelengths after the signal is absorbed by the gas;

[0028] Specifically, the following steps are included:

[0029] 3.1) select and configure the corresponding number of lasers to ensure that each laser produces light signal wavelength matched with the best detection wavelength; connect the components such as laser, gas chamber, combiner, coupler, delay fiber and photodetector according to the system architecture to ensure smooth signal transmission; the number of laser, gas chamber, coupler and delay fiber matches the number of gas detection points;

[0030] 3.2) use pulse driving module to drive laser to produce pulse signal at the same time;

[0031] 3.3) the combiner combines the signals of each laser to a fiber for transmission, and uses delay fiber to distinguish the signals of each laser to ensure that they do not overlap in time domain;

[0032] 3.4) the coupler transmits the light source signal to the main road, and transmits the signal light reflected from the gas chamber to the photodetector and data acquisition module, measures and records the initial light intensity at each detection wavelength in the absence of gas ;

[0033] 3.5) after the gas chamber is filled with the mixed gas to be tested, the signal peak intensity at each detection wavelength is measured again , by comparing the signal intensity in the absence of gas with the signal intensity after filling with gas, the relative change of light intensity is calculated.

[0034] In step 4), the concentration of each component gas in the mixed gas to be tested at different detection points is calculated, which includes the following steps:

[0035] 4.1) the host computer collects the light intensity change data of different detection wavelengths at all detection points;

[0036] For simultaneous detection of several gases, the absorption coefficient of each gas to be tested at each detection wavelength is measured, and the absorption coefficient of the gas at wavelength is recorded as ;

[0037]

[0038] In the formula, is the change of light intensity after the gas absorption at the wavelength , and the absorption coefficient of the gas at the wavelength is denoted as ( , and the concentration of each component gas of the mixed gas to be measured is respectively , is the effective absorption length of the gas chamber;

[0039] 4.2) According to the Beer-Lambert law and the light intensity change amount, combined with the absorption coefficient matrix, a gas concentration calculation matrix is constructed, which can convert the light intensity change amount into the concentration of each component gas; the concentration calculation matrix of each component gas is as follows:

[0040]

[0041] According to the above obtained parameters, combined with the matrix, the concentration of each component gas in the mixed gas to be measured at different detection points can be calculated, and the distributed detection of the mixed gas with overlapping absorption spectrum is realized.

[0042] Compared with the prior art, the technical scheme of the present application has the following outstanding technical effects and advantages:

[0043] 1. Solve the problem of spectral overlap: The present application proposes an effective solution for the detection of mixed gas with overlapping absorption spectrum, calculates the concentration of each component gas by using the light absorption amount of the mixed gas to be measured at different detection wavelengths, and can accurately distinguish and calculate the concentration of each component gas in the mixed gas by selecting the optimal detection wavelength combination, even if the absorption spectrum overlaps.

[0044] 2. Optimize the selection of detection wavelength: By calculating the condition number of the gas absorption matrix, the present application determines the optimal detection wavelength combination. It can reduce the measurement error and improve the detection accuracy. At the same time, it also avoids the problem of inaccurate detection caused by improper wavelength selection.

[0045] 3. Reduce cost and easy to expand: Compared with the prior art, the system device of the present application is simpler and has lower cost. Through the ingenious combination of wave combiner, coupler, delay optical fiber and other components, the transmission and differentiation of signals are realized without the need for complex light splitting devices and multiple detection systems. At the same time, the system architecture of the present application is easy to expand and maintain. By increasing the number of lasers, gas chambers, couplers and delay optical fibers and other components, the number of detection points can be easily expanded. In addition, the connection between each component of the system is simple, easy to disassemble and replace, and the maintenance cost is reduced.

[0046] ​4. Multipoint multiplexing detection: the present application is based on the time division multiplexing principle, and utilizes the delay optical fiber to distinguish the signals of different wavelengths and different detection points in the time domain, so as to realize the distributed measurement of the multi-component gas with overlapping absorption spectrum; the present application is suitable for the multipoint multiplexing detection of gas, and can simultaneously monitor the gas concentration of multiple detection points. It has important significance for the real-time online distributed measurement of mixed gas with overlapping spectrum, and can meet the wide needs in the fields of industry, environmental protection and the like. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 It is a schematic diagram of the mixed gas distributed sensing system with overlapping absorption spectrum.

[0048] Figure 2 It is the absorption spectrum of propane and isobutane in the HITRAN database.

[0049] Figure 3 It is the value of the reciprocal of the condition number of the absorption coefficient matrix at each detection wavelength.

[0050] Figure 4 It is the pulse signal returned by the test point.

[0051] Figure 5 It is the pulse signal returned by the test point 1. DETAILED DESCRIPTION

[0052] In order to make the purpose, technical scheme and advantages of the present application more clear, the following embodiments will be further described in combination with the drawings. It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application. On the contrary, the present application covers any substitution, modification, equivalent method and scheme defined by the claims within the essence and scope of the present application.

[0053] The mixed gas distributed sensing system with overlapping absorption spectrum described in the embodiments of the present application has a system block diagram as shown in Figure 1 , which comprises a pulse driving module 1, a laser, a gas chamber, a combiner, a coupler, a delay optical fiber, a photoelectric detection and data acquisition module, and an upper computer.

[0054] The pulse driving module is used to drive a series of lasers, which are sequentially laser 1, laser 2, and laser n. . Each of the lasers is connected with the input port of the combiner through a corresponding delay line (labeled as L1, L2, and Ln respectively); The couplers are sequentially numbered as coupler 1, coupler 2, and coupler n. ; and The delay fibers are sequentially labeled l1, l2, up to ln. Port 101 of coupler 1 is connected to the output port of the multiplexer, and port 102 of coupler 1 is connected to the photoelectric detection and data acquisition module. Port 103 of coupler 1 is connected to port 201 of coupler 2 via delay fiber l1; port 202 of coupler 2 is connected to air chamber 1, port 203 of coupler 2 is connected to port 301 of coupler 3 via delay fiber l2, port 302 of coupler 3 is connected to air chamber 2, and so on, forming the corresponding connection relationships.

[0055] The pulse drive module is the power source for the entire system; the pulse drive module is... Each laser provides a stable and synchronized pulsed current; these pulsed currents drive the lasers to simultaneously generate light pulses of a specific wavelength.

[0056] The laser, acting as a light source, is driven by a pulse driving module to generate pulse signals; each laser can emit light of a specific wavelength, and the signals of each laser are distinguished by a delay fiber. Lasers (laser 1, laser 2 to laser 3) The emitted light pulses are transmitted through multiple optical fibers to a multiplexer, forming a light source network. These light pulses interact with the components in the gas mixture, producing different absorptions, thereby enabling the detection of the gas components.

[0057] The multiplexer is a key component of the system of this invention. The multiplexer is used to combine the signals required for gas sensing from… Signals from different lasers, each with a specific wavelength, are combined and transmitted onto a single optical fiber. This simplifies the system structure significantly, requiring only one fiber to transmit all the signals. However, since the light pulses from different lasers may overlap in the time domain, a delay fiber is needed to differentiate them, ensuring that the pulses from different lasers are clearly distinguishable in the time domain when they reach the photoelectric detection and data acquisition module. The combined signal is then transmitted to each gas chamber via optical fiber and couplers.

[0058] A gas chamber can be used to contain the gas mixture to be detected. The light pulse can propagate in the gas chamber and reflect the light signal. Several gas chambers, delay fibers and couplers are required for several gas detection points. After the gas chamber is filled with the gas mixture to be detected, the light pulse enters the gas chamber and interacts with the components in the gas mixture, causing the intensity of the light pulse to change, thereby carrying information about the gas composition.

[0059] After passing through the gas chamber, the light pulse is reflected back and transmitted to the photoelectric detection and data acquisition module through the coupler. The coupler 1 can transmit the light source signal to the subsequent main road, and also transmit the signal light reflected from the gas chamber to the photoelectric detection and data acquisition module. Other couplers are used to transmit part of the main road light to the gas chamber, and transmit the signal light reflected from the gas chamber back to the main road. The splitting ratio of the coupler can be set according to the actual situation. In the embodiment, the coupler 1 adopts a component with a splitting ratio of 50:50. The splitting ratios of other couplers (couplers 2 to couplers ) in the system are adjusted according to the specific situation. The pulse signals reflected from the gas chambers to the photoelectric detection and data acquisition module are distinguished in the time domain by the delay optical fiber.

[0060] The photoelectric detection and data acquisition module receives the reflected signals from the gas chambers and converts them into electrical signals for collection and recording. The photoelectric detection and data acquisition module transmits the collected electrical signals to the host computer for processing.

[0061] The host computer is a powerful computer that receives signals from the photoelectric detection and data acquisition module and processes and analyzes them. By performing operations such as superposition averaging on the collected time domain signals, the host computer can improve the signal-to-noise ratio and improve the detection limit of the system for gas concentration, thereby more accurately detecting the concentration of each component in the mixed gas. The host computer can calibrate the gas to be measured by calculating the relative change of the received reflected signals, and then detect the concentration of the mixed gas according to the calibrated coefficient to obtain the concentration of each component in the mixed gas.

[0062] The entire sensing system connects the gas chamber, coupler, delay optical fiber and photoelectric detection and data acquisition module through optical fiber to form a complete and distributed gas sensing system.

[0063] The detection principle of the present application is based on the Beer-Lambert law:

[0064] (1)

[0065] wherein, is the initial light intensity without passing through the gas absorption, is the light intensity after passing through the gas absorption, the absorption coefficient is related to the type of gas and the detection wavelength, is the concentration of the gas to be measured, is the optical path of the laser and the gas. The difference between the relative intensity values can be used to represent the ratio of the absolute value of the light intensity, so formula (1) can be expressed as:

[0066] (2)

[0067] This represents the difference in relative intensity. , It is the relative intensity value, and the unit is dB.

[0068] The principle of the mixed gas concentration detection is that, assuming the type of gas to be measured is... The types are denoted as gases 1, 2, 3… At least required Lasers with wavelengths of , denoted as , . The number of lasers required corresponds to the number of gases to be tested. The signal intensities at different wavelengths when there is no gas initially are denoted as follows: When detecting a gas mixture, the measured signal intensity values ​​at different wavelengths are recorded as follows: , Let be the relative change in light intensity after signals of different wavelengths are absorbed by the gas. Then, for... For simultaneous detection of several gases, formula (2) can be evolved into:

[0069] (3)

[0070] The above formula, To be at wavelength The change in light intensity after absorption by the gas, gas At wavelength The absorption coefficient below is denoted as ( The concentrations of each component gas in the gas mixture to be tested are as follows: Furthermore, the calculation matrix for the gas concentration of each component can be derived by inversion:

[0071] (4)

[0072] The process of determining the types of gases whose absorption spectra overlap, determining the absorption spectral ranges of the gases to be tested, and determining the optimal detection wavelength based on the absorption spectral data of the gases to be tested includes: determining the types of gases in the gas mixture to be tested, determining the absorption spectral ranges based on the types of gases to be tested, determining the gas absorption coefficient matrix based on the absorption spectral data of each gas to be tested, and calculating the condition number of the absorption coefficient matrix. The wavelengths corresponding to the minimum condition number are the optimal detection wavelengths. Specifically, this involves: determining the range of the absorption spectrum of the gases to be tested; and establishing the absorption coefficient matrix of the gases to be tested. coefficient matrix elements in Indicates the first Gas at wavelength Lower absorption cross section ( Within the absorption spectral range, this data is available from the HITRAN database.

[0073] (5)

[0074] right Each term in the matrix is ​​iterated over within the absorption spectral range, and the condition number of the absorption coefficient matrix is ​​calculated to select the wavelength combination that minimizes the condition number. This is the optimal wavelength for detecting mixed gases.

[0075] The absorption coefficient of each gas to be tested was measured at each detection wavelength. At wavelength The absorption coefficient below is denoted as ,in, .

[0076] Multiple wavelength lasers combined with time-division multiplexing technology are used to obtain light intensity information at different wavelengths at various detection points. The signals at each detection point are separated in the time domain by a delay fiber. For a gas cell at a certain detection point, the host computer can obtain the peak signal intensity of the reflected signal at different detection wavelengths when the gas cell is initially empty, denoted as follows: When the gas chamber is filled with the gas mixture to be tested, the measured peak signal intensity values ​​at different detection wavelengths reflected by the gas chamber are denoted as follows: .

[0077] Based on the parameters obtained above, and combined with formula (4), the concentrations of each component gas in the gas mixture to be tested at different detection points can be calculated, thereby realizing distributed detection of gas mixtures with overlapping absorption spectra. This includes obtaining the changes in light intensity caused by gas absorption at different detection wavelengths returned from each detection point in the host computer, and calculating the concentrations of each component gas in the gas mixture to be tested at each detection point based on the changes in light intensity at each detection point and the absorption coefficients of each component gas at each detection wavelength.

[0078] The following uses a mixture of propane and isobutane with overlapping absorption spectra as an example to illustrate the method for detecting the concentration of mixed gases in this invention.

[0079] In this embodiment, there are two types of gases to be tested, requiring two lasers of different wavelengths. The distributed detection method for mixed gases with overlapping absorption spectra provided in this embodiment of the invention is implemented by the following steps:

[0080] The absorption coefficient matrix of the gas to be measured is as follows:

[0081] (6)

[0082] , This represents the detection wavelength of propane and isobutane. The gas absorption cross section below, , This represents the detection wavelength of propane and isobutane. The gas absorption cross section below. Based on the HITRAN database, propane (C3H8) and isobutane (C4H8) are analyzed. 10 Absorption spectral data, such as Figure 2 As shown. Data in the wavelength range of 1680–1700 nm was selected for analysis. To determine the optimal detection wavelength, the absorption cross-section data within this range were traversed, and the condition number of the gas absorption coefficient matrix was calculated. The wavelength combination corresponding to the minimum condition number is the optimal detection wavelength. In this embodiment, the condition number can be infinitely large; the minimum value of the condition number can be found by calculating its reciprocal. Figure 3 As shown, the reciprocal of the condition number of the absorption coefficient matrix is ​​plotted at various detection wavelengths. By comparing the reciprocals of the condition number at different wavelengths, it can be seen that the reciprocal of the condition number reaches its maximum value, i.e., the condition number is at its minimum, when the detection wavelengths are selected at 1686.34 nm and 1689.09 nm, respectively. Therefore, these two wavelengths are selected as the optimal detection wavelengths. Thus, in this embodiment, the two laser wavelengths are 1686.34 nm and 1689.09 nm, respectively.

[0083] The absorption coefficients of propane and isobutane gases at each detection wavelength were measured and calculated. Formula (2) above can be rewritten as:

[0084] (7)

[0085] in, The gas equivalent absorption coefficient contains the effective optical path information of the laser passing through the gas cell. By calculating the equivalent absorption coefficient of each gas at each detection wavelength, the concentration of each component in the gas mixture to be tested can also be calculated. Formula (4) can be rewritten as:

[0086] (8)

[0087] For gases At wavelength The equivalent absorption coefficient is denoted as ( .

[0088] In this embodiment, two test points are set, and the pulse signals returned from the two test points are acquired in the host computer, such as... Figure 4 As shown, each test point contains two pulse signals, with the first pulse signal corresponding to the detection wavelength. =1686.34 nm, the second pulse signal corresponds to the detection wavelength = 1689.09 nm. Taking the gas chamber of test point 1 as an example, during the calibration of the gas absorption coefficient, the equivalent absorption coefficients of the gas chamber of test point 1 for propane and isobutane at the two detection wavelengths can be measured: the equivalent absorption coefficient of propane at the detection wavelength = 1686.34 nm is = 6.054, the equivalent absorption coefficient of isobutane is = 4.656; the equivalent absorption coefficient of propane at the detection wavelength = 1689.09 nm is = 6.818, and the equivalent absorption coefficient of isobutane is = 10.41. The calibration process of test point 2 is consistent with that of test point 1, and the same method and steps are followed.

[0089] The test point gas chamber is filled with a mixed gas of 10% propane and 5% isobutane, and the light intensity is compared with that of air (as a reference gas). In the host computer, the amount of change in light intensity caused by the absorption of the mixed gas of propane and isobutane at the two detection wavelengths of test point 1 is obtained. Figure 5 The pulse signals returned for test point 1 before and after the mixed gas is filled at the two detection wavelengths are obtained, and the amount of change in light intensity of the gas chamber at the detection wavelength = 1686.34 nm is = 0.4647 dB, and the amount of change in light intensity at the detection wavelength = 1689.09 nm is = 1.1905 dB. According to the equivalent absorption coefficients of propane and isobutane at the respective detection wavelengths and the amount of change in light intensity caused by the absorption of the mixed gas, the concentrations of the respective component gases in the mixed gas of propane and isobutane to be measured at the test point can be calculated: 9.74% propane and 5.06% isobutane.

[0090] The above examples are only preferred embodiments of the present application and should not be considered as limiting the scope of the present application. Any equivalent changes and improvements made in accordance with the scope of the present application should still be included in the patent coverage of the present application.

Claims

1. A method for selecting light source wavelength and detecting gas concentration, characterized by employing a distributed sensing system for mixed gases with overlapping absorption spectra, the system comprising a pulse drive module, a laser, a gas chamber, a combiner, a coupler, a delay fiber, a photoelectric detection and data acquisition module, and a host computer; The pulse driving module is used to drive the laser to generate a stable pulse signal; The laser serves as a light source, and multiple lasers can be set. Each laser generates light of a specific wavelength, which is selected according to the absorption spectrum characteristics of the gas to be tested. Multiple air chambers are provided, each of which is used to contain the gas to be tested and serves as a channel for the propagation and reflection of light signals. The number of air chambers corresponds to the number of detection points. The multiplexer is used to combine the light source signal required for gas sensing onto a single optical fiber for transmission, thereby simplifying the system structure. Multiple couplers are provided, wherein the first coupler is used to transmit the light source signal to the main path and also transmit the signal light reflected from the air chamber to the photoelectric detection and data acquisition module. The other couplers are used to transmit a portion of the main path light to the air chamber and transmit the signal light reflected from the air chamber back to the main path. The splitting ratio of the coupler is set according to the actual situation to optimize the signal transmission efficiency. Multiple delay fibers are provided to distinguish the signals of different lasers and prevent the signals from overlapping in the time domain. The signal corresponding to each laser is transmitted through delay fibers of different lengths, thereby forming a clear distinction in the time domain. The photoelectric detection and data acquisition module is used to convert the received signal into an electrical signal and acquire it, and then transmit the signal to the host computer for processing. By superimposing and averaging the acquired time-domain signals, the signal-to-noise ratio can be improved, thereby increasing the detection limit of the system for gas concentration. The host computer is used to process and analyze the collected data and calculate the concentration of each component gas. The pulse signals reflected back to the photoelectric detection and data acquisition module from each gas chamber are separated in the time domain by the delay fiber. The host computer calculates the relative change of the received reflected signals to calibrate the gas to be tested, and then detects the concentration of the mixed gas based on the calibrated coefficients. The method includes the following steps: 1) Determine the absorption spectral range of the gas to be tested, especially the parts that may overlap, based on the type and absorption spectral characteristics of the gas to be tested; establish the absorption coefficient matrix of the gas to be tested, and determine multiple optimal detection wavelengths based on the absorption spectral data of the gas to be tested. The method for determining multiple optimal detection wavelengths specifically includes the following steps: 1.1) Determine the type of gas to be tested: Identify which types of gases are included in the gas mixture; 1.2) Determine the absorption spectral range: Based on the HITRAN database, obtain the characteristic absorption peaks and absorption spectral ranges of each gas to be measured, especially the parts that may overlap; 1.3) Establishing the absorption coefficient matrix: Within the defined absorption spectral range, establish the absorption coefficient matrix A for each gas at each possible detection wavelength. The elements A in the absorption coefficient matrix A are... m,i The absorption coefficient of the i-th gas at the m-th detection wavelength is represented by λ, where m,i = 1 to n; the detection wavelength is λ. m Within the absorption spectral range, assuming there are n analytes and n detection wavelengths, the absorption coefficient matrix A of the analytes is expressed as: 1.4) Calculate the condition number and select the optimal detection wavelength: Iterate through all possible wavelength combinations and calculate the condition number of the absorption coefficient matrix; the wavelength combination with the smallest condition number is the optimal detection wavelength. Specifically: for λ1, λ2, ..., λ n Each term in the matrix is ​​iterated over within the absorption spectral range, and the condition number of the absorption coefficient matrix is ​​calculated to select the wavelength combination λ1,λ2,…,λ that minimizes the condition number. n This is the optimal wavelength for detecting mixed gases; 2) At the selected optimal detection wavelength, measure the absorption coefficient of each gas to be tested at each detection wavelength; 3) A pulse drive module is used to drive multiple lasers to generate pulse signals of each optimal detection wavelength. A multiplexer combines the signals from each laser onto a single optical fiber for transmission. Different delay fibers are used to ensure that the signals from each laser do not overlap in the time domain. Based on time division multiplexing technology, light intensity information of different wavelengths at different detection points is obtained from the received signals. 4) The signals reflected by each gas chamber are transmitted to the photoelectric detection and data acquisition module via the first coupler. The photoelectric detection and data acquisition module converts them into electrical signals for acquisition and transmits them to the host computer for processing to improve the signal-to-noise ratio. The host computer uses the measured absorption coefficient and the acquired signal strength and other parameters to calculate the concentration of each component gas in the mixed gas to be tested at different detection points, so as to realize the distributed detection of mixed gases with overlapping absorption spectra.

2. The method for selecting light source wavelength and detecting gas concentration as described in claim 1, characterized in that... In step 3), the light intensity information of different wavelengths at different detection points is obtained. Assuming there are n types of gases to be tested, denoted as gas 1, 2, 3…n; at least n laser wavelengths are required, denoted as λ1, λ2, λ3…λ… n If there are several types of gases to be tested, then several lasers are needed. The signal intensities at different wavelengths when there is no gas initially are denoted as I1, I2…I… n When detecting a mixed gas, the measured signal intensity values ​​at different wavelengths are recorded as I1', I2'…I… n ',ΔI1,ΔI2…ΔI n This represents the relative change in light intensity after signals of different wavelengths are absorbed by the gas.

3. The method for selecting light source wavelength and detecting gas concentration as described in claim 1, characterized in that... In step 3), the process of obtaining light intensity information of different wavelengths at different detection points from the received signal based on time-division multiplexing technology specifically includes the following steps: 3.1) Select and configure an appropriate number of lasers to ensure that the wavelength of the light signal generated by each laser is the optimal detection wavelength; connect the components such as lasers, gas cells, multiplexers, couplers, delay fibers, and photodetectors according to the system architecture to ensure smooth signal transmission; the number of lasers, gas cells, couplers, and delay fibers should match the number of gas detection points. 3.2) Use a pulse drive module to drive multiple lasers to generate pulse signals simultaneously; 3.3) The multiplexer combines the signals from each laser onto a single optical fiber for transmission, using different delay fibers to distinguish the signals from each laser, ensuring that the signals from each laser do not overlap in the time domain; 3.4) The coupler transmits the light source signal to the main circuit and transmits the signal light reflected from the gas chamber to the photoelectric detection and data acquisition module. In the absence of gas, it measures and records the initial light intensity I1, I2…I at each detection wavelength. n ; 3.5) After filling the gas chamber with the gas mixture to be tested, measure the peak signal intensity I1', I2'…I at each detection wavelength again. n By comparing the signal intensity when there is no gas with the signal intensity after the gas is filled, the relative changes in light intensity ΔI1, ΔI2…ΔI are calculated. n .

4. The method for selecting light source wavelength and detecting gas concentration as described in claim 1, characterized in that... In step 4), calculating the concentration of each component gas in the gas mixture at different detection points specifically includes the following steps: 4.1) The host computer collects data on the change in light intensity at all detection points under different detection wavelengths; For simultaneous detection of n gases, the absorption coefficient of each gas is measured at each detection wavelength. Gas i at wavelength λ... m The absorption coefficient under the given condition is denoted as α. m,i ; In the formula, ΔI m For wavelength λ m The change in light intensity after absorption by the gas, where gas i is at wavelength λ. m The absorption coefficient under the given condition is denoted as α. m,i (m, i = 1 to n), the concentrations of each component gas in the gas mixture to be tested are C1, C2...C... n L is the effective absorption length of the gas chamber; 4.2) Based on Beer-Lambert's law and the change in light intensity, combined with the absorption coefficient matrix, a gas concentration calculation matrix is ​​constructed. This matrix can convert the change in light intensity into the concentration of each component gas. The gas concentration calculation matrices for each component gas are as follows: Based on the parameters obtained above, the concentration of each component gas in the gas mixture at different detection points can be calculated by combining the matrix, thus realizing distributed detection of gas mixtures with overlapping absorption spectra.

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