Light source wavelength selection and gas concentration detection method of absorption spectrum overlapped mixed gas distributed sensing system

Abstract

The invention discloses a light source wavelength selection and gas concentration detection method for an absorption spectrum overlapped mixed gas distributed sensing system, and relates to the field of distributed gas detection. The method comprises the following steps: determining an optimal detection wavelength through an algorithm provided by the invention according to the type of each to-be-detected gas and corresponding absorption spectrum data; respectively measuring the absorption coefficient of each to-be-detected gas at each detection wavelength; pulse modulation is carried out on the selected lasers with different wavelengths, and light intensity information corresponding to the gas detection points is obtained by combining a time division multiplexing technology; according to the obtained data, the concentration of each component gas in the mixed gas at different detection points is calculated according to the method provided by the invention, and multi-point distributed detection of various gases is realized. The method provided by the invention has the capability of detecting the concentration of each component of the mixed gas with overlapped absorption spectrums in a multi-point distributed manner, provides a new solution for multi-point distributed detection of the multi-component mixed gas, and has the advantages of low cost, no need of power supply at detection points, safety, reliability and the like.

Description

Technical Field

[0001] The present invention relates to the field of distributed gas detection, and in particular to a method for selecting a light source wavelength and detecting a gas concentration in a distributed sensing system of mixed gases with overlapping absorption spectra. Background Art

[0002] Tunable diode laser absorption spectroscopy (TDLAS) is a detection technology that measures the concentration of trace gases through the absorption information of gas to lasers of specific wavelengths. Compared with traditional electrochemical detection methods, it 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. It has broad application prospects in the fields of coal, gas, and petrochemicals.

[0003] For multi-component gases with non-overlapping absorption spectra, since the wavelengths corresponding to the characteristic absorption peaks of each gas do not overlap, the concentration of each gas in the mixed gas can be measured by monitoring the absorption of each corresponding wavelength; however, for mixed gases with overlapping absorption spectra, since two or more gases absorb at the same wavelength, the concentration of each component in the mixed gas cannot be obtained through the absorption information of a single wavelength. The use of multiple wavelengths for measurement raises issues such as how to determine the optimal working wavelength and how to quantitatively evaluate the pros and cons of wavelength selection. On the other hand, current gas sensing solutions based on TDLAS can usually only perform single-point or limited-point gas monitoring, and cannot achieve multi-point distributed multi-component gas sensing with overlapping absorption spectra. For example, Wang Yin et al. proposed a method for detecting the concentration of each component in a mixed gas with overlapping absorption spectra based on TDLAS. By establishing a relationship model between the "weight" of the absorption spectrum and the concentration of each component gas in the gas mixture, single-point detection of mixed gases with overlapping absorption spectra can be achieved (Wang Yin, Wei Yubin, Zhang Tingting, Hu Jie, Zhao Weisong, Li Yanfang, Wang Zhaowei, Liu Tongyu. Gas concentration monitoring method, device and system: CN106525742A [P]. 20191001), but this solution using a single laser requires that the laser wavelength can be scanned over a large range, which has high requirements on device performance and control system. More importantly, if there is no obvious difference in the absorption spectra of different gases in the mixed gas within the scannable wavelength range, it will cause problems such as poor detection effect. In addition, this solution can only detect single-point gas. Dong Xiaopeng's research group at Xiamen University proposed a multi-point gas detection scheme based on time division multiplexing technology (Dong Xiaopeng, Yang Jiahua. A cascade multi-point gas detection method with branched gas chambers: CN 114965289 B [P]. 20240823), but the scheme did not involve the measurement of mixed gases with overlapping absorption spectra and the related wavelength selection issues. For gases with non-overlapping absorption spectra and multiple distinct peaks, Zhang et al. proposed the use of wavelength division multiplexing technology for multi-point and multi-gas measurements (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 on the use of optical switches and multiple detectors to separate gas signals at different detection points, and at the same time modulate multiple lasers at different frequencies to distinguish gas signals of different components in the mixed gas, thereby realizing the measurement of mixed gas of methane and water vapor (Wang Z, Chang J, Yu H, Tian C, Zhang H, Zhang X, Tang L, ZhangQ, 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). However, this scheme is also not suitable for multi-point detection of mixed gases with overlapping absorption spectra. Summary of the invention

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

[0005] The present invention provides a mixed gas distributed sensing system with overlapping absorption spectra, including a pulse drive module, a laser, a gas chamber, a combiner, a coupler, a delay optical 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 a stable pulse signal;

[0007] The laser is used as a light source, and the laser can be set to multiple, each laser generates light of a specific wavelength, and 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 tested, light propagates in the gas chamber and can reflect the light signal, and the number of the gas chambers corresponds to the number of the detection points;

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

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

[0011] The delay optical fiber is used to distinguish the signals of different lasers to prevent the signals from overlapping in the time domain. The signal corresponding to each laser is transmitted through delay optical fibers of different lengths, thereby forming a clear distinction in the time domain;

[0012] The photoelectric detection and data acquisition module is used to convert the received signal into an electrical signal and collect it, and transmit the signal to the host computer for processing. By superimposing and averaging the collected time domain signals to improve the signal-to-noise ratio, the detection limit of the system for gas concentration can be improved;

[0013] The host computer is used to process and analyze the collected data and calculate the concentration of each component gas; the pulse signal reflected by each gas chamber back to the photoelectric detection and data acquisition module is distinguished in the time domain by the delayed optical fiber. The host computer calculates the relative change of the received reflected signal to calibrate the gas to be tested, and then the concentration of the mixed gas can be detected according to the calibrated coefficient.

[0014] Furthermore, the laser, gas chamber, coupler and delay fiber can be set to multiple, and the number of gas chambers, delay fibers and couplers corresponds to the number of detection points; if there are several gas detection points, several gas chambers, delay fibers and couplers are required, and each coupler connects an air chamber and a delay fiber.

[0015] Furthermore, the coupler is provided in plurality, wherein the splitting ratio of the first coupler can be 50:50.

[0016] The present invention provides a method for selecting a light source wavelength and detecting a gas concentration, using the mixed gas distributed sensing system with overlapping absorption spectra, and the specific method includes the following steps:

[0017] 1) According to the type and absorption spectrum characteristics of the gas to be tested, determine the absorption spectrum range of the gas to be tested, especially the possible overlapping parts; establish the absorption coefficient matrix of the gas to be tested, and determine multiple optimal detection wavelengths according to the absorption spectrum data of the gas to be tested;

[0018] 2) Under the selected optimal detection wavelength, measure the absorption coefficient of each gas to be tested at each detection wavelength;

[0019] 3) Use the pulse drive module to drive multiple lasers to generate pulse signals of each optimal detection wavelength. The combiner combines the signals from each laser into one optical fiber for transmission. Through different delay optical fibers, ensure that the signals from each laser do not overlap in the time domain. Based on the time division multiplexing technology, obtain the light intensity information of different wavelengths at different detection points from the received signal.

[0020] 4) The signals reflected by 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 electrical signals for collection 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 collected signal intensity and other parameters to calculate the concentration of each component gas in the mixed gas to be tested at different detection points, thereby realizing distributed detection of mixed gases with overlapping absorption spectra.

[0021] In step 1), the method for determining multiple optimal detection wavelengths specifically includes the following steps:

[0022] 1.1) Determine the type of gas to be tested: clarify what types of gas are included in the mixed gas.

[0023] 1.2) Determine the absorption spectrum range: According to relevant databases (such as HITRAN), obtain the characteristic absorption peaks and absorption spectrum ranges of each gas to be tested, especially the possible overlapping parts.

[0024] 1.3) Establish an absorption coefficient matrix: In the determined absorption spectrum range, establish the absorption coefficient matrix of the gas to be tested for each gas at each possible detection wavelength. , the elements in the coefficient matrix For the Gas at wavelength Lower absorption cross section, In the absorption spectrum range ( ), the data is obtained from the HITRAN database; assuming that The gas to be tested and detection wavelength, then the absorption coefficient matrix of the gas to be measured is It is expressed 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: Each item in is traversed in the absorption spectrum range, and the condition number of the absorption coefficient matrix is ​​calculated, so that the wavelength combination corresponding to the minimum condition number is selected This is the optimal wavelength for mixed gas detection.

[0027] In step 3), the light intensity information of different wavelengths at different detection points is obtained. Assuming that the type of gas to be detected is , denoted as gas 1, 2, 3… ; At least The wavelengths of the laser are denoted by , there are several types of gases to be tested, so several lasers are needed. For a certain detection point, The pulse signals of different wavelengths are recorded as When detecting mixed gases, the signal intensity measurements at different wavelengths are recorded as , is the relative change in light intensity of signals of different wavelengths after being absorbed by the gas;

[0028] The specific steps include:

[0029] 3.1) Select and configure the appropriate number of lasers to ensure that the wavelength of the optical signal generated by each laser matches the optimal detection wavelength; connect components such as lasers, gas chambers, combiners, couplers, delay fibers, and photodetectors according to the system architecture to ensure smooth signal transmission; the number of lasers, gas chambers, couplers, and delay fibers matches the number of gas detection points;

[0030] 3.2) Use the pulse drive module to drive the laser and generate pulse signals at the same time;

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

[0032] 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, the initial light intensity at each detection wavelength is measured and recorded. ;

[0033] 3.5) After the gas chamber is filled with the mixed gas to be tested, measure the signal peak intensity at each detection wavelength again. By comparing the initial signal intensity without gas with the signal intensity after filling with gas, the relative change in 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 specifically includes the following steps:

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

[0036] for Simultaneous detection of gases, measuring the absorption coefficient of each gas at each detection wavelength. In wavelength The absorption coefficient under ;

[0037]

[0038] In the formula, For the wavelength The change in light intensity after being absorbed by the gas. In wavelength The absorption coefficient under ( The concentrations of the components of the mixed gas to be tested are , is the effective absorption length of the air chamber;

[0039] 4.2) According to the Beer-Lambert 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 calculation matrix of each component gas concentration is as follows:

[0040]

[0041] According to the parameters obtained above, combined with the matrix, the concentration of each component gas in the mixed gas to be tested at different detection points can be calculated, thereby realizing distributed detection of mixed gases with overlapping absorption spectra.

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

[0043] 1. Solve the problem of spectral overlap: The present invention proposes an effective solution to the problem of mixed gas detection with overlapping absorption spectra. The concentration of each component gas is calculated by using the light absorption of the mixed gas to be tested at different detection wavelengths. By selecting the optimal detection wavelength combination, the concentration of each component gas in the mixed gas can be accurately distinguished and calculated, even if there is overlap in the absorption spectra.

[0044] 2. Optimize the detection wavelength selection: By calculating the condition number of the gas absorption matrix, the present invention determines the best 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. Reduced cost and easy expansion: Compared with the prior art, the system device of the present invention is simpler and has lower cost. Through the clever combination of components such as combiners, couplers, and delay fibers, signal transmission and differentiation are achieved without the need for complex spectrometers and multiple detection systems. At the same time, the system architecture of the present invention is easy to expand and maintain. By increasing the number of components such as lasers, air chambers, couplers, and delay fibers, the number of detection points can be easily expanded. In addition, the various components of the system are simply connected and easy to disassemble and replace, reducing maintenance costs.

[0046] 4. Multi-point multiplexing detection: Based on the principle of time division multiplexing, the present invention uses delayed optical fiber to distinguish signals of different wavelengths and different detection points in the time domain, and realizes the distributed measurement of multi-component gases with overlapping absorption spectra; the present invention is suitable for multi-point multiplexing detection of gases, and can simultaneously monitor the gas concentration of multiple detection points. It is of great significance for real-time online distributed measurement of mixed gases with overlapping spectra, and can meet the wide needs of industry, environmental protection and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 Schematic diagram of the principle of a distributed sensing system for mixed gases with overlapping absorption spectra.

[0048] Figure 2 These are the absorption spectra of propane and isobutane in the HITRAN database.

[0049] Figure 3 is the value of the inverse 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 test point 1. DETAILED DESCRIPTION

[0052] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the following embodiments will further illustrate the present invention in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. On the contrary, the present invention covers any substitution, modification, equivalent method and scheme made on the essence and scope of the present invention as defined by the claims.

[0053] The system block diagram of the mixed gas distributed sensing system with overlapping absorption spectra according to the embodiment of the present invention is as follows: Figure 1 As shown, it includes a pulse drive module 1, Lasers, A gas chamber, a combiner, Couplers, Delay optical fiber, photoelectric detection and data acquisition module, host computer;

[0054] The pulse drive module is used to drive a series of lasers, laser 1, laser 2, and so on. . Each laser is connected to the input port of the combiner through a corresponding delay line (labeled as L1, L2 to Ln respectively); The couplers are numbered as Coupler 1, Coupler 2, and so on. ;and The delay optical fibers are marked as l1, l2, and so on. Port 101 of coupler 1 is connected to the output port of the combiner, 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 through delay optical 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 through delay optical fiber l2, port 302 of coupler 3 is connected to air chamber 2, and so on, forming corresponding connection relationships.

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

[0056] The laser is used as a light source and is driven by a pulse drive module to generate a pulse signal at the same time; each laser can emit light of a specific wavelength, and the laser signals are distinguished by a delay optical fiber; Lasers (Laser 1, Laser 2 to Laser ) The light pulses emitted by the detector are transmitted to the combiner through multiple optical fibers to form a light source network. These lights interact with the components in the mixed gas to produce different absorption, thereby realizing the detection of gas components;

[0057] The combiner is a key component of the system of the present invention. The combiner is used to combine the The light source signals with specific wavelengths from different lasers are combined into one optical fiber for transmission. The system only needs one optical fiber to transmit all the light source signals, which greatly simplifies the structure of the system. However, since the light pulses emitted by different lasers may overlap in the time domain, it is necessary to use a delay fiber to distinguish them to ensure that the light pulses from different lasers can be clearly distinguished in the time domain when they reach the photoelectric detection and data acquisition module. The signal after the combiner is transmitted to each gas chamber through the optical fiber and each coupler.

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

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

[0060] The photoelectric detection and data acquisition module receives the reflected signal from the air chamber and converts it into an electrical signal for collection and recording. The photoelectric detection and data acquisition module transmits the collected electrical signal to the host computer for processing.

[0061] The host computer is a powerful computer that receives signals from the photoelectric detection and data acquisition modules and processes and analyzes them. By performing operations such as superposition and averaging on the collected time domain signals, the host computer can improve the signal-to-noise ratio and 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 tested by calculating the relative change of the received reflection signal, 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 components such as the gas chamber, coupler, delay fiber, and photoelectric detection and data acquisition modules through optical fibers to form a complete, distributed gas sensing system.

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

[0064] (1)

[0065] in, is the initial light intensity before gas absorption, is the light intensity after being absorbed by the gas, and the absorption coefficient It is related to the gas type and detection wavelength. is the concentration of the gas to be measured, is the optical path of the laser and the gas. Usually, the difference in relative intensity values ​​can be used to express the ratio of the absolute values ​​of light intensity, so formula (1) can be expressed as:

[0066] (2)

[0067] Represents the relative strength difference, here , It is the relative intensity value in dB.

[0068] The principle of mixed gas concentration detection is that, assuming the type of gas to be tested is , denoted as gas 1, 2, 3… ; At least The wavelengths of the laser are denoted by , if there are several gases to be measured, several lasers are needed. When there is no gas initially, the signal intensities of different wavelengths are recorded as When detecting mixed gases, the signal intensity measurements at different wavelengths are recorded as , is the relative change in light intensity of signals of different wavelengths after being absorbed by the gas. For simultaneous detection of gases, formula (2) can be evolved into:

[0069] (3)

[0070] The above formula, For the wavelength The change in light intensity after being absorbed by the gas. In wavelength The absorption coefficient under ( The concentrations of the components of the mixed gas to be tested are . The calculation matrix of the gas concentration of each component can be further inverted as follows:

[0071] (4)

[0072] The method of determining the types of gases to be tested whose absorption spectra overlap, determining the absorption spectrum interval of the gases to be tested, and determining the optimal detection wavelength according to the absorption spectrum data of the gases to be tested includes: determining the types of gases in the mixed gas to be tested, determining the absorption spectrum interval according to the types of gases to be tested, determining the gas absorption coefficient matrix according to the absorption spectrum 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: Determine the absorption spectrum interval of the gases to be tested. Establish the absorption coefficient matrix of the gases to be tested. , the coefficient matrix Elements in Indicates Gas at wavelength Lower absorption cross section, ( In the absorption spectrum range), the data can be obtained from the HITRAN database.

[0073] (5)

[0074] right Each item in is traversed in the absorption spectrum range, and the condition number of the absorption coefficient matrix is ​​calculated, so that the wavelength combination corresponding to the minimum condition number is selected This is the optimal wavelength for mixed gas detection.

[0075] Measure the absorption coefficient of each gas to be tested at each detection wavelength. In wavelength The absorption coefficient under ,in, .

[0076] The light intensity information of different wavelengths at different detection points is obtained by using multiple wavelength lasers combined with time division multiplexing technology. The signals of each detection point are separated in the time domain by delayed optical fibers. For the gas chamber at a certain detection point, the host computer can obtain the signal peak intensities at different detection wavelengths reflected by the gas chamber when there is no gas initially, which are recorded as When the gas chamber is filled with the mixed gas to be tested, the signal peak intensity measurement values ​​at different detection wavelengths reflected by the gas chamber are recorded as .

[0077] According to the above-mentioned parameters, combined with formula (4), the concentration of each component gas in the mixed gas to be tested at different detection points can be calculated, and the distributed detection of mixed gases with overlapping absorption spectra can be realized. This includes obtaining the light intensity change caused by gas absorption at different detection wavelengths returned by each detection point in the host computer, and calculating the concentration of each component gas in the mixed gas to be tested at each detection point according to the light intensity change at each detection point and the absorption coefficient of each component gas at each detection wavelength.

[0078] The following describes the mixed gas concentration detection method in the embodiment of the present invention by taking a propane and isobutane mixed gas with overlapping absorption spectra as an example.

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

[0080] First, the absorption coefficient matrix of the gas to be measured is:

[0081] (6)

[0082] , Represents propane and isobutane gases at the detection wavelength The gas absorption cross section under , Represents propane and isobutane gases at the detection wavelength According to the gas absorption cross sections provided by the HITRAN database for propane (C3H8), isobutane (C4H 10 ) absorption spectrum data, such as Figure 2 As shown. Data in the wavelength range of 1680-1700nm are selected for analysis. In order to determine the optimal detection wavelength, the absorption cross-section data in the wavelength range of 1680-1700nm are traversed to calculate the condition number of the gas absorption coefficient matrix. When the condition number is the smallest, the corresponding wavelength combination is the optimal detection wavelength. In this embodiment, the condition number is infinite, and the minimum value of the condition number can be found by calculating the inverse of the condition number. Figure 3 As shown, the reciprocal of the absorption coefficient matrix condition number is plotted at each detection wavelength. By comparing the reciprocal of the condition number at different wavelengths, it can be seen that when the detection wavelength is selected at 1686.34 nm and 1689.09 nm, the reciprocal of the condition number reaches the maximum value, that is, the condition number is the minimum. Therefore, the two wavelengths are selected as the optimal detection wavelengths, so the two laser wavelengths described in this embodiment 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 respectively. The above formula (2) can be rewritten as:

[0084] (7)

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

[0086] (8)

[0087] For gas In wavelength The equivalent absorption coefficient under ( .

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

[0089] A mixture of 10% propane and 5% isobutane is filled into the test point gas chamber, and the light intensity is compared with that of air (as the reference gas). The light intensity change caused by the absorption of the propane and isobutane mixture at two detection wavelengths at test point 1 is obtained in the host computer. Figure 5 is the pulse signal returned by test point 1 before and after the mixed gas is filled at two detection wavelengths, and it can be obtained that the gas chamber is = Change in light intensity at 1686.34 nm =0.4647 dB, at the detection wavelength = Change in light intensity at 1689.09 nm =1.1905 dB. Based on the equivalent absorption coefficients of propane and isobutane gases at each detection wavelength and the change in light intensity caused by the absorption of the mixed gas, the concentration of each component gas in the propane and isobutane mixed gas to be tested at the detection point can be calculated: 9.74% propane, 5.06% isobutane.

[0090] The above embodiments are only preferred embodiments of the present invention and cannot be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the present invention.

Claims

1. A mixed gas distributed sensing system with overlapping absorption spectra, characterized in that It includes pulse drive module, laser, gas chamber, combiner, coupler, delay fiber, photoelectric detection and data acquisition module, and host computer; The pulse driving module is used to drive the laser to generate a stable pulse signal; The laser is used as a light source. Multiple lasers can be provided. Each laser generates light of a specific wavelength, and the wavelength is selected according to the absorption spectrum characteristics of the gas to be measured; There are multiple gas chambers, each of which is used to contain the gas to be tested and serves as a channel for the propagation and reflection of the optical signal. The number of gas chambers corresponds to the number of detection points. The combiner is used to combine the light source signals required for gas sensing onto one optical fiber for transmission, so as to simplify the system structure; The couplers are provided in plurality, 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, and the other couplers are used to transmit a part 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; The delay optical fibers are provided in plurality to distinguish signals of different lasers and prevent signals from overlapping in the time domain. The signal corresponding to each laser is transmitted through delay optical 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 collect it, and transmit the signal to the host computer for processing. By superimposing and averaging the collected time domain signals to improve the signal-to-noise ratio, the detection limit of the system for gas concentration can be improved; The host computer is used to process and analyze the collected data and calculate the concentration of each component gas; the pulse signal reflected by each gas chamber back to the photoelectric detection and data acquisition module is distinguished in the time domain by the delayed optical fiber, and the host computer calculates the relative change of the received reflected signal, calibrates the gas to be tested, and then detects the concentration of the mixed gas according to the calibrated coefficient.

2. The mixed gas distributed sensing system with overlapping absorption spectra as claimed in claim 1, characterized in that The number of the gas chambers, delay optical fibers and couplers corresponds to the number of detection points; if there are several gas detection points, several gas chambers, delay optical fibers and couplers are set, and each coupler connects an air chamber and a delay optical fiber.

3. The mixed gas distributed sensing system with overlapping absorption spectra as claimed in claim 1, characterized in that The splitting ratio of the first coupler is 50:

50.

4. A method for selecting a light source wavelength and detecting a gas concentration, characterized in that A mixed gas distributed sensing system with overlapping absorption spectra as claimed in any one of claims 1 to 3 is used, and the specific method comprises the following steps: 1) According to the type and absorption spectrum characteristics of the gas to be tested, determine the absorption spectrum range of the gas to be tested, especially the possible overlapping parts; establish the absorption coefficient matrix of the gas to be tested, and determine multiple optimal detection wavelengths according to the absorption spectrum data of the gas to be tested; 2) Under the selected optimal detection wavelength, measure the absorption coefficient of each gas to be tested at each detection wavelength; 3) Use the pulse drive module to drive multiple lasers to generate pulse signals of each optimal detection wavelength. The combiner combines the signals from each laser into one optical fiber for transmission. Through different delay optical fibers, ensure that the signals from each laser do not overlap in the time domain. Based on the time division multiplexing technology, obtain the light intensity information of different wavelengths at different detection points from the received signal. 4) The signals reflected by 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 electrical signals for collection 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 collected signal intensity and other parameters to calculate the concentration of each component gas in the mixed gas to be tested at different detection points, thereby realizing distributed detection of mixed gases with overlapping absorption spectra.

5. A method for selecting a light source wavelength and detecting a gas concentration as claimed in claim 4, characterized in that In step 1), the method for determining multiple optimal detection wavelengths specifically includes the following steps: 1.1) Determine the type of gas to be tested: clarify what types of gas are included in the mixed gas; 1.2) Determine the absorption spectrum range: According to the HITRAN database, obtain the characteristic absorption peaks and absorption spectrum ranges of each gas to be tested, especially the possible overlapping parts; 1.3) Establish an absorption coefficient matrix: In the determined absorption spectrum range, establish the absorption coefficient matrix of the gas to be tested for each gas at each possible detection wavelength. , the absorption coefficient matrix Medium Element Indicates The gas in The absorption coefficient at the detection wavelength is ; Detection wavelength In the absorption spectrum range, it is assumed that The gas to be tested and detection wavelength, then the absorption coefficient matrix of the gas to be measured is It is expressed as: ； 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: Each item in is traversed in the absorption spectrum range, and the condition number of the absorption coefficient matrix is ​​calculated, so that the wavelength combination corresponding to the minimum condition number is selected This is the optimal wavelength for mixed gas detection.

6. A method for selecting a light source wavelength and detecting a gas concentration as claimed in claim 4, characterized in that In step 3), the light intensity information of different wavelengths at different detection points is obtained. Assuming that the type of gas to be detected is , denoted as gas 1, 2, 3… ; At least The wavelengths of the laser are denoted by , if there are several gases to be tested, several lasers are needed. The signal intensities of different wavelengths when there is no gas at the beginning are recorded as When detecting mixed gases, the signal intensity measurements at different wavelengths are recorded as , It is the relative change in light intensity of signals of different wavelengths after being absorbed by the gas.

7. A method for selecting a light source wavelength and detecting a gas concentration as claimed in claim 4, characterized in that In step 3), the light intensity information of different wavelengths at different detection points is obtained from the received signal based on the time division multiplexing technology, which specifically includes the following steps: 3.1) Select and configure the corresponding number of lasers to ensure that the wavelength of the optical signal generated by each laser is the optimal detection wavelength; connect the components such as lasers, gas chambers, combiners, couplers, delay fibers and photodetectors according to the system architecture to ensure smooth signal transmission; the number of lasers, gas chambers, couplers and delay fibers should match the number of gas detection points; 3.2) Use the pulse drive module to drive multiple lasers to generate pulse signals simultaneously; 3.3) The combiner combines the signals of each laser into one optical fiber for transmission, and uses different delay fibers to distinguish the signals of 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, the initial light intensity at each detection wavelength is measured and recorded. ; 3.5) After the gas chamber is filled with the mixed gas to be tested, measure the signal peak intensity at each detection wavelength again. By comparing the initial signal intensity without gas with the signal intensity after filling with gas, the relative change in light intensity is calculated. .

8. A method for selecting a light source wavelength and detecting a gas concentration as claimed in claim 4, characterized in that In step 4), the concentration of each component gas in the mixed gas to be tested at different detection points is calculated, which specifically includes the following steps: 4.1) The host computer collects the light intensity change data of all detection points at different detection wavelengths; for Simultaneous detection of gases, measuring the absorption coefficient of each gas at each detection wavelength. In wavelength The absorption coefficient under ; In the formula, For the wavelength The change in light intensity after being absorbed by the gas. In wavelength The absorption coefficient under ( The concentrations of the components of the mixed gas to be tested are , is the effective absorption length of the air chamber; 4.2) According to the Beer-Lambert 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 calculation matrix of each component gas concentration is as follows: According to the parameters obtained above, combined with the matrix, the concentration of each component gas in the mixed gas to be tested at different detection points can be calculated, thereby realizing distributed detection of mixed gases with overlapping absorption spectra.

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