CN119880848A - Gas concentration detection method of low-pressure detection system based on TDLAS - Google Patents
Gas concentration detection method of low-pressure detection system based on TDLAS Download PDFInfo
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- G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
- G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
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Abstract
The application discloses a gas concentration detection method of a low-pressure detection system based on TDLAS, which comprises the steps of discharging gas to be detected into an optical coupling air chamber through an input needle valve, reducing pressure by a vacuum pump, collecting a light intensity curve at the moment after the pressure in the optical coupling air chamber is reduced, and inverting to obtain the gas concentration. Meanwhile, in the step-down process, the working temperature of the laser is kept unchanged, and the position of the gas absorption peak is kept unchanged in the step-down state by controlling the closed-loop adjustment of the current driving module of the laser. Therefore, under low pressure, overlapping interference between gas absorption peaks and cross interference of water vapor on the gas to be measured can be placed, and gas measurement accuracy is improved.
Description
Technical Field
The application relates to the technical field of gas concentration detection, in particular to a gas concentration detection method.
Background
The tunable laser diode absorption spectroscopy (TDLAS) is a gas detection technology based on laser spectroscopy, which uses a narrow bandwidth laser emitted by a tunable semiconductor laser to scan a specific absorption spectrum line of a specific gas molecule, thereby realizing high-sensitivity and high-selectivity detection of gas concentration. The technology measures the absorption degree of gas molecules on laser energy by precisely controlling the wavelength of a laser to be matched with the wavelength of a gas absorption spectrum line, and then calculates the concentration of gas according to the beer-lambert law. The TDLAS technology is widely used in the fields of environmental monitoring, industrial process control, safety detection, medical diagnosis, and the like, due to its characteristics of high resolution, rapid response, capability of monitoring various gas components in real time, and the like.
When the TDLAS technology is actually used for measuring gas, particularly overlapping interference among gas absorption peaks and cross interference of water vapor on the gas to be measured easily occur in a middle infrared band, and the gas measurement accuracy is affected. Therefore, around the cross interference of the gas, a gas concentration correction method is generally adopted, that is, the concentration of the interfering gas (such as water vapor) is measured, then the concentration of the measured gas is corrected, or a table look-up method is used for correction, and linear, nonlinear and polynomial fitting is needed for the interference during correction. However, when the cross interference is generated by the gas, the matrix correction is difficult to realize along with the change of the temperature and the pressure, and meanwhile, the gas concentration measurement and the fitting gas concentration are required to be within a specific range, otherwise, the fitting polynomial can have great deviation. Thus, how to provide a high-precision gas measurement method for reducing and suppressing gas cross interference is a problem to be solved.
Disclosure of Invention
Based on the above, a method for detecting the gas concentration of a low-pressure detection system based on TDLAS is provided to solve the problems of overlapping interference between gas absorption peaks and cross interference of water vapor to the gas to be detected in the prior art, thereby affecting the accuracy of gas measurement.
The gas concentration detection method of the TDLAS-based low-voltage detection system comprises a laser (7), an optical input collimator (1) of the laser, an optical coupling air chamber (3), an input needle valve (6), a gas input end (4 a), a gas output end (4 b), a vacuum pump (5), a photoelectric detector (2), a laser temperature control module (8), a laser current driving module (9), a signal processing module and an embedded control module (12);
The optical coupling air chamber (3) is provided with the air input end (4 a) and the air output end (4 b), the input needle valve (6) is connected with the air input end (4 a), the air output end (4 b) is connected with the vacuum pump (5), and the input needle valve (6) is used for being connected with an air bottle of air to be detected;
The optical coupling air chamber (3) is provided with an optical input collimator (1) of a laser, and the output end of the laser (7) is connected with the optical input collimator (1) of the laser, and the laser (7) is also connected with a laser temperature control module (8) and a laser current driving module (9);
the optical coupling air chamber (3) is also provided with a photoelectric detector (2), and the output end of the photoelectric detector (2) is connected with the embedded control module (12) through the signal processing module, and the embedded control module (12) is also connected with the laser current driving module (9);
The detection method comprises the following steps:
opening an input needle valve (6) to enable gas to be detected to be discharged into the optical coupling air chamber (3) through the air input end (4 a), and closing the input needle valve (6) after inputting a certain volume of gas to be detected;
-turning on the laser (7) by means of the embedded control module (12);
starting the vacuum pump (5) to reduce the voltage, starting the photoelectric detector (2) to convert the optical signal into a current signal, processing the current signal by using the signal processing module, and calculating the processed current information by using the embedded control module (12) to obtain a curve of a 2f signal;
In the step-down process, the central peak position of the curve of the 2f signal is obtained by calculation through the embedded control module (12) in real time, and the laser current driving module (9) is subjected to closed-loop adjustment through the embedded control module (12) so as to ensure that the central peak position of the curve of the 2f signal is unchanged in the step-down process;
And when the pressure of the optical coupling air chamber (3) is detected to be reduced to 0.1atm, carrying out inversion calculation on the curve of the 2f signal correspondingly acquired by the embedded control module (12) to obtain the gas concentration.
In the above scheme, optionally, the opening of the input needle valve (6) enables the gas to be tested to be discharged into the optical coupling gas chamber (3) through the gas input end (4 a) further comprises:
Evacuating the residual gas in the optical coupling gas cell (3);
The input needle valve (6) is connected into a nitrogen bottle, and the optical coupling air chamber (3) is flushed by utilizing nitrogen.
In the scheme, the signal processing module comprises a current conversion voltage module (10) and a phase-locked amplifying module (11);
the output end of the photoelectric detector (2) is connected with the embedded control module (12) through the current conversion voltage module (10) and the phase-locked amplifying module (11) in sequence.
In the above scheme, further optionally, the processing of the current signal by the signal processing module comprises converting the current signal into a voltage signal by a current conversion voltage module (10), and performing 2f frequency phase-locked amplification on the voltage signal by a phase-locked amplification module (11).
In the scheme, optionally, the method further comprises the step that the embedded control module (12) guarantees that the working temperature of the laser (7) is unchanged by controlling the laser temperature control module (8) in the step-down process.
In the above scheme, optionally, the pressure of the optical coupling air chamber (3) is detected by a pressure gauge arranged on the vacuum pump (5).
In the above scheme, optionally, the laser (7) is a DFB laser.
The application has at least the following beneficial effects:
According to the application, after the gas to be detected is discharged into the optical coupling air chamber (3) through the input needle valve (6), the pressure is reduced by the vacuum pump (5), and after the pressure in the optical coupling air chamber (3) is reduced, the light intensity curve at the moment is collected and inverted to obtain the gas concentration. Meanwhile, in the step-down process, the working temperature of the laser (7) is kept unchanged, and the position of a gas absorption peak is kept unchanged in the step-down state by controlling the closed-loop adjustment of the laser current driving module. Therefore, under low pressure, overlapping interference between gas absorption peaks and cross interference of water vapor on the gas to be measured can be placed, and gas measurement accuracy is improved.
Drawings
FIG. 1 is a graph showing the absorbance profile of 1% Vol water vapor at 1 atm;
FIG. 2 is a graph showing the absorbance profile of 1% Vol water vapor at 0.1 atm;
FIG. 3 is the absorbance of 0.1% Vol COS at1 atm;
FIG. 4 is the absorbance of 0.1% Vol COS at 0.1 atm;
FIG. 5 is the absorbance of 0.5% Vol moisture with 10ppb ethane at1 atm;
FIG. 6 is the absorbance of 0.5% Vol moisture with 10ppb ethane at 0.1atm pressure;
Fig. 7 is a schematic structural diagram of a TDLAS-based low voltage detection system according to an embodiment of the present application;
fig. 8 is a flowchart of a gas concentration detection method of a TDLAS based low pressure detection system according to an embodiment of the application.
Detailed Description
The present application will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present application more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the application.
The gas absorption line type is divided into natural stretching and Gaussian stretching, and the natural stretching line type function expressed by wavelength is Lorentz line type, and the formula is shown in formula (1).
Wherein g N represents a Lorentz absorption linear function, gamma is the half-width of the absorption spectrum of the gas to be detected, lambda represents the wavelength, and lambda 0 represents the gas absorption center wavelength;
When the air pressure is reduced, the absorption line changes from Lorentz line to a sharp Voigt line function as shown in equation (2):
Wherein g v represents a Voigt linear function, α L represents the half-width of the line, λ represents a wavelength, λ 0 represents a gas absorption center wavelength, m is the mass of absorbed gas molecules, T is the gas temperature, k is the boltzmann constant, and c is the light velocity in vacuum;
By reducing the pressure and the gas absorption spectrum width, the low-pressure state can be subjected to description fitting through the formula (2), so that cross interference between two gas absorption peaks caused by the mutual overlapping of spectrum broadening is avoided, the cross interference effect is reduced, and the gas measurement capability is further improved.
At an optical path length of 0.1m and a temperature of 25 ℃ (all simulations are performed under the conditions of the patent), the gas is under the pressure of 1atm and 0.1atm, the wave beam 2015-2020 cm < -1 >, the absorbance of 1%Vol vapor under 1atm is shown in figure 1, the absorbance of 1%Vol vapor under 0.1atm is shown in figure 2, the molecular number of the gas is reduced due to the reduction of the gas pressure, but the absorption waveform accords with the Voigt line type due to the low pressure, so the deformation of the absorption wave is sharp, and the spread of the vapor absorption spectrum is reduced.
For the case that the gas absorption spectrum is dense and the measurement accuracy is easily affected by cross interference of the side gas during scanning, taking COS carbonyl sulfide gas as an example, the absorbance of 0.1%Vol gas under the pressure of 1atm and 0.1atm is in the range of 2061-2063 cm < -1 >, the absorbance of 0.1%Vol gas under the pressure of 1atm is shown as shown in figure 3, the overlapping part generates very high 'bottom' due to the expansion of adjacent gas absorption, the 'zero background' without gas absorption cannot be scanned by TDLAS technology, thus the accurate measurement cannot be performed, the absorbance of 0.1%Vol gas under the pressure of 0.1atm is shown as shown in figure 4, the absorbance peak of the 'zero background' can be measured between adjacent wavelengths by low-pressure measurement, and therefore 2061.75 or 2062.25cm < -1 > absorption spectrum is selected as the gas measurement wavelength, and the cross interference influence of the adjacent absorption peak is reduced.
For mid-infrared gas absorption spectroscopy, when high-precision gas measurement (in ppb order) is required, the gas to be measured is mostly subject to cross-interference of water vapor. Taking ethane as an example, although the water vapor absorption is weak near ethane, the water vapor content in the air is basically 0.2% -1% vol or higher, and the generated water vapor absorption affects the trace amount of ethane measurement. For the example of ethane at 3.37um absorption wavelength, at 1atm pressure, the absorption peak broadening generated at 0.5% vol for water vapor affects the ethane absorption, as can be seen from FIG. 5, because the 0.5% vol water vapor is present, the absorption spectrum will superimpose 10ppb ethane with the 0.5% vol water vapor absorption spectrum, producing significant cross-interference. In this case, the gas measurement was performed by setting the atmosphere of 0.1atm, and as shown in FIG. 6, the absorbance of 0.5% Vol vapor at 0.1atm and 10ppb ethane was found to be such that the absorption spectra of ethane and vapor were not overlapped, and thus the ethane content could be accurately measured with high accuracy.
In one embodiment, as shown in fig. 7 and 8, a gas concentration detection method of a low-voltage detection system based on a TDLAS is provided, wherein the detection system comprises a laser (7), an optical input collimator (1) of the laser, an optical coupling air chamber (3), an input needle valve (6), a gas input end (4 a), a gas output end (4 b), a vacuum pump (5), a photoelectric detector (2), a laser temperature control module (8), a laser current driving module (9), a signal processing module and an embedded control module (12);
The optical coupling air chamber (3) is provided with the air input end (4 a) and the air output end (4 b), the input needle valve (6) is connected with the air input end (4 a), the air output end (4 b) is connected with the vacuum pump (5), and the input needle valve (6) is used for being connected with an air bottle of air to be detected;
The optical coupling air chamber (3) is provided with an optical input collimator (1) of a laser, and the output end of the laser (7) is connected with the optical input collimator (1) of the laser, and the laser (7) is also connected with a laser temperature control module (8) and a laser current driving module (9);
the optical coupling air chamber (3) is also provided with a photoelectric detector (2), and the output end of the photoelectric detector (2) is connected with the embedded control module (12) through the signal processing module, and the embedded control module (12) is also connected with the laser current driving module (9);
The detection method comprises the following steps:
opening an input needle valve (6) to enable gas to be detected to be discharged into the optical coupling air chamber (3) through the air input end (4 a), and closing the input needle valve (6) after inputting a certain volume of gas to be detected;
-turning on the laser (7) by means of the embedded control module (12);
starting the vacuum pump (5) to reduce the voltage, starting the photoelectric detector (2) to convert the optical signal into a current signal, processing the current signal by using the signal processing module, and calculating the processed current information by using the embedded control module (12) to obtain a curve of a 2f signal;
In the step-down process, the central peak position of the curve of the 2f signal is obtained by calculation through the embedded control module (12) in real time, and the laser current driving module (9) is subjected to closed-loop adjustment through the embedded control module (12) so as to ensure that the central peak position of the curve of the 2f signal is unchanged in the step-down process;
And when the pressure of the optical coupling air chamber (3) is detected to be reduced to 0.1atm, carrying out inversion calculation on the curve of the 2f signal correspondingly acquired by the embedded control module (12) to obtain the gas concentration.
Specifically, the needle valve is opened, the flow of the entering gas is limited, the gas is introduced through 4 (a), the gas is discharged through 4 (b), 5 is a vacuum pump, and different rotation speeds are set to achieve the pressure of 1 atm-0.1 atm.
The low-pressure calibration method comprises the following steps:
(1) The input needle valve (6) is connected with a standard gas cylinder or a gas distribution instrument to output standard concentration gas, and the vacuum pump (5) is not opened;
(2) Directly exhausting gas through the gas input end 4 (a), after the gas concentration is stable (when the gas quantity is more than 10 times of the volume of the gas chamber and the gas is stable for 5 min), closing the input needle valve (6), and opening the vacuum valve (5);
(3) A vacuum valve (5) is arranged to control the pressure to be reduced from 1atm to the low-pressure to be measured, the concentration proportion of the low-pressure gas in the optical coupling air chamber (3) is considered to be unchanged, but the molecular number of the gas is reduced according to the pressure proportion, and the gas is used as the standard gas concentration under the pressure to carry out instrument calibration.
(4) In the depressurization process, starting the TDLAS gas sensing equipment, detecting the central peak position of the 2f signal in real time, and performing closed-loop adjustment through a laser current driving module (9) to ensure that the central peak position is unchanged, so that the locked wavelength is prevented from changing due to the fact that the gas absorption peak is inconsistent with the gas absorption peak under normal pressure after voltage transformation.
Note that, changing the gas concentration can not directly be connected to the standard gas or the gas distribution instrument to output the gas, but the gas is emptied, and after the nitrogen is introduced to flush, the step (1) is repeated, other concentration standard gases are introduced, and then the calibration of different gas concentrations is realized according to the steps (2), (3) and (4).
In the gas concentration detection method of the low-pressure detection system based on the TDLAS, after the gas to be detected is discharged into the optical coupling air chamber (3) through the input needle valve (6), the pressure is reduced by the vacuum pump (5), and after the pressure in the optical coupling air chamber (3) is reduced, the light intensity curve at the moment is collected and inverted to obtain the gas concentration. Meanwhile, in the step-down process, the working temperature of the laser (7) is kept unchanged, and the position of a gas absorption peak is kept unchanged in the step-down state by controlling the closed-loop adjustment of the laser current driving module. Therefore, under low pressure, overlapping interference between gas absorption peaks and cross interference of water vapor on the gas to be measured can be placed, and gas measurement accuracy is improved.
In one embodiment, the opening of the input needle valve (6) so that the gas to be measured is discharged into the optical coupling gas chamber (3) through the gas input end (4 a) further comprises:
Evacuating the residual gas in the optical coupling gas cell (3);
The input needle valve (6) is connected into a nitrogen bottle, and the optical coupling air chamber (3) is flushed by utilizing nitrogen.
In one embodiment, the signal processing module comprises a current conversion voltage module (10) and a phase-locked amplifying module (11);
the output end of the photoelectric detector (2) is connected with the embedded control module (12) through the current conversion voltage module (10) and the phase-locked amplifying module (11) in sequence.
In one embodiment, the processing of the current signal by the signal processing module comprises converting the current signal into a voltage signal by a current conversion voltage module (10), and performing 2f frequency phase-locked amplification on the voltage signal by a phase-locked amplification module (11).
In one embodiment, the method further comprises the embedded control module (12) ensuring that the operating temperature of the laser (7) is unchanged by controlling the laser temperature control module (8) during the depressurization.
In one embodiment, the pressure of the optical coupling air chamber (3) is detected by a pressure gauge provided on the vacuum pump (5).
In this embodiment, a pressure gauge may also be provided on the gas path to detect the pressure of the optically coupled gas cell.
In one embodiment, the laser (7) is a DFB laser.
The application provides a low-pressure gas calibration method, which reduces cross interference among gases, realizes overlapping interference of adjacent gas absorption peaks and suppression effect of water vapor gas cross interference, and improves gas measurement accuracy.
The technical features of the above embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.
The above examples illustrate only a few embodiments of the application, which are described in detail and are not to be construed as limiting the scope of the application. It should be noted that it will be apparent to those skilled in the art that several variations and modifications can be made without departing from the spirit of the application, which are all within the scope of the application. Accordingly, the scope of protection of the present application is to be determined by the appended claims.
Claims (7)
1. The gas concentration detection method of the low-voltage detection system based on the TDLAS is characterized in that the detection system comprises a laser (7), an optical input collimator (1) of the laser, an optical coupling air chamber (3), an input needle valve (6), a gas input end (4 a), a gas output end (4 b), a vacuum pump (5), a photoelectric detector (2), a laser temperature control module (8), a laser current driving module (9), a signal processing module and an embedded control module (12);
The optical coupling air chamber (3) is provided with the air input end (4 a) and the air output end (4 b), the input needle valve (6) is connected with the air input end (4 a), the air output end (4 b) is connected with the vacuum pump (5), and the input needle valve (6) is used for being connected with an air bottle of air to be detected;
The optical coupling air chamber (3) is provided with an optical input collimator (1) of a laser, and the output end of the laser (7) is connected with the optical input collimator (1) of the laser, and the laser (7) is also connected with a laser temperature control module (8) and a laser current driving module (9);
the optical coupling air chamber (3) is also provided with a photoelectric detector (2), and the output end of the photoelectric detector (2) is connected with the embedded control module (12) through the signal processing module, and the embedded control module (12) is also connected with the laser current driving module (9);
The detection method comprises the following steps:
Opening an input needle valve (6) to enable gas to be detected to be discharged into the optical coupling air chamber (3) through a gas input end (4 a), and closing the input needle valve (6) after inputting a certain volume of gas to be detected;
starting the vacuum pump (5) to reduce the voltage, starting the photoelectric detector (2) to convert the optical signal into a current signal, processing the current signal by using the signal processing module, and calculating the processed current information by using the embedded control module (12) to obtain a curve of a 2f signal;
In the step-down process, the central peak position of the curve of the 2f signal is obtained by calculation through the embedded control module (12) in real time, and the laser current driving module (9) is subjected to closed-loop adjustment through the embedded control module (12) so as to ensure that the central peak position of the curve of the 2f signal is unchanged in the step-down process;
And when the pressure of the optical coupling air chamber (3) is detected to be reduced to 0.1atm, carrying out inversion calculation on the curve of the 2f signal correspondingly acquired by the embedded control module (12) to obtain the gas concentration.
2. The method for detecting the gas concentration of the TDLAS based low pressure detection system according to claim 1, wherein the opening of the input needle valve (6) so that the gas to be detected is discharged into the optical coupling gas chamber (3) through the gas input end (4 a) further comprises:
Evacuating the residual gas in the optical coupling gas cell (3);
The input needle valve (6) is connected into a nitrogen bottle, and the optical coupling air chamber (3) is flushed by utilizing nitrogen.
3. The method for detecting the gas concentration of the low-voltage detection system based on the TDLAS according to claim 1, wherein the signal processing module comprises a current conversion voltage module (10) and a lock-in amplifying module (11);
the output end of the photoelectric detector (2) is connected with the embedded control module (12) through the current conversion voltage module (10) and the phase-locked amplifying module (11) in sequence.
4. The method for detecting the gas concentration of the TDLAS based low voltage detection system according to claim 2, wherein processing the current signal by the signal processing module includes converting the current signal into a voltage signal by a current conversion voltage module (10), and performing 2f frequency lock-in amplification on the voltage signal by a lock-in amplification module (11).
5. The method for detecting the gas concentration of the low-voltage detection system based on the TDLAS according to claim 1, further comprising the step that the embedded control module (12) ensures that the working temperature of the laser (7) is unchanged by controlling the laser temperature control module (8) in the depressurization process.
6. The method for detecting the gas concentration of the TDLAS based low pressure detection system according to claim 1, characterized in that the pressure of the optical coupling gas chamber (3) is detected by a pressure gauge provided on the vacuum pump (5).
7. The method for detecting the gas concentration of a TDLAS based low voltage detection system according to claim 1, characterized in that the laser (7) is a DFB laser.
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| CN121027042A (en) * | 2025-10-27 | 2025-11-28 | 吉林大学 | A high-precision gas detection adaptive light intensity noise suppression system and method |
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| CN121027042A (en) * | 2025-10-27 | 2025-11-28 | 吉林大学 | A high-precision gas detection adaptive light intensity noise suppression system and method |
| CN121027042B (en) * | 2025-10-27 | 2026-01-30 | 吉林大学 | A high-precision gas detection adaptive light intensity noise suppression system and method |
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