Gas concentration detection method of low-pressure detection system based on TDLAS (Tunable Diode Laser Absorption Spectroscopy)

Abstract

The invention discloses a TDLAS (tunable diode laser absorption spectroscopy)-based gas concentration detection method of a low-pressure detection system, which comprises the following steps: discharging to-be-detected gas into an optical coupling gas chamber through an input needle valve, reducing the pressure by using a vacuum pump, after the pressure in the optical coupling gas chamber is reduced, acquiring a light intensity curve at the moment, and performing inversion to obtain the gas concentration. Meanwhile, in the step-down process, it is guaranteed that the working temperature of the laser is not changed, and the position of a gas absorption peak in the step-down state is not changed by controlling closed-loop adjustment of a laser current driving module. Therefore, under low pressure, overlapping interference between gas absorption peaks and cross interference of water vapor to the gas to be measured can be prevented, so that the gas measurement accuracy is improved.

Description

Technical Field

[0001] The present application relates to the technical field of gas concentration detection, and in particular to a gas concentration detection method. Background Art

[0002] Tunable Laser Diode Absorption Spectroscopy (TDLAS) is a gas detection technology based on laser spectroscopy. It uses narrow-bandwidth lasers emitted by tunable semiconductor lasers to scan specific absorption lines of specific gas molecules, thereby achieving highly sensitive and selective detection of gas concentrations. This technology precisely controls the wavelength of the laser to match the wavelength of the gas absorption line, measures the degree of absorption of laser energy by gas molecules, and then calculates the gas concentration based on the Beer-Lambert law. TDLAS technology is widely used in environmental monitoring, industrial process control, safety testing, medical diagnosis and other fields due to its high resolution, fast response and ability to monitor multiple gas components in real time.

[0003] When actually using TDLAS technology to measure gas, especially in the mid-infrared band, overlapping interference between gas absorption peaks and cross-interference of water vapor on the measured gas are prone to occur, affecting the accuracy of gas measurement. Therefore, around gas cross-interference, a gas concentration correction method is generally used, that is, the concentration of the interfering gas (such as water vapor) is measured, and then the concentration of the measured gas is corrected, or the table lookup method is used for correction. When correcting, the interference needs to be fitted linearly, nonlinearly, or polynomially. However, when the gas produces cross-interference, it is difficult to make a matrix correction as the temperature and pressure change. At the same time, the measured gas concentration and the fitted gas concentration must be within a specific range, otherwise the fitted polynomial will have a large deviation. Therefore, how to propose a high-precision gas measurement method that reduces and suppresses gas cross-interference has become an urgent problem to be solved. Summary of the invention

[0004] Based on this, in response to the above technical problems, a gas concentration detection method 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 on the gas to be measured in the prior art, thereby affecting the accuracy of gas measurement.

[0005] In a first aspect, a gas concentration detection method of a low-pressure detection system based on TDLAS is provided, wherein the detection system comprises: a laser (7), an optical input collimator (1) of the laser, an optical coupling gas chamber (3), an input needle valve (6), a gas input end (4a), a gas output end (4b), a vacuum pump (5), a photodetector (2), a laser temperature control module (8), a laser current driving module (9), a signal processing module, and an embedded control module (12);

[0006] The optical coupling gas chamber (3) is provided with the gas input end (4a) and the gas output end (4b); the input needle valve (6) is connected to the gas input end (4a), and the gas output end (4b) is connected to the vacuum pump (5); the input needle valve (6) is used to be connected to a gas cylinder of the gas to be measured;

[0007] The optical coupling gas chamber (3) is provided with a laser optical input collimator (1), and the output end of the laser (7) is connected to the laser optical input collimator (1); the laser (7) is also connected to a laser temperature control module (8) and a laser current driving module (9);

[0008] A photodetector (2) is also provided in the optical coupling gas chamber (3); an output end of the photodetector (2) is connected to the embedded control module (12) via the signal processing module; the embedded control module (12) is also connected to the laser current driving module (9);

[0009] The detection method comprises:

[0010] Opening the input needle valve (6) to allow the gas to be measured to be discharged into the optical coupling gas chamber (3) through the gas input end (4a); after a certain volume of the gas to be measured is input, closing the input needle valve (6);

[0011] Turning on the laser (7) via the embedded control module (12);

[0012] The vacuum pump (5) is turned on to reduce the pressure, the photodetector (2) is turned on to convert the optical signal into a current signal, the current signal is processed by the signal processing module, and the embedded control module (12) calculates the processed current information to obtain a curve of the 2f signal;

[0013] During the voltage reduction process, the central peak position of the curve of the 2f signal calculated by the embedded control module (12) is monitored in real time, and the laser current driving module (9) is closed-loop regulated by the embedded control module (12) to ensure that the central peak position of the curve of the 2f signal remains unchanged during the voltage reduction process;

[0014] When the pressure of the optical coupling gas chamber (3) is detected to be reduced to 0.1 atm, the gas concentration is obtained by inverting the curve of the 2f signal corresponding to the acquired signal of the embedded control module (12).

[0015] In the above solution, optionally, before the opening of the input needle valve (6) to allow the gas to be measured to be discharged into the optical coupling gas chamber (3) through the gas input end (4a) further comprises:

[0016] exhausting the gas remaining in the optical coupling gas chamber (3);

[0017] The input needle valve (6) is connected to a nitrogen bottle, and the optical coupling gas chamber (3) is flushed with nitrogen.

[0018] In the above solution, optionally, the signal processing module includes: a current-to-voltage conversion module (10) and a phase-locked amplification module (11);

[0019] The output end of the photoelectric detector (2) is connected to the embedded control module (12) via the current-to-voltage conversion module (10) and the phase-locked amplifier module (11) in sequence.

[0020] In the above scheme, further optionally, using the signal processing module to process the current signal includes: using a current-to-voltage conversion module (10) to convert the current signal into a voltage signal, and then using a phase-locked amplification module (11) to perform 2f frequency phase-locked amplification on the voltage signal.

[0021] In the above solution, optionally, the method further comprises: during the voltage reduction process, the embedded control module (12) controls the laser temperature control module (8) to ensure that the operating temperature of the laser (7) remains unchanged.

[0022] In the above solution, optionally, the pressure of the optical coupling chamber (3) is detected by a pressure gauge arranged on the vacuum pump (5).

[0023] In the above solution, optionally, the laser (7) is a DFB laser.

[0024] This application has at least the following beneficial effects:

[0025] The present application discharges the gas to be measured into the optical coupling chamber (3) through the input needle valve (6), reduces the pressure using a vacuum pump (5), waits for the pressure in the optical coupling chamber (3) to decrease, collects the light intensity curve at this time and performs inversion to obtain the gas concentration. At the same time, during the pressure reduction process, the operating temperature of the laser (7) is ensured to remain unchanged, and the position of the gas absorption peak remains unchanged under the pressure reduction state by controlling the closed-loop regulation of the laser current driving module. Therefore, under low pressure, the overlapping interference between gas absorption peaks and the cross interference of water vapor on the gas to be measured can be prevented, thereby improving the accuracy of gas measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is the distribution diagram of 1% Vol water vapor absorbance under 1atm pressure;

[0027] Figure 2 This is the distribution diagram of 1% Vol water vapor absorbance at 0.1 atm pressure;

[0028] Figure 3 It is the absorbance of 0.1% Vol COS at 1 atm pressure;

[0029] Figure 4 It is the absorbance of 0.1% Vol COS at 0.1 atm pressure;

[0030] Figure 5 It is the absorbance of 0.5% Vol water vapor and 10 ppb ethane at 1 atm pressure;

[0031] Figure 6 It is the absorbance of 0.5% Vol water vapor and 10 ppb ethane at 0.1 atm pressure;

[0032] Figure 7 A schematic diagram of the structure of a low voltage detection system based on TDLAS provided in one embodiment of the present application;

[0033] Figure 8 A flow chart of a gas concentration detection method of a low-pressure detection system based on TDLAS provided in one embodiment of the present application. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. 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.

[0035] Gas absorption line types are divided into natural broadening and Gaussian broadening. The natural broadening line function expressed in wavelength is the Lorentz line type, and its formula is shown in equation (1).

[0036]

[0037] where g N represents the Lorentz absorption line function, γ is the half-height width of the absorption spectrum of the gas to be measured, λ represents the wavelength, and λ0 represents the central wavelength of gas absorption;

[0038] When the gas pressure decreases, the absorption spectrum changes from the Lorentz line to a sharp Voigt line function, as shown in formula (2):

[0039]

[0040] where g v represents the Voigt linear function, α L is the half-height width of the line type, λ is the wavelength, λ0 is the central wavelength of gas absorption, m is the mass of the absorbing gas molecules, T is the gas temperature, k is the Boltzmann constant, and c is the speed of light in vacuum;

[0041] By reducing the pressure, the width of the gas absorption spectrum is reduced. The low-pressure state can be described and fitted by formula (2), avoiding the cross-interference caused by the overlap of the two gas absorption peaks due to spectrum broadening, reducing the cross-interference effect, and further improving the gas measurement capability.

[0042] When the optical path length is 0.1m and the temperature is 25℃ (all simulations in this patent application are carried out under this condition), the gas is at 1atm and 0.1atm pressure, and the beam is in the range of 2015~2020cm-1. The absorbance of 1%Vol water vapor at 1atm is as follows Figure 1 As shown, the absorbance of 1% Vol water vapor at 0.1 atm is Figure 2 As shown in the figure, the number of gas molecules decreases due to the decrease in gas pressure, but because the pressure is low, the absorption waveform conforms to the Voigt line shape, so the absorption waveform becomes sharper, reducing the broadening of the water vapor absorption spectrum.

[0043] For the case where the gas absorption spectrum is dense, it is easy to be affected by the cross interference of the surrounding gas during scanning, affecting the measurement accuracy. Taking COS carbonyl sulfide gas as an example, at 1atm and 0.1atm pressure, the beam is in the range of 2061~2063cm-1, and the absorbance of 0.1%Vol gas at 1atm pressure is as follows Figure 3 As shown in the figure, it can be seen that the absorption of adjacent gases is broadened, resulting in a very high "bottom" in the overlapping part. The "zero background" without gas absorption cannot be scanned by TDLAS technology, so accurate measurement cannot be performed. The absorbance of 0.1% Vol gas at 0.1atm pressure is as follows: Figure 4 As shown, by low-pressure measurement, the absorption peak between adjacent wavelengths that can measure "zero background" is found, so the 2061.75 or 2062.25cm-1 absorption spectrum is selected as the gas measurement wavelength to reduce the cross-interference effect of adjacent absorption peaks.

[0044] For mid-infrared gas absorption spectroscopy, when high-precision gas measurement (ppb level) is required, most of the gases to be measured are subject to cross-interference from water vapor. Taking ethane as an example, although the water vapor absorption near ethane is very weak, the water vapor content in the air is basically 0.2% to 1% Vol or even higher, and the resulting water vapor absorption will affect the measurement of trace amounts of ethane. Taking ethane with an absorption wavelength of 3.37um as an example, at a pressure of 1atm, when the water vapor content is 0.5% Vol, the resulting absorption peak broadening partially affects the absorption of ethane, from Figure 5 It can be seen that due to the presence of 0.5% Vol water vapor, the absorption spectrum will overlap the absorption spectrum of 10ppb ethane and 0.5% Vol water vapor, resulting in obvious cross-interference. In view of this situation, a 0.1atm pressure environment is set for gas measurement, such as Figure 6As shown, it is the absorbance of 0.5% Vol water vapor and 10 ppb ethane at 0.1 atm. It can be seen that the absorption spectra of ethane and water vapor do not overlap, so the ethane content can be accurately determined with high precision.

[0045] In one embodiment, Figure 7 and Figure 8 As shown, a gas concentration detection method of a low-pressure detection system based on TDLAS is provided, wherein the detection system comprises: a laser (7), an optical input collimator (1) of the laser, an optical coupling gas chamber (3), an input needle valve (6), a gas input end (4a), a gas output end (4b), a vacuum pump (5), a photodetector (2), a laser temperature control module (8), a laser current driving module (9), a signal processing module, and an embedded control module (12);

[0046] The optical coupling gas chamber (3) is provided with the gas input end (4a) and the gas output end (4b); the input needle valve (6) is connected to the gas input end (4a), and the gas output end (4b) is connected to the vacuum pump (5); the input needle valve (6) is used to be connected to a gas cylinder of the gas to be measured;

[0047] The optical coupling gas chamber (3) is provided with a laser optical input collimator (1), and the output end of the laser (7) is connected to the laser optical input collimator (1); the laser (7) is also connected to a laser temperature control module (8) and a laser current driving module (9);

[0048] A photodetector (2) is also provided in the optical coupling gas chamber (3); an output end of the photodetector (2) is connected to the embedded control module (12) via the signal processing module; the embedded control module (12) is also connected to the laser current driving module (9);

[0049] The detection method comprises:

[0050] Opening the input needle valve (6) to allow the gas to be measured to be discharged into the optical coupling gas chamber (3) through the gas input end (4a); after a certain volume of the gas to be measured is input, closing the input needle valve (6);

[0051] Turning on the laser (7) via the embedded control module (12);

[0052] The vacuum pump (5) is turned on to reduce the pressure, the photodetector (2) is turned on to convert the optical signal into a current signal, the current signal is processed by the signal processing module, and the embedded control module (12) calculates the processed current information to obtain a curve of the 2f signal;

[0053] During the voltage reduction process, the central peak position of the curve of the 2f signal calculated by the embedded control module (12) is monitored in real time, and the laser current driving module (9) is closed-loop regulated by the embedded control module (12) to ensure that the central peak position of the curve of the 2f signal remains unchanged during the voltage reduction process;

[0054] When the pressure of the optical coupling gas chamber (3) is detected to be reduced to 0.1 atm, the gas concentration is obtained by inverting the curve of the 2f signal corresponding to the acquired signal of the embedded control module (12).

[0055] Specifically, by opening the needle valve and limiting the incoming gas flow, the gas is taken in through 4 (a) and out through 4 (b). 5 is a vacuum pump with different speeds set to achieve a pressure ranging from 1 atm to 0.1 atm.

[0056] The low pressure calibration includes the following steps:

[0057] (1) Connect the input needle valve (6) to the standard gas bottle or the gas distributor outputting the standard concentration gas, and do not turn on the vacuum pump (5);

[0058] (2) Directly discharge gas through the gas input terminal 4 (a), wait for the gas concentration to stabilize (when the gas volume is more than 10 times the volume of the gas chamber, stabilize for 5 minutes), close the input needle valve (6), and open the vacuum valve (5);

[0059] (3) A vacuum valve (5) is set to control the pressure to drop from 1 atm to the low pressure to be measured. It is assumed that the concentration ratio of the low-pressure gas in the optical coupling chamber (3) remains unchanged, but the number of gas molecules decreases in proportion to the pressure, and the instrument is calibrated as the standard gas concentration at this pressure.

[0060] (4) During the depressurization process, the TDLAS gas sensor device is turned on to detect the center peak position of the 2f signal in real time. The laser current driving module (9) performs closed-loop regulation to ensure that the center peak position remains unchanged, thereby avoiding changes in the locked wavelength caused by the gas absorption peak after the pressure change being inconsistent with that under normal pressure.

[0061] It should be noted that to change the gas concentration, the standard gas or the gas output by the gas distributor cannot be directly introduced. Instead, the gas should be emptied and flushed with nitrogen. Then, step (1) should be repeated to introduce standard gas of other concentrations. Then, steps (2), (3), and (4) should be followed to achieve calibration of different gas concentrations.

[0062] In the above-mentioned gas concentration detection method of the low-pressure detection system based on TDLAS, after the gas to be measured is discharged into the optical coupling gas chamber (3) through the input needle valve (6), the pressure is reduced by using a vacuum pump (5), and after the pressure in the optical coupling gas chamber (3) becomes smaller, the light intensity curve at this time is collected and inverted to obtain the gas concentration. At the same time, during the pressure reduction process, the operating temperature of the laser (7) is ensured to remain unchanged, and the position of the gas absorption peak remains unchanged under the pressure reduction state by controlling the closed-loop regulation of the laser current driving module. Therefore, under low pressure, the overlapping interference between gas absorption peaks and the cross interference of water vapor on the gas to be measured can be prevented, thereby improving the accuracy of gas measurement.

[0063] In one embodiment, before the step of opening the input needle valve (6) to allow the gas to be measured to be discharged into the optical coupling gas chamber (3) through the gas input end (4a), the step further comprises:

[0064] exhausting the gas remaining in the optical coupling gas chamber (3);

[0065] The input needle valve (6) is connected to a nitrogen bottle, and the optical coupling gas chamber (3) is flushed with nitrogen.

[0066] In one embodiment, the signal processing module comprises: a current-to-voltage conversion module (10) and a phase-locked amplification module (11);

[0067] The output end of the photoelectric detector (2) is connected to the embedded control module (12) via the current-to-voltage conversion module (10) and the phase-locked amplifier module (11) in sequence.

[0068] In one embodiment, using the signal processing module to process the current signal includes: using a current-to-voltage conversion module (10) to convert the current signal into a voltage signal, and then using a phase-locked amplification module (11) to perform 2f frequency phase-locked amplification on the voltage signal.

[0069] In one embodiment, the method further comprises: during the voltage reduction process, the embedded control module (12) controls the laser temperature control module (8) to ensure that the operating temperature of the laser (7) remains unchanged.

[0070] In one embodiment, the pressure of the optical coupling chamber (3) is detected by a pressure gauge arranged on the vacuum pump (5).

[0071] In this embodiment, a pressure gauge may be provided on the gas circuit to detect the pressure of the optical coupling gas chamber.

[0072] In one embodiment, the laser (7) is a DFB laser.

[0073] The present application proposes a low-pressure gas calibration method, which reduces cross-interference between gases, achieves the suppression effect of overlapping interference of adjacent gas absorption peaks and cross-interference of water vapor gas, and improves the accuracy of gas measurement.

[0074] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0075] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A gas concentration detection method based on a low-pressure detection system of TDLAS, characterized in that: The detection system comprises: a laser (7), an optical input collimator (1) of the laser, an optical coupling gas chamber (3), an input needle valve (6), a gas input end (4a), a gas output end (4b), a vacuum pump (5), a photodetector (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 gas chamber (3) is provided with the gas input end (4a) and the gas output end (4b); the input needle valve (6) is connected to the gas input end (4a), and the gas output end (4b) is connected to the vacuum pump (5); the input needle valve (6) is used to be connected to a gas cylinder of the gas to be measured; The optical coupling gas chamber (3) is provided with a laser optical input collimator (1), and the output end of the laser (7) is connected to the laser optical input collimator (1); the laser (7) is also connected to a laser temperature control module (8) and a laser current driving module (9); A photodetector (2) is also provided in the optical coupling gas chamber (3); an output end of the photodetector (2) is connected to the embedded control module (12) via the signal processing module; the embedded control module (12) is also connected to the laser current driving module (9); The detection method comprises: Opening the input needle valve (6) to allow the gas to be measured to be discharged into the optical coupling gas chamber (3) through the gas input end (4a); after a certain volume of the gas to be measured is input, closing the input needle valve (6); and turning on the laser (7) through the embedded control module (12); The vacuum pump (5) is turned on to reduce the pressure, the photodetector (2) is turned on to convert the optical signal into a current signal, the current signal is processed by the signal processing module, and the embedded control module (12) calculates the processed current information to obtain a curve of the 2f signal; During the voltage reduction process, the central peak position of the curve of the 2f signal calculated by the embedded control module (12) is monitored in real time, and the laser current driving module (9) is closed-loop regulated by the embedded control module (12) to ensure that the central peak position of the curve of the 2f signal remains unchanged during the voltage reduction process; When the pressure of the optical coupling gas chamber (3) is detected to be reduced to 0.1 atm, the gas concentration is obtained by inverting the curve of the 2f signal corresponding to the acquired signal of the embedded control module (12).

2. The gas concentration detection method based on the low-pressure detection system of TDLAS according to claim 1 is characterized in that: The step of opening the input needle valve (6) to allow the gas to be measured to be discharged into the optical coupling gas chamber (3) through the gas input end (4a) further comprises: exhausting the gas remaining in the optical coupling gas chamber (3); The input needle valve (6) is connected to a nitrogen bottle, and the optical coupling gas chamber (3) is flushed with nitrogen.

3. The gas concentration detection method of the low-pressure detection system based on TDLAS according to claim 1, characterized in that: The signal processing module comprises: a current-to-voltage conversion module (10) and a phase-locked amplification module (11); The output end of the photoelectric detector (2) is connected to the embedded control module (12) via the current-to-voltage conversion module (10) and the phase-locked amplifier module (11) in sequence.

4. The gas concentration detection method of the low-pressure detection system based on TDLAS according to claim 2 is characterized in that: Using the signal processing module to process the current signal comprises: using a current-to-voltage conversion module (10) to convert the current signal into a voltage signal, and then using a phase-locked amplification module (11) to perform 2f frequency phase-locked amplification on the voltage signal.

5. The gas concentration detection method of the low-pressure detection system based on TDLAS according to claim 1, characterized in that: The method further comprises: during the voltage reduction process, the embedded control module (12) controls the laser temperature control module (8) to ensure that the operating temperature of the laser (7) remains unchanged.

6. The gas concentration detection method of the low-pressure detection system based on TDLAS according to claim 1, characterized in that: The pressure of the optical coupling gas chamber (3) is detected by a pressure gauge arranged on the vacuum pump (5).

7. The gas concentration detection method of the low-pressure detection system based on TDLAS according to claim 1, characterized in that: The laser (7) is a DFB laser.

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