CN121164236A - A dual-path gas concentration detection method based on TDLAS - Google Patents

Abstract

The invention provides a double-optical-path gas detection method based on a Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology, which realizes multi-gas detection with high sensitivity and wide dynamic range by inverting gas concentration through double-wavelength laser output, double-optical-path design and a maximum variable data processing algorithm, and is particularly suitable for distinguishing and inverting the concentration of gases with similar absorption peaks (such as methane and carbon monoxide). The method has the core that the movement of the reflecting mirror is controlled by the stepping motor so as to realize the double-optical-path design of the air chamber, the similar gas of the absorption peak can be effectively distinguished by combining the output of the double-wavelength laser, the light intensity signals of the long optical path and the short optical path are synchronously processed by utilizing the least square method, and the gas concentration is inverted by the calculation of the light intensity ratio and the matrix operation, so that the detection precision is improved. The dual-optical-path gas detection method realizes dual-wavelength dual optical paths, can dynamically adjust the optical path length, and can be suitable for different scenes. The system has the advantages of high sensitivity, wide dynamic range, capability of distinguishing multiple gases, flexibility, low complexity and strong anti-interference capability.

Description

TDLAS-based double-optical-path gas concentration detection method

Field of the art

The invention relates to the technical field of gas detection, in particular to double-optical-path gas detection by utilizing a Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology, which can realize multi-gas detection with high sensitivity and wide dynamic range.

(II) technical background

The gas detection technology has important application in the fields of environmental monitoring, industrial process control, medical diagnosis, safety protection and the like. With the acceleration of the industrialization process and the improvement of environmental protection requirements, higher requirements are put on the sensitivity, selectivity and instantaneity of the gas detection technology. Conventional gas detection methods include electrochemical sensors, infrared absorption spectroscopy, gas chromatography, and the like, but these methods have certain limitations in practical applications. Such as electrochemical sensing, has low cost and easy integration, but has limited sensitivity, is easily influenced by ambient temperature and humidity, and has short service life. The infrared absorption spectrometry has higher selectivity and sensitivity, but the resolution of the traditional non-dispersive infrared (NDIR) technology is lower, and the gas with similar absorption peaks is difficult to distinguish. Although the gas chromatography can realize high-precision detection, the equipment is complex, the cost is high, the detection speed is low, and the requirement of real-time monitoring is difficult to meet.

In recent years, tunable Diode Laser Absorption Spectroscopy (TDLAS) technology has become one of the mainstream technologies in the field of gas detection due to its characteristics of high sensitivity, high selectivity, rapid response, and the like. TDLAS technology inverts gas concentration by measuring the intensity of the absorption spectrum based on the absorption characteristics of a gas molecule for a particular wavelength of laser light. The core principle is Beer-lambert law (Beer-LambertLaw), i.e. the light intensity decay is proportional to the gas concentration and the optical path length.

However, the conventional TDLAS system still faces the challenge in practical application that the sensitivity of low-concentration detection is insufficient, that is, the absorption signal is weak and is easy to be interfered by noise during detection of low-concentration gas, so that the detection accuracy is reduced. To increase sensitivity, it is often desirable to increase the optical path length. In the prior art, in order to improve the sensitivity, a multiple reflection cell (such as a Herriott cell or White cell) is generally used to increase the optical path length. The dynamic range is limited, and the detection requirements of low-concentration gas and high-concentration gas are difficult to meet by the traditional single-optical path system. High concentration gas may cause saturation of the signal, while low concentration gas signals may be swamped by noise. The difficulty of multi-gas detection is great, and the absorption spectrum lines of different gases can be overlapped (such as the absorption peaks of methane and carbon monoxide are similar), so that the contribution of each gas is difficult to distinguish when the concentration is inverted.

Various improvements have been proposed in the prior art. For example, multi-wavelength lasers are used to achieve multi-gas detection, or complex signal processing algorithms are used to improve detection accuracy. However, these schemes often have problems of complex system, high cost, poor flexibility and the like, and are difficult to meet the requirements of practical application. Particularly in the detection of gases with similar absorption peaks (such as methane and carbon monoxide), the conventional method has difficulty in effectively distinguishing the absorption signals of the gases, so that the detection accuracy is reduced.

Aiming at the problems, the invention realizes the following functions of 1. High sensitivity detection by combining a dual-wavelength laser output and dual-optical path design and a least square concentration inversion algorithm, wherein the high sensitivity detection is realized by remarkably enhancing the absorption signal of low concentration gas and improving the detection sensitivity by the long-optical path design. 2. And the detection in a wide dynamic range is flexibly suitable for gas detection in different concentration ranges through the design of adjustable distance of the short-path reflector, and signal saturation or noise interference is avoided. 3. And the multi-gas distinguishing capability is to combine the dual-wavelength laser output and the least square method to effectively distinguish the gases with similar absorption peaks (such as methane and carbon monoxide) so as to realize multi-gas detection. Meanwhile, the distance of the optical path reflector is adjustable, so that the optical path length can be dynamically adjusted according to actual requirements, and the optical path reflector is suitable for different application scenes.

(III) summary of the invention

The invention aims to provide a dual-optical-path gas detection method based on TDLAS, which realizes multi-gas detection with high sensitivity and wide dynamic range through dual-wavelength laser output, dual-optical-path design and a least square concentration inversion algorithm, and is particularly suitable for distinguishing and inverting the concentration of gas with similar absorption peaks. The method has the advantages of strong adaptability to system scenes, high flexibility and moderate cost while ensuring the detection performance.

The purpose of the invention is realized in the following way:

Firstly, designing a double-optical-path optical structure, constructing a set of TDLAS detection system comprising a variable optical path, emitting laser beams with specific wavelengths by a stable laser source, entering an optical-path variable air chamber after collimation and focusing of the laser beams, and firstly, detecting concentration and storing data under a short optical path. When receiving the short optical path detection signal, the step motor controls the reflector to move, and the detection optical path is accurately changed. Then, the gas concentration detection under the long optical path is performed and the detection data is stored.

Based on the TDLAS technical principle, the selected laser wavelength is accurately modulated. By controlling the driving current or temperature of the laser source, the emitted laser wavelength is rapidly scanned near the specific absorption spectrum line of the target gas. When laser passes through the long-short optical path air chamber, the gas molecules can absorb the laser with specific wavelength, and the detector receives the light signal absorbed by the gas and converts the light signal into an electric signal to be output.

And an advanced digital lock-in amplifier is adopted to process the long and short optical path detection electric signals which are sequentially output by the detector. The phase-locked amplifier can extract weak signals related to gas absorption from complex background noise through phase locking with the modulation signals, amplify and filter the weak signals, so that the signal-to-noise ratio of detection signals is remarkably improved, and the detection capability of the system on the weak signals is enhanced. After receiving the processed light intensity signal of the long and short optical path gas detection, comprehensively analyzing and processing signal data, inverting the gas concentration by using a least partial square method, and displaying and storing the concentration value.

The method for detecting the gas concentration of the double optical paths based on the TDLAS has the characteristics and effects that:

Compared with the traditional TDLAS gas detection, the TDLAS-based double-optical-path gas concentration detection method has the main difference that the gas detection sensitivity is reserved and the gas detection range is enlarged by the double-optical-path design, so that the method can be suitable for detecting gases with different concentrations. By combining the dual-wavelength laser output and the least square method, the gas (such as methane and carbon monoxide) with similar absorption peaks can be effectively distinguished. The optical path length can be dynamically adjusted according to actual requirements, and the method is suitable for different application scenes. The position movement of the reflecting mirror controlled by the stepping motor is utilized, the complexity and the cost are not obviously increased while the detection performance of the equipment is improved, and the practical application is facilitated.

(IV) description of the drawings

FIG. 1 is a flow chart of a method for detecting gas concentration by double optical paths

FIG. 2 is a schematic diagram of a dual optical path gas detection device

Reference numerals in the figure, (a) a tunable laser transmitter, (b) an optical reflection system, (c) a reflector moving slide rail, (d) a reflector, (e) a stepping motor, (f) a lock-in amplifier and a signal converter, and (g) a data monitoring center.

(Fifth) detailed description of the invention

1. The double-optical-path gas detection device is built, so that the installation positions of the laser light source emitter, the variable-optical-path gas chamber, the detector, the lock-in amplifier, the signal processing unit and other components are ensured to be accurate, strict debugging is carried out, and the optical alignment and the electrical connection between the components are ensured to be normal.

2. The double-optical path detection air chamber is designed, installed and debugged, as shown in (c) of the attached figure 2, the design of the two reflectors is controlled in the two groove buckles, the rollers at the upper ends of the reflectors are connected into the track and can move, and the rollers at the lower ends of the movable reflectors are connected with the stepping motor to control the movement of the reflectors.

3. According to the type of the gas to be detected and the absorption characteristic of the gas in the infrared band, a spectrum database or related literature data is utilized to accurately select the proper laser wavelength. Meanwhile, according to the selected laser wavelength and detection requirements, the modulation parameters of the laser light source, including modulation frequency, modulation amplitude and the like, are reasonably set so as to ensure that the laser wavelength can effectively scan near the absorption spectrum line of the target gas.

4. Starting the equipment to detect the gas concentration, sucking the gas to be detected into the gas chamber, emitting light wave with wavelength lambda ₁ by the laser, and recording the initial light intensity valueMeanwhile, the short optical path length L _short is determined, the light wave passes through the detection air chamber and then reaches the detector, and the detector receives and processes the light intensity to obtain the light intensityThe stepper motor controls the reflector to move, the air chamber is changed into a long optical path with the length of L _long, the light wave passes through the detection air chamber and then reaches the detector, and the detector receives and processes the light intensity to obtain the light intensity

5. The laser emits light wave with wavelength lambda ₂, and the initial light intensity value is recordedThe detector receives and processes the signal to obtain the light intensity through the short-path air chamberThe stepper motor controls the reflector to move, the air chamber is changed into a long optical path with the length of L _long, the light wave passes through the detection air chamber and then reaches the detector, and the detector receives and processes the light intensity to obtain the light intensity

6. And processing the acquired electric signals by using signal processing software in a computer. The signal is amplified, filtered and phase locked by a digital lock-in amplifier, an effective signal related to gas absorption is extracted, the light intensity ratio of a long-short optical path is calculated, and the gas concentration is inverted according to a least square method. Taking two mixed gas concentrations as examples:

calculating the light intensity ratio of the long optical path to the short optical path:

Inversion concentration C ₁/C₂:

the mixed gas concentration C ₁、C₂ is solved.

7. And displaying the calculated gas concentration data in real time through a display screen, and simultaneously storing the data into a computer hard disk for subsequent data analysis and processing. In addition, the data can be transmitted to a remote monitoring center through a network communication interface, so that remote real-time monitoring is realized.

Claims (6)

1. A dual-optical-path gas detection method based on tunable diode laser absorption spectroscopy (TDLAS) technology, characterized by the following steps: emitting two specific wavelengths of laser light using a tunable diode laser, corresponding to the absorption spectral lines of the target gas respectively; controlling the movement of a reflector via a stepper motor to achieve a dual-optical-path design of the gas chamber; combining the dual-wavelength laser output to obtain the light intensity changes of the corresponding wavelengths at different optical paths; filtering and denoising the light intensity signals of different wavelengths at different optical paths received by a lock-in amplifier; using least squares matrix operations to invert the concentration of the mixed gas; outputting the gas concentration value based on the inversion result; and displaying or storing it in real time. 2. The dual-optical-path gas detection method based on tunable diode laser absorption spectroscopy (TDLAS) technology according to claim 1, characterized in that: a specific wavelength of laser is tuned to address the different absorption peaks of different gases for light waves. When it is necessary to distinguish the concentration of mixed gases with similar absorption peaks, two specific wavelengths of laser light are tuned to correspond to the absorption lines of the main target gas, respectively. 3. The dual-optical-path gas detection method based on tunable diode laser absorption spectroscopy (TDLAS) technology according to claim 1, characterized in that: unlike other motors used for control purposes, the stepper motor can accept digital control signals and convert them into corresponding linear displacements, enabling precise control of the reflector's movement distance, thereby accurately adjusting the optical path length. Inputting a control signal yields a reflector position change. Compared to traditional DC control systems, this control system significantly reduces costs and almost eliminates the need for system adjustments. 4. The dual-optical-path gas detection method based on tunable diode laser absorption spectroscopy (TDLAS) technology according to claim 1, characterized in that: to address the measurement of mixed gas concentrations, especially in cases where single-optical-path absorption peaks easily overlap, the method utilizes dual-wavelength lasers to detect the corresponding light intensity change signals through long and short optical-path gas cells, with the light intensity attenuation following Lambert-Beer's law. After passing through the long and short optical-path gas cells, a lock-in amplifier is used to filter and reduce noise in the long and short optical-path light intensity change signals. 5. A dual-optical-path gas detection method based on tunable diode laser absorption spectroscopy (TDLAS) technology according to claim 1, characterized in that: the ratio of light intensity of the long and short optical paths is calculated using the collected wavelength value, optical path length, incident light intensity, and corresponding absorption coefficient. The gas concentration is then inverted using least-squares matrix operations. 6. The dual-optical-path gas detection method based on tunable diode laser absorption spectroscopy (TDLAS) technology according to claim 1, characterized in that: the dual-optical-path detection method is highly flexible, maintaining measurement accuracy while adapting to gas detection under different environments, and is particularly suitable for distinguishing and retrieving the concentration of gases with similar absorption peaks.