Ultraviolet gas analyzer and analysis method based on multistage temperature control

Abstract

The invention discloses an ultraviolet gas analyzer based on multistage temperature control. The ultraviolet gas analyzer comprises a case, a gas pump, a gas capacitor, a washer, a flow meter, an ultraviolet differential gas analysis module, an ultraviolet module signal processing board, a main control circuit board, an Internet of Things card, a power supply module and a temperature control system, the gas pump is connected with the gas source and the gas capacitor, gas to be detected is introduced into the ultraviolet gas analyzer and extracted into the gas capacitor, and the gas capacitor is connected with the washer; the flowmeter is mounted in front of the ultraviolet differential gas analysis module; the ultraviolet differential gas analysis module is used for emitting and receiving ultraviolet light and measuring the absorption characteristic in a gas sample; the temperature control system is used for carrying out temperature adjustment treatment on the air chamber module, a two-stage PID control mode is adopted when the temperature adjustment treatment is carried out on the air chamber module, the first-stage PID is used for primarily adjusting the temperature, and the temperature is made to be close to the set temperature; according to the high-precision temperature control system, high-precision temperature control is achieved, the temperature fluctuation is smaller than + / -0.1 DEG C, and the gas concentration testing precision is improved.

Description

Technical Field

[0001] The invention relates to the technical field of ultraviolet gas analysis, and in particular to an ultraviolet gas analyzer and an analysis method based on multi-stage temperature control. Background Art

[0002] Due to pollution in production and life, benzene series such as toluene, meta-xylene, o-xylene, and p-xylene can be widely detected in human living and survival environments. They are highly harmful to the human blood, nerves, and reproductive systems. Nitrogen dioxide, nitric oxide, vinyl chloride, isobutylene, etc. are also major air pollutants, which are highly irritating and corrosive to the human body. At present, the above pollutants have been included in the routine monitoring of the atmospheric environment, and indoor and outdoor air quality standards have been stipulated.

[0003] The UV gas analyzer is an instrument used to measure and analyze gas composition. It is based on the absorption characteristics of ultraviolet light and determines the concentration of the gas by measuring the degree of absorption of ultraviolet light of a specific wavelength by the gas in the sample.

[0004] The working principle of UV gas analyzer is to measure the concentration of gas by using the energy difference generated when UV light is absorbed by gas. When UV light passes through the gas, the gas molecules absorb UV light of a specific wavelength and cause the intensity of the light to decrease. The measuring instrument compares the difference between the incident light and the transmitted light to calculate the gas concentration. At present, UV gas analyzer is widely used in environmental monitoring, industrial process control and safety testing. It can detect a variety of gases, such as sulfur dioxide, carbon monoxide, nitrogen oxides, etc. At the same time, UV gas analyzer has the advantages of fast response, high precision and no need to use chemical reagents, making it a commonly used tool in the field of gas analysis.

[0005] However, in order to achieve ultra-low concentration gas detection, a long optical path design is adopted to achieve the design requirement of 20m optical path in a smaller chamber. The optical path is more sensitive to mechanical deformation caused by temperature, and conventional PID control cannot meet the temperature control requirements. Summary of the invention

[0006] In order to solve the above problems in the prior art, the present invention proposes an ultraviolet gas analyzer and an analysis method based on multi-stage temperature control.

[0007] The complete technical solution of the present invention includes:

[0008] A UV gas analyzer based on multi-stage temperature control, the UV gas analyzer comprising a chassis, an air pump, a gas container, a scrubber, a flow meter, a UV differential gas analysis module, a UV module signal processing board, a main control circuit board, an Internet of Things card, a power module and a temperature control system;

[0009] The chassis includes a chassis sheet metal structure, a front panel, a rear panel, an air chamber module and a hardware installation structure;

[0010] The gas pump is connected to the gas source and the gas container, and the gas to be tested is introduced into the ultraviolet gas analyzer and extracted into the gas container, and the gas container is connected to the scrubber;

[0011] The flow meter is installed before the ultraviolet differential gas analysis module;

[0012] The UV differential gas analysis module is used to transmit and receive UV light and measure the absorption characteristics in the gas sample;

[0013] The temperature control system performs temperature control on the air chamber module, and adopts a two-stage PID control mode when performing temperature control on the air chamber module, wherein the first-stage PID is used for primary temperature adjustment to make the temperature close to the set temperature; and the second-stage PID is used for fine adjustment of small temperature fluctuations.

[0014] Furthermore, the temperature control system adopts adaptive temperature control parameter adjustment technology to achieve high-precision temperature control.

[0015] Furthermore, the temperature control system makes the temperature fluctuation of the air chamber module less than ±0.1°C.

[0016] Furthermore, the ultraviolet differential gas analysis module includes a light source, a gas absorption cell, a spectrometer, a temperature sensor, a pressure sensor and an embedded system, and the embedded system is located on a data acquisition and signal processing board.

[0017] Furthermore, the temperature control system performs constant temperature treatment on the spectrometer, light source and gas absorption cell, including a first temperature control device and a second temperature control device. The first temperature control device performs constant temperature heating at the bottom of the spectrometer, and the second temperature control device heats the light source and the gas absorption cell to maintain a constant temperature.

[0018] Furthermore, the spectrometer is kept at a constant temperature of 45°C, and the light source and the gas absorption cell are kept at a constant temperature of 45°C.

[0019] Furthermore, the temperature control system uses a polyimide electric heating film to perform overall temperature control and insulation on the air chamber module.

[0020] Furthermore, the temperature control system also includes a gas preheating unit, which preheats the gas by means of a pipe winding before the gas enters the gas chamber, thereby ensuring that the temperature entering the gas chamber is substantially stable.

[0021] Furthermore, the temperature sensor is used to monitor the temperature of the gas in the gas absorption cell.

[0022] Furthermore, the method for ultraviolet gas analysis using the ultraviolet gas analyzer comprises the following steps:

[0023] (1) Turn on the gas source and air pump to introduce the gas to be tested into the gas container;

[0024] (2) Clean the gas through a scrubber to remove impurities in the gas to be tested to ensure that the gas is pure and pollution-free;

[0025] (3) Control and monitor the flow rate of the gas through a flow meter to ensure that the sample flow rate and flow rate are stable;

[0026] (4) selecting a suitable wavelength in the ultraviolet differential gas analysis module and turning on the light source; introducing and inputting the gas sample into the ultraviolet differential gas analysis module and allowing it to interact with the ultraviolet light;

[0027] (5) The UV differential gas analysis module measures the intensity of the transmitted light and calculates the concentration of the gas;

[0028] (6) Process and record the data to obtain the results of gas composition.

[0029] The present invention adopts a long optical path design to achieve ultra-low concentration gas detection and realize the design requirement of 20m optical path in a smaller chamber, because the optical path is more sensitive to mechanical deformation caused by temperature. In order to reduce the impact of ambient temperature, the gas chamber is temperature-controlled and the temperature is controlled at 45°C, which improves the accuracy of gas concentration testing. In terms of control method, two-stage PID control is adopted. The first-stage PID is used to preliminarily adjust the temperature so that the temperature reaches the set temperature as soon as possible; the second-stage PID is used to fine-tune the temperature fluctuation; at the same time, combined with technologies such as adaptive temperature control parameter adjustment, high-precision temperature control is achieved, and the temperature fluctuation is less than ±0.1°C. The gas is preheated by winding the pipeline to ensure that the temperature entering the gas chamber is basically constant, reducing the impact of low temperature on the gas chamber. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a structural diagram of the ultraviolet gas analyzer of the present invention.

[0031] Figure 2 It is a diagram of system temperature fluctuation under the temperature control system of the present invention.

[0032] Figure 3 It is the structural diagram of the double-head pump of the present invention.

[0033] Figure 4 This is a structural diagram of the elastic gas container of the present invention.

[0034] Figure 5 It is a schematic diagram of the ultraviolet gas analysis module of the present invention.

[0035] Figure 6 This is the schematic diagram of the multiple reflection long optical path gas cell.

[0036] Figure 7 This is a diagram of the combination of a beam splitter and a coupling reflector.

[0037] Figure 8 It is the target standard spectrum and the automatically selected characteristic peaks.

[0038] Fig. 9 is the measured spectrum and the characteristic peaks that match the target spectrum.

[0039] Fig.10 This is the wavelength alignment diagram of the measured spectrum and the target spectrum after absolute calibration.

[0040] Fig.11 This is the matching coefficient diagram of the absorption curve of each concentration before and after calibration and the standard absorption curve.

[0041] Fig.12 This is the SO2 ultraviolet differential signal and its selected characteristic absorption band diagram. DETAILED DESCRIPTION

[0046] The present invention is described in detail below in conjunction with the embodiments and drawings, but it should be understood that the embodiments and drawings are only used to exemplify the present invention and do not constitute any limitation on the protection scope of the present invention. All reasonable changes and combinations within the scope of the inventive concept of the present invention fall within the protection scope of the present invention.

[0047] Figure 1 This is a structural diagram of the ultraviolet gas analysis of the present invention. As shown in the figure, the basic structure of the ultraviolet gas analyzer includes a chassis, an air pump, a gas container, a scrubber, a flow meter, an ultraviolet differential gas analysis module, an ultraviolet module signal processing board, a main control circuit board, an Internet of Things card and a power module, etc., and a temperature control system. The chassis mainly includes a chassis sheet metal structure, a front panel structure, a rear panel structure, a gas chamber module, a hardware installation structure, etc. The overall size of the chassis: length × width × height = 1070mm × 240mm × 171mm; weight is about 19Kg. The remaining components are arranged inside the chassis.

[0048] The gas pump is connected to the gas source and the gas container, and is responsible for introducing the gas to be tested into the system and extracting it into the gas container. The gas container is connected to the scrubber to clean the gas and remove impurities in the gas to be tested, and ensure that the gas is pure and pollution-free.

[0049] The flow meter is installed before the UV analysis module to control the gas flow rate and monitor the gas flow to ensure the stability and consistency of the sample.

[0050] The UV differential gas analysis module is the key component of the UV gas analyzer, including a light source, a gas absorption cell, a spectrometer, a temperature control module, a temperature sensor, a pressure sensor and an embedded system (located on the data acquisition and signal processing board), which is used to emit and receive UV light and measure the absorption characteristics in the gas sample.

[0051] The main innovative features of the ultraviolet gas analyzer of the present invention include a high-precision temperature control system, a high-precision flow control system, a wireless control system, a gas path system and an Internet of Things transmission system. The above systems are described below.

[0052] (1) Temperature control system

[0053] The present invention adopts a long optical path design to realize ultra-low concentration gas detection and achieves a 20m optical path in a smaller chamber. Therefore, the optical path is more sensitive to mechanical deformation caused by temperature. In order to reduce the impact of ambient temperature, the gas chamber module needs to be temperature controlled (temperature controlled at 45°C). Test results show that the size of the temperature control fluctuation directly affects the gas concentration test results. Therefore, in order to achieve high-precision temperature control of the gas chamber, the present invention adopts a two-stage PID control mode. The first-stage PID is used to preliminarily adjust the temperature so that the temperature reaches near the set temperature as soon as possible; the second-stage PID is used to fine-tune the temperature fluctuation; at the same time, combined with the adaptive temperature control parameter adjustment technology, high-precision temperature control is achieved, and the temperature fluctuation is less than ±0.1°C, such as Figure 2 shown.

[0054] (2) Flow control system

[0055] The air pump, air volume and flow control program constitute the high-precision flow control system of the present invention, which specifically includes:

[0056] Considering the flow stability and air pump design, the present invention adopts a double-head pump as the gas extraction power source, such as Figure 3 As shown, the dual-head pump is driven by one motor with two pump heads, and the two pump heads are used in parallel. When operating in this parallel mode, the two pump heads work alternately, which can make the gas flow smoother and the pulse characteristics weaker; at the same time, it also achieves a larger flow rate when using a smaller volume gas pump, meeting the flow design requirements of the instrument.

[0057] The gas container is mainly used to reduce the impact of air pump pulse fluctuations and act as a buffer for the gas. The traditional gas container adopts the fixed volume double air chamber airflow buffering principle, but as the flow rate increases, the volume requirement is larger. Considering the portability requirements of the present invention, in order to solve the problem of insufficient space for placing large gas containers in order to reduce volume and weight, and the small gas container bottle cannot achieve the technical effect of buffering a larger flow rate. The present invention innovatively develops an elastic gas container, such as Figure 4As shown, it includes an air container body 5, an air container end cover 3, an elastic air container film 4 is provided between the body and the end cover, the film divides the elastic air container into two air chambers, the air container film, the air container body and the air container end cover are fixed by a stainless steel cross slot self-tapping screw 1, the specification is ST2.9x13, and a stainless steel flat washer 2 is provided between the screw 1 and the end cover. The two air chambers of the elastic air container of the present invention are different from conventional air containers of fixed volume. Due to the addition of the elastic air container film, the volume of the two air chambers can change with the fluctuation of the air flow generated by the pump body, and the pressure change generated by the pump body is balanced by the elastic force of the elastic air container film itself and the atmospheric pressure, thereby reducing the pressure fluctuation and stabilizing the air flow rate.

[0058] The flow control program adopts PID control, which realizes high-stability flow control by real-time monitoring of gas flow, feedback control of the air pump, and effective adjustment of the deviation proportional (P) coefficient, integral (I) coefficient and differential (D) coefficient.

[0059] (3) Gas system

[0060] In view of the sensitivity of the long optical path gas chamber to temperature and considering the convenience of on-site use of the instrument, the present invention studies the instrument gas path design, which mainly includes:

[0061] ① Gas preheating

[0062] Aiming at the temperature-sensitive period of the long optical path gas chamber, a gas preheating function is added. The gas is heated by temperature-controlled heating before entering the gas chamber. The gas is preheated by winding the pipe to ensure that the temperature entering the gas chamber is basically constant, reducing the impact of low temperature on the gas chamber.

[0063] ②Add zero adjustment gas circuit

[0064] In order to improve the portability of the instrument for on-site use, the instrument is designed to add a zeroing gas circuit, which uses a solenoid valve to automatically switch the zeroing gas circuit, and uses activated carbon and Purafil filters to filter VOCs and NO on-site. x Wait for the gas to be filtered to obtain zero gas.

[0065] ③Measurement by negative pressure in air chamber

[0066] In view of the strong adsorption and low concentration of VOCs gases and gases such as NH3 and NO2, the present invention places the air pump, gas volume, flow meter, etc. behind the air chamber, and the air chamber is in a negative pressure state to avoid possible contamination and adsorption of the gas by the air pump and flow meter, thereby improving the measurement accuracy of the instrument.

[0067] (4) Wireless control system

[0068] In order to improve the convenience of instrument operation, the present invention adopts a wireless handheld operator to control the instrument. At the same time, in view of the harsh on-site environment, an 8-inch industrial-grade tablet computer is selected, which has anti-interference, shockproof, dustproof and moisture-proof functions, stronger temperature adaptability, supports 50m long-distance control, and is more convenient to operate.

[0069] Focusing on the wireless control requirements, the present invention uses Wi-Fi direct connection with stronger anti-interference ability as the communication method between the host and the handheld operator. It enables devices to connect to each other without a wireless access point, giving full play to the speed advantage of Wi-Fi. The transmission speed is fast and efficient, and the transmission speed is nearly 100 times that of Bluetooth, and it has stronger anti-electromagnetic interference ability.

[0070] (5) Internet of Things Transmission System

[0071] In response to the transmission needs of the Internet of Things, the present invention adopts the NB-IoT / GPRS environmental monitoring instrument Internet of Things embedded gateway, which is used for real-time remote uploading of instrument measurement process data and status data, as well as remote command issuance, so that it can seamlessly access various environmental monitoring big data analysis cloud platforms. It provides advanced and mature product support for reliable cloud access, trusted monitoring, traceable diagnosis, and controllable traceability of various environmental monitoring instruments.

[0072] The ultraviolet differential gas analysis module of the present invention is described below:

[0073] The ultraviolet differential gas analysis module is mainly composed of a light source, a gas absorption cell, a spectrometer, a temperature control module, a temperature sensor, a pressure sensor and an embedded system (data acquisition and signal processing board). The light source and the spectrometer are synchronously controlled by the embedded system to collect the absorption spectrum of the gas in the gas absorption cell. The two sets of temperature control devices are not controlled by the embedded system, and each independently controls the temperature of the spectrometer and the gas absorption cell to keep the spectrometer at a constant temperature of 45°C and the gas absorption cell at a constant temperature of 45°C. The sensors controlled by the embedded system of the data acquisition and signal processing board also include: 1 sensor for monitoring the gas temperature in the gas absorption cell and 1 sensor for monitoring the gas pressure in the gas absorption cell. The structure is as follows: Figure 5 The design of the UV differential gas analysis module mainly includes electronic design, optical and mechanical design and software.

[0074] (1) Electronics design includes:

[0075] ①Synchronous control of light source and spectrometer and raw signal acquisition

[0076] The synchronization signal is output by the data acquisition and signal processing board and input into the light source and spectrometer respectively. First, the spectrometer is triggered to start exposure, and then the light source is triggered to flash. The number of times the light is on can be adjusted. After the flashing ends, the spectrometer stops exposing, collects one spectrum, and then proceeds to collect the next spectrum.

[0077] ②The temperature control device realizes the constant temperature heating of the spectrometer and the gas absorption cell

[0078] The temperature control devices of the spectrometer and the gas absorption cell are independent temperature control devices and are not controlled by the data acquisition and signal processing board. One set of temperature control devices heats the bottom of the spectrometer to maintain a constant temperature, and the other set heats the gas absorption cell to maintain a high temperature. PID control is used to stabilize the temperature of the spectrometer and the gas absorption cell at the set temperature respectively. The spectrometer is kept at a constant temperature of 45°C, and the light source and the gas absorption cell are kept at a constant temperature of 45°C.

[0079] ③Control of temperature / pressure sensor

[0080] The temperature sensor and pressure sensor respectively obtain the temperature and pressure of the gas in the gas absorption cell for calculating the concentration correction; sensor interfaces such as oxygen and flow monitoring, pump control interfaces, etc. are reserved.

[0081] ④Data acquisition, signal processing board, main control circuit board communication

[0082] Execute the commands of the main control circuit board module, obtain parameters, and upload the measured concentration.

[0083] ⑤ Data acquisition and signal processing board When the data acquisition and signal processing board is running, the spectrum collected by the spectrometer can be obtained synchronously, which is convenient for debugging and on-site viewing of the original spectrum.

[0084] (2) Optical and mechanical design

[0085] The optical machine mainly involves the construction of the optical path of the light source, gas absorption cell and spectrometer, which is used to test and calculate the absorbance of the gas in the gas absorption cell, and then calculate the concentration. The light source is directly coupled to the light inlet of the gas absorption cell and the light outlet of the gas absorption cell is directly coupled to the spectrometer. The optical path length of the return light path in the gas absorption cell is about 20 meters. The design of the gas absorption cell needs to consider the placement of the inlet and outlet positions to avoid water vapor contamination of the lenses in the gas absorption cell during ventilation, to ensure smooth gas circulation in the gas absorption cell to avoid dead corners as much as possible, and to keep the volume of the gas absorption cell as small as possible to improve the ventilation response time.

[0086] The gas absorption cell needs to be heated, temperature controlled and covered with insulation. At the same time, the pressure inside the gas chamber needs to be measured to compensate the measurement results.

[0087] (3) Software Design

[0088] The software part of the UV differential gas analysis module in the present invention mainly controls each component through the embedded system running on the data acquisition and signal processing board, collects data and calculates concentration. The main functions include:

[0089] ①Control the synchronous work of light source and spectrometer and collect spectral data;

[0090] ②Read the values ​​of sensors such as temperature / pressure sensors;

[0091] ③ Communicate with the main control circuit board module, execute the main control circuit board module command, obtain parameters from the main control circuit board module, and send parameter values ​​to the main control circuit board module;

[0092] ④ Analyze the data collected by the spectrometer in real time, calculate the concentration, use the measurement parameters of each sensor to correct the calculated value, and send the concentration value to the main control circuit board module.

[0093] In the ultraviolet light source part, the present invention selects a pulsed xenon lamp as the light source. Compared with the deuterium lamp used in some existing technologies, the deuterium lamp is a continuous working light source. During use, there are problems such as weak light intensity, short life, long preheating time, and narrow spectral range. Especially for the detection of NO2, because the energy of the deuterium lamp is very weak above 400nm, the ability to detect ultra-low concentrations of NO2 is limited. The present invention selects a pulsed xenon lamp as the light source, which is a pulse mode, with a wavelength range of 185nm to 2000nm (more advantageous when used for low-concentration NO2 detection), and a life of 1×10 9 The maximum discharge frequency is equivalent to more than 10,000 hours. It can adjust the single discharge energy and the number of flashes during the exposure time. Its luminous wavelength covers the ultraviolet absorption peak of common pollutants to be tested (generally required to be 200-500nm); and has high luminous stability and low cost.

[0094] After selecting the xenon lamp, the present invention innovatively designs its circuit structure according to its characteristics and ultraviolet requirements. In terms of xenon lamp power supply, a dual-inductor SEPIC buck-boost circuit is used in conjunction with a linear voltage regulator circuit to achieve a wide input voltage range and high output stability.

[0095] ② In terms of the discharge voltage fine-tuning circuit, a DA converter controlled by a signal processing board is used in conjunction with an analog voltage adjustment circuit to generate an analog voltage of 3.2V to 4.8V and provide it to the xenon lamp, thereby realizing digital control fine-tuning of the xenon lamp discharge voltage by the signal processing board program.

[0096] ③At the same time, the present invention can also adjust the flashing times of the pulse xenon lamp within the integration time of the spectrometer, thereby achieving more flexible adjustment of light energy.

[0097] The UV differential gas analysis module works as follows:

[0098] The light emitted by the light source enters the gas absorption cell after being collimated, passes through the gas with absorption characteristics, and is focused by the collimated light path (rear end) at the other end of the gas absorption cell before being sent to the spectrometer. The received light is split by the spectrometer, then converted into an electrical signal, and finally sent to the computer after A / D conversion and the spectrum of the corresponding band is displayed. Finally, the computer is used to solve their respective concentrations based on the absorption characteristics of the target gas to be measured contained in the spectrum.

[0099] The concentration calculation is based on the Lambert-Beer law. The specific workflow is as follows:

[0100] (1) After the module is powered on, it is preheated: the external trigger starts working, the spectrometer starts collecting spectra (preheating), and the gas chamber heater starts heating;

[0101] (2) The first N spectra collected are preprocessed by removing dark current, wavelength calibration, spectral filtering, etc. and saved as (temporary) background spectra;

[0102] (3) Enter the spectrum cycle acquisition mode: each spectrum is collected and preprocessed such as dark current removal, wavelength calibration, and spectrum filtering. At the same time, the temperature / pressure value in the gas chamber is collected, the gas concentration value is calculated, and the message queue is checked to see if there is a command from the host computer. If so, the corresponding message response function is executed; if not, the next cycle acquisition is continued.

[0103] In terms of the gas absorption cell, the present invention designs a long optical path ultraviolet gas absorption cell. Since the gas chamber is the gas sensing component of the optical gas analyzer and is the key to realizing the detection of gases of different concentrations, the light beam in the gas chamber is selectively absorbed by the gas flowing through the gas chamber, and the light beam passing through the gas chamber carries the type and concentration information of the gas. By analyzing the spectrum of the light beam passing through the gas chamber, the type and concentration of the gas can be solved, thereby achieving the purpose of gas measurement.

[0104] Theoretically, the longer the optical path, the lower the gas concentration that can be detected, that is, the smaller the detection limit. In order to meet the emergency monitoring requirements of this project and realize ultra-low concentration gas detection, the present invention designs and develops a multiple reflection long optical path gas cell.

[0105] At present, the multi-reflection gas pools used on the market are mostly White gas pools. White gas pools are mainly composed of three spherical mirrors with the same curvature. The optical path is determined by the number of reflections and the distance between the left and right lenses. White gas pools are particularly suitable for point light source gas pools (different from laser gas pools), but White gas pools have some unavoidable disadvantages. Since the lenses of the light source utilization of the White gas pool are at the same end of the gas chamber, in order to increase the light source utilization, the aperture of the reflector must be increased at the same time. Increasing these two apertures means that the aperture of the gas pool will increase rapidly; second, the volume utilization of the gas pool is low, that is, the light is not evenly distributed in the gas pool, one end is more sparse, and the other end is more dense; third, the structural design is more complicated, because the structural design must consider both the position requirements of the gas pool optical lens (adjustable) and the long-term stability of the position of the optical lens. As a result, the White gas chamber is often bulky and extremely troublesome to assemble and debug.

[0106] Based on the above problems, the present invention designs a multi-reflection long optical path gas cell. Figure 6 As shown: It is mainly composed of two reflectors with the same radius of curvature. There are light holes for the incident and outgoing light beams in the center of the reflector, which are bonded to the through hole 2 with a window sheet. The other parts are air chamber structural parts used to fix the reflectors and form a sealed chamber. Adjusting the distance between the two reflectors can produce different reflection times. Its principle mainly utilizes the reflection properties of spherical mirrors. We know that the light emitted from the center of the spherical mirror will return to the original path after reaching the spherical reflector. When the incident light point deviates from the center of the sphere, the reflected light will converge to a point symmetrical with the center of the sphere. When there are two symmetrically distributed spherical mirrors, the light emitted from any point on the line connecting the centers of the spheres will eventually fall on its symmetrical point.

[0107] Compared with White gas pool, the main advantages are:

[0108] ① If the number of reflections is the same, the lens distance is the same, and the numerical aperture (divergence angle of the light source) is also the same, the volume of the gas pool of the present invention must be much smaller. If the number of reflections is the same, the lens distance is the same, and the volume is the same, the light energy utilization rate of the new gas pool must be high.

[0109] ② Since the optical lens of the gas pool of the present invention is a concentric structure, the requirements of the gas pool can be met only by processing the structural parts, and there is no need to think too much about assembly and debugging. Therefore, the structural design of the gas pool of the present invention is more flexible and convenient, and it can be connected with optical fiber, or the light source and spectrometer can be directly connected.

[0110] ③ Improve the dimensional stability of the air chamber structure and improve the spectral stability

[0111] The stability of the structural dimensions of the multi-reflection long optical path gas pool has a significant impact on the optical path of the gas chamber, the size of the light intensity received by the spectrometer, and the stability of the spectrum. The light emitted by the light source is reflected multiple times between the first and second reflectors at a certain distance to reach the target optical path, and converges at the reflector light outlet at the spectrometer end to enter the spectrometer. When the curvature of the reflector is determined, the number of reflections is determined by the distance between the reflectors; the more reflections, the smaller the error tolerance of the distance between the reflectors. At the same time, the change in the distance between the reflectors will cause the change in the position of the convergence point, causing the convergence point to deviate from the slit of the spectrometer, causing the light intensity to drop. The gas chamber size is relatively long, and the thermal expansion and contraction phenomenon is the main reason for the stability of the axial size of the gas pool. In response to the above problems, a low expansion coefficient quartz tube is used as a structural component solution, but the quartz tube processing size accuracy consistency is poor and is not suitable for mass production. In order to facilitate mass production, the gas pool adopts a structure solution with two end mirror seats plus six low expansion coefficient 4J32 Invar alloy tie rods. The 4J32 Invar alloy tie rod has a low thermal expansion coefficient, and the axial size changes slightly due to changes in ambient temperature, and the parts are easy to meet the processing accuracy requirements. At the same time, three reinforcement rings are arranged on the tie rod to improve the rigidity of the tie rod.

[0112] ④Reduce the impact of temperature and air flow pressure fluctuations

[0113] It was found in the experimental test that changes in the external environment temperature and fluctuations in the airflow pressure will cause fluctuations in the spectrum, thereby causing measurement errors. In response to the above problems, measures were taken to control the temperature of the gas chamber tube and stabilize the airflow. In response to changes in ambient temperature, a polyimide electric heating film is used to control the overall temperature of the gas chamber tube, and the module is insulated as a whole, so that the gas pool works in a relatively stable working environment. In terms of reducing airflow pressure fluctuations, a gas nozzle with a larger inner diameter is used to reduce changes in airflow velocity; the center hole of the reflector is sealed with a quartz window so that the airflow does not pass through the center light hole of the reflector, reducing the disturbance of the airflow to the spectrum, and the gas pool intake and exhaust channels are set around the reflector. Through the above technical solutions, the stability of the spectrum is improved.

[0114] For ease of installation, traditional ultraviolet differential gas detection modules generally use optical fibers to transmit optical signals from the light source to the gas chamber and from the gas chamber to the spectrometer. However, conventional optical fibers are easily damaged by high temperatures and easily contaminated by gases, while high-temperature resistant optical fibers are expensive. Especially for heat and moisture principle gas detection, the heating temperature of the gas pool is usually above 100°C. The existing use of optical fibers to transmit optical signals has major drawbacks.

[0115] In response to the above problems, the present invention innovatively develops a fiber-free detection solution that combines a beam splitter and a coupling reflector: the light from the light source enters the coupling reflector after passing through the beam splitter (semi-transmissive and semi-reflective), and the reflector focuses the light at the entrance of the optical path of the multiple-reflection air chamber. After multiple reflections in the air chamber, the light returns from the air chamber and is reflected again by the coupling reflector to the beam splitter, and the beam splitter reflects the light into the spectrometer.

[0116] This solution has changed the traditional optical fiber technology, greatly improved the reliability of the instrument, reduced the instrument failure rate, and also reduced the instrument production cost.

[0117] Wavelength Calibration Solution for UV Gas Analyzer

[0118] During use, the UV differential gas analyzer may encounter drift in the wavelength of the measured spectrum due to changes in environmental conditions (such as temperature and vibration) (the drift may be a whole pixel drift or less than a pixel, i.e., sub-pixel), which will ultimately affect the accuracy of the measurement results. Specifically, the UV differential gas analyzer needs to be calibrated before leaving the factory, and the standard spectrum, calculation model and parameters are stored in the software and hardware. After the UV differential gas analyzer is calibrated and put into actual use, if the wavelength of the measured spectrum shifts left or right during use, if the drift cannot be calibrated in real time, the test spectrum will be misaligned relative to the wavelength of the standard spectrum during calibration, and the inaccurate band used for calculation will introduce calculation errors, resulting in inaccurate indications.

[0119] The existing main methods for wavelength drift include: first, using mercury lamp for calibration, which requires the spectrometer to be removed from the system, which wastes time and resources and brings great inconvenience to users; second, the wavelength can be recalibrated by introducing standard gas of the measured gas, which is a one-time calibration. After calibration, the calibration amount is always used, and it is impossible to compensate in real time according to the wavelength drift of the current measurement; third, the wavelength drift compensation method based on the xenon lamp spectrum provides a real-time dynamic compensation for the wavelength, but the drift amount can only be calculated in real time when nitrogen is introduced. When the gas to be measured is introduced in the actual test, it cannot be calculated in real time. At the same time, the calibration accuracy should belong to pixel-level calibration, and no sub-pixel-level calibration is mentioned; fourth, the measured spectrum data of the measured gas and the absorption cross-section database of the measured gas measured in the laboratory can be used to calculate the wavelength drift of the spectrometer and calibrate in real time to eliminate its influence, but when the measured gas is a mixed gas or the concentration of the measured gas is low (near the lower limit of determination), the use of this method is subject to certain restrictions.

[0120] In order to overcome the above-mentioned deficiencies of the prior art, the present ultraviolet differential gas analyzer adopts a three-level real-time wavelength calibration method, which can align the real-time measured spectrum wavelength with the standard spectrum wavelength during calibration, thereby ensuring the accuracy of the calculation results.

[0121] Specific methods include:

[0122] (1) Primary pixel calibration: All spectra are calibrated to a uniform wavelength pixel point to obtain absolute wavelength calibration. Absolute wavelength calibration calibrates the wavelength to align with the reference standard spectrum according to the spectral characteristics of the light source used in the spectrometer, which facilitates the unification of calculation models and parameters. This calibration process does not require the spectrometer to be disassembled, but can be directly calibrated in the analyzer;

[0123] (2) Secondary pixel calibration: Pixel-level wavelength drift calculation is performed when the instrument is zeroed. This calibration value can be reused in subsequent tests. Pixel-level calibration is performed only once when the instrument is zeroed.

[0124] (3) Three-level sub-pixel calibration: During the measurement of the gas to be tested, the calibration value is calculated and calibrated in real time. Since the pixel-level calibration value is known, sub-pixel calibration only needs to be performed within one or two pixels, and the real-time calculation amount is smaller.

[0125] After three levels of calibration, the spectra are aligned, which can maximize the accuracy of subsequent calculation results.

[0126] The following is a detailed introduction to the specific calibration process using SO2 gas inversion as an example:

[0127] (1) Primary pixel calibration: Select the target standard spectrum and find the peak point, and the measured spectrum and find the peak point in the spectrum that matches the target standard spectrum. Establish the pixel mapping relationship between the measured spectrum and the standard spectrum through fitting.

[0128] Figure 8 is the standard spectrum and the peak point found automatically. Fig. 9 In order to measure the spectrum and automatically find the peak point that matches the target standard spectrum, the pixel mapping relationship between the measured spectrum and the standard spectrum is established by fitting.

[0129] According to the pixel mapping relationship between the target standard spectrum and the measured spectrum, the spectrum obtained after interpolation of the measured spectrum is as follows: Fig.10 As shown, the wavelength alignment between the measured spectrum and the target spectrum is achieved.

[0130] (2) Secondary pixel calibration

[0131] Secondary pixel calibration is performed when the instrument is zeroed. The zero gas spectrum is first calibrated for wavelengths of more than one pixel, and then the wavelength calibration is performed at the sub-pixel level.

[0132] (3) Level 3 calibration

[0133] The three-level pixel calibration is a real-time sub-pixel calibration, which adopts the scheme of calibrating each inversion band separately (aligning the wavelength with the corresponding inversion band during calibration). Taking the wavelength calibration of the SO2 inversion band as an example, the details are as follows:

[0134] During real-time measurement, the test spectrum is first calibrated for wavelengths above one pixel according to the zero gas calibration amount, and then a sub-pixel wavelength calibration is performed in real time.

[0135] like Fig.11 As shown, the matching rate (correlation coefficient) between the differential absorbance calculated from the calibrated spectrum and the standard differential absorbance spectrum has been significantly improved, thereby improving the effectiveness and accuracy of the detection.

[0136] Gas Inversion Calculation Method for Ultraviolet Gas Analyzer

[0137] The commonly used methods for calculating the concentration of ultraviolet differential absorption gas mainly include the following categories: ① Using polynomial fitting to obtain the slowly varying part of the absorption spectrum, deducting it to obtain the differential absorption spectrum, and then using the cumulative differential absorbance of the characteristic spectrum to perform least squares fitting to obtain the concentration inversion formula; ② Using the multivariate linear regression method to decompose the differential absorption spectrum to solve the concentration; ③ Using the Fourier transform method to extract the high-frequency part and inverse transform to obtain the differential absorption spectrum, and then performing the cumulative absorbance least squares fitting or multivariate linear regression to solve the concentration.

[0138] However, the aforementioned method ① cannot effectively utilize the spectral shape information because it uses the cumulative differential absorbance to calculate the concentration. The calculated value of the spectral noise introduced when there is no detection gas is always positive, thus raising the detection limit. The second method makes full use of the spectral shape information and can effectively avoid noise interference and cross-interference between gases, but it is very sensitive to the wavelength drift of the spectrum. The spectral drift of one or two pixels can significantly reduce the inversion accuracy. The third method also cannot avoid the problems of high detection limit or sensitivity to wavelength drift encountered by the aforementioned two methods.

[0139] In view of the problems that the existing ultraviolet differential absorption flue gas concentration calculation method cannot effectively utilize the spectral shape information and is sensitive to the wavelength drift of the spectrum, this method proposes the following innovative solutions: different concentration calculation methods are used according to the absorbance characteristics of different gases:

[0140] ①Multivariate linear regression

[0141] The conventional gas inversion calculation method is based on the Lambert-Beer law to establish the differential signal of gas per unit concentration at different wavelengths, gas concentration, and gas differential absorption spectrum equation, as shown in the following formula.

[0142]

[0143] D=A·C

[0144] C=(A′·A) -1 ·A′·D

[0145] Where: D is the differential absorbance matrix (measured data); A is the unit concentration differential absorption matrix (determined during instrument calibration); C is the gas concentration matrix. The gas concentration information can be obtained by solving the overdetermined equation using least squares. The specific processing process is as follows:

[0146] When a certain gas standard gas (taking SO2 as an example) is introduced, an absorption spectrum is obtained, such as Fig.12 As shown, - intercept its characteristic spectrum and divide it by the gas concentration to obtain the unit concentration differential absorbance, and use this data as the first column of matrix A; similarly, obtain the unit concentration characteristic differential absorbance information of NO, benzene, toluene and other gases in turn, and use them as the second column, third column and so on of matrix A, and so on, in sequence to obtain matrix A.

[0147] When measuring, the absorbance data of the characteristic absorption band corresponding to each gas is intercepted in sequence according to the gas type order in matrix A to form a differential absorbance matrix D, and then the concentration matrix C of each gas is obtained technically.

[0148] Among them, NO, benzene and toluene gases were analyzed using the multivariate linear regression method.

[0149] ②Fourier transform method

[0150] Due to the difference in the quasi-periodic absorption lines of SO2 and NH3 gases in the ultraviolet band, the spectrum information corresponding to the gas is obtained by performing fast Fourier transform (FFT) on the gas absorption spectrum. Based on the relationship between the intensity of the characteristic spectrum (as shown in the figure, SO2 selects the 9th sequence maximum value, NH3 selects the 3rd sequence maximum value) and the gas concentration, the concentrations of SO2 and NH3 gases are inversely calculated.

[0151] ③Matched filtering

[0152] NO2 gas signal absorption is weak, and the traditional least square calculation method is very susceptible to noise interference, resulting in poor detection limit. Based on the characteristics of NO2 gas absorption being located at the back end of spectral measurement, with few cross-interference gases and being susceptible to noise interference, the present invention innovatively introduces the matched filtering method in the signal processing method into the spectral concentration calculation to perform gas identification and calculation.

[0153] As a signal processing method, matched filtering is a kind of optimal linear filter. When the input signal has a special waveform, the criterion of the filter is to maximize the signal-to-noise ratio of the output signal and reduce the influence of noise. The matched filter is composed of the input signal arranged in reverse order, and the amplitude characteristics of the filter are consistent with the amplitude spectrum of the signal. Therefore, the matched filtering of the signal is equivalent to performing a cross-correlation operation on the signal.

[0154] Specifically, in the present invention, the NO2 characteristic absorption spectrum inverse sequence is used as a filter to perform matched filtering (convolution) on the received NO2 differential spectrum to be measured, and the filtered signal is obtained. Then, the calculated peak value is used as the characteristic value to establish a fitting relationship with the gas concentration to achieve the inversion calculation of NO2 gas concentration. Compared with the traditional method, the detection limit is further improved, as shown in Table 1 (1.2m gas chamber).

[0155] Table 1 Comparison of NO2 detection limits by different methods (unit: mg / m 3 )

[0156]

[0157] Multi-component gas cross-interference processing algorithm for ultraviolet gas analyzer

[0158] Nonlinear cross-interference between multi-component gases is a difficult problem in gas analysis. Compared with conventional UV analyzers, this project requires the measurement of more types of gases, and ammonia, benzene, and toluene are all located at the front end of the UV spectrum. The cross-interference of various gases is extremely serious, affecting the detection accuracy of the instrument. In order to solve the problem of multi-gas cross-interference, the multivariate linear regression method is conventionally used to eliminate cross-interference. However, due to the influence of factors such as device nonlinearity and noise, this method usually cannot completely eliminate the influence of cross-interference, and residual gas exists.

[0159] In response to the above problems, this project adopts a two-stage interference elimination method to eliminate linear cross-interference and nonlinear cross-interference caused by nonlinear response of devices, etc., which effectively solves the cross-interference problem between test gases. The specific method is as follows:

[0160] ① Multiple linear regression to eliminate cross-interference: This method is the same as the multiple linear regression method in the gas concentration inversion technology, that is, extracting the characteristic values ​​of various gases in the absorbance curve of the gas to be measured as the differential absorbance matrix, and by solving the overdetermined equation, while obtaining the gas concentration, eliminate the linear cross-interference between gases;

[0161] ② Secondary nonlinear cross-interference elimination: When the above method still has residual gas interference after eliminating the linear cross-interference between gases, polynomial fitting is further used to obtain the relationship between the characteristic value of the interfering gas and the characteristic contribution value of the gas to be measured, and the interfering gas contribution value is deducted from the characteristic value of the gas to be measured to obtain the final characteristic value of the gas to be measured.

[0162] Through two-stage processing, direct cross-interference of gases is effectively removed. The following table shows the comparison results before and after interference removal.

[0163] Table 2 Concentration data of some gases before and after removal of cross-interference

[0164]

[0165] The above applications are only some implementation methods of the present application. For those skilled in the art, several modifications and improvements can be made without departing from the creative concept of the present application, and these all belong to the protection scope of the present application.

Claims

1. An ultraviolet gas analyzer based on multi-stage temperature control, characterized in that: The ultraviolet gas analyzer includes a chassis, an air pump, a gas container, a scrubber, a flow meter, an ultraviolet differential gas analysis module, an ultraviolet module signal processing board, a main control circuit board, an Internet of Things card, a power module and a temperature control system; The chassis includes a chassis sheet metal structure, a front panel, a rear panel, an air chamber module and a hardware installation structure; The gas pump is connected to the gas source and the gas container, and the gas to be tested is introduced into the ultraviolet gas analyzer and extracted into the gas container, and the gas container is connected to the scrubber; The flow meter is installed before the ultraviolet differential gas analysis module; The UV differential gas analysis module is used to transmit and receive UV light and measure the absorption characteristics in the gas sample; The temperature control system performs temperature control on the air chamber module, and adopts a two-stage PID control mode when performing temperature control on the air chamber module, wherein the first-stage PID is used for primary temperature adjustment to make the temperature close to the set temperature; and the second-stage PID is used for fine adjustment of small temperature fluctuations.

2. The ultraviolet gas analyzer based on multi-stage temperature control according to claim 1, characterized in that: The temperature control system adopts adaptive temperature control parameter adjustment technology to achieve high-precision temperature control.

3. The ultraviolet gas analyzer based on multi-stage temperature control according to claim 2, characterized in that: The temperature control system makes the temperature fluctuation of the gas chamber module less than ±0.1°C.

4. The ultraviolet gas analyzer based on multi-stage temperature control according to claim 1, characterized in that: The ultraviolet differential gas analysis module comprises a light source, a gas absorption cell, a spectrometer, a temperature sensor, a pressure sensor and an embedded system, wherein the embedded system is located on a data acquisition and signal processing board.

5. The ultraviolet gas analyzer based on multi-stage temperature control according to claim 4, characterized in that: The temperature control system performs constant temperature treatment on the spectrometer, light source and gas absorption cell, and includes a first temperature control device and a second temperature control device. The first temperature control device performs constant temperature heating at the bottom of the spectrometer, and the second temperature control device heats the light source and the gas absorption cell to maintain a constant temperature.

6. The ultraviolet gas analyzer based on multi-stage temperature control according to claim 5, characterized in that: The constant temperature of the spectrometer is 45°C, and the constant temperature of the light source and the gas absorption cell is 45°C.

7. The ultraviolet gas analyzer based on multi-stage temperature control according to claim 6, characterized in that: The temperature control system uses a polyimide electric heating film to perform overall temperature control and heat preservation on the gas chamber module.

8. The ultraviolet gas analyzer based on multi-stage temperature control according to claim 7, characterized in that: The temperature control system also includes a gas preheating unit, which preheats the gas by means of a pipeline winding before the gas enters the gas chamber, thereby ensuring that the temperature entering the gas chamber is substantially stable.

9. The ultraviolet gas analyzer based on multi-stage temperature control according to claim 8, characterized in that: The temperature sensor is used to monitor the temperature of the gas in the gas absorption cell.

10. A method for ultraviolet gas analysis using the ultraviolet gas analyzer according to any one of claims 1 to 9, characterized in that: The steps include: (1) Turn on the gas source and air pump to introduce the gas to be tested into the gas container; (2) Clean the gas through a scrubber to remove impurities in the gas to be tested to ensure that the gas is pure and pollution-free; (3) Control and monitor the flow rate of the gas through a flow meter to ensure that the sample flow rate and flow rate are stable; (4) selecting a suitable wavelength in the ultraviolet differential gas analysis module and turning on the light source; introducing and inputting the gas sample into the ultraviolet differential gas analysis module and allowing it to interact with the ultraviolet light; (5) The UV differential gas analysis module measures the intensity of the transmitted light and calculates the concentration of the gas; (6) Process and record the data to obtain the results of gas composition.

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