CN222353779U

CN222353779U - A gas sensor

Info

Publication number: CN222353779U
Application number: CN202323656089.0U
Authority: CN
Inventors: 祝铭; 陈珂
Original assignee: Dalian University of Technology; Shenzhen Institute of Advanced Technology of CAS
Current assignee: Dalian University of Technology; Shenzhen Institute of Advanced Technology of CAS
Priority date: 2023-12-31
Filing date: 2023-12-31
Publication date: 2025-01-14
Anticipated expiration: 2033-12-31

Abstract

The utility model relates to a gas sensor which comprises the steps of obtaining a gas mixed solute in a solution to be measured, selecting one point/section from an infrared wave band as an irradiation wave band, placing the gas mixed solute in a photoacoustic cavity, irradiating the gas mixed solute by an excitation light source of the irradiation wave band, obtaining a sound signal in the photoacoustic cavity, and processing the sound signal to obtain the concentration of the gas to be measured in the solution to be measured. Through the steps, the excitation light source can directly irradiate the gas mixed solute in the photoacoustic cavity, so that the gas mixed solute absorbs electromagnetic radiation of the excitation light source under the irradiation of the excitation light source, the molecular temperature of the gas mixed solute is increased, the pressure in the photoacoustic cavity is changed, the concentration of the gas to be detected in the solution to be detected can be obtained after the photoacoustic pressure is detected and then processed, the equipment is not required to be heated or calibrated in advance, and the purposes of simplifying preparation work and improving detection efficiency can be achieved.

Description

Gas sensor

Technical Field

The utility model relates to the technical field of gas detection, in particular to a gas sensor.

Background

With the development of modern industrial society, human activity has led to the emission of large amounts of carbon dioxide (CO ₂) into the atmosphere. For many years, the concentration of CO ₂ in the atmosphere has steadily increased from 285ppm before the 60 s of the 19 th century to over 400ppm today. The ocean plays an irreplaceable role in the global carbon cycle, absorbing approximately one quarter of the total amount of CO ₂ emitted by humans into the atmosphere annually through sea gas exchange. However, as the concentration of CO ₂ in the atmosphere continues to increase, the rate at which the ocean absorbs CO ₂ increases, resulting in a decrease in the pH of the seawater, causing ocean acidification, affecting the ocean ecosystem. In order to evaluate the effect of climate change on the sea, it is important to measure the concentration of CO ₂ in seawater.

Photoacoustic spectroscopy (PAS) is an indirect absorption spectroscopy technique that differs from Tunable Diode Laser Absorption Spectroscopy (TDLAS). And (3) reversely pushing out the concentration of the gas to be detected by detecting the amplitude of the photoacoustic pressure wave generated by the absorption of the light energy by the gas molecules. This is an extremely sensitive, background-free, low-drift detection method. In the prior art, a solution for directly measuring the concentration of CO ₂ in a gas-liquid solution based on PZT piezoelectric-photoacoustic spectroscopy (PAS) is disclosed. Its advantages are no need of water-gas separating unit, high response time and 60 seconds. However, the sensitivity is insufficient and the device requires 30 minutes of pre-heating before normal use. In addition, a small QEPAS gas sensor for detecting dissolved gases in a deep sea environment is disclosed, equipped with a compact sonographer. It has a low gas consumption of about 300. Mu.l and a minimum detection limit of CH4 of 1.1ppm. But the resonant frequency and responsiveness of the gas sensor tend to drift and therefore require calibration prior to use. In summary, although the infrared absorption spectrum technology is utilized to research the dissolved gas in the seawater to show higher accuracy in the prior art, the challenge of low detection efficiency caused by complex preparation work when the equipment is operated is still faced.

Disclosure of utility model

In order to solve the defect of low detection efficiency caused by complex preparation work in the running process of equipment, the utility model provides a gas sensor.

The utility model adopts the technical scheme that the gas sensor comprises:

A separation unit for obtaining a gas mixed solute in the solution to be detected;

The excitation unit is used for selecting one point/section from the infrared wave band as an irradiation wave band;

The photoacoustic cavity is used for placing the gas-mixed solute in the photoacoustic cavity, and the gas-mixed solute is irradiated by an excitation light source of an irradiation wave band in the excitation unit;

The sound receiving unit is used for acquiring sound signals in the photoacoustic cavity;

And the processing unit is used for processing the sound signals to obtain the concentration of the gas to be detected in the solution to be detected.

Preferably, the excitation unit and the sound receiving unit are respectively positioned at two sides of the photoacoustic cavity, the separation unit is communicated with the photoacoustic cavity, and the processing unit is in signal connection with the excitation unit and the sound receiving unit.

Preferably, an optical filter is arranged between the excitation unit and the photoacoustic cavity.

Preferably, a window lens is arranged on one side of the photoacoustic cavity, which is close to the excitation unit, and the window lens is in sealing connection with the photoacoustic cavity.

Preferably, a window lens is arranged on one side of the photoacoustic cavity, which is close to the excitation unit, and the window lens is a barium fluoride window lens.

Preferably, the separation unit is a polymer membrane, one side of the polymer membrane is communicated with the photoacoustic cavity, and the other side of the polymer membrane is communicated with the solution to be detected.

Preferably, the separation unit communicates with the photoacoustic cell through a diffusion channel having a radius in the range of 0.2mm to 0.8mm.

Preferably, the photoacoustic cell is made of brass and the inner wall is polished.

Compared with the prior art, the utility model has the following beneficial effects:

The application also discloses a gas sensor which comprises a separation unit for obtaining the gas mixed solute in the solution to be detected, an excitation unit for selecting one point/section from the infrared wave band as an irradiation wave band, a photoacoustic cavity for placing the gas mixed solute in the photoacoustic cavity, an excitation light source for irradiating the gas mixed solute in the irradiation wave band of the excitation unit, a sound receiving unit for obtaining sound signals in the photoacoustic cavity, and a processing unit for processing the sound signals to obtain the concentration of the gas to be detected in the solution to be detected. Through the steps, the excitation light source can directly irradiate the gas-mixed solute in the photoacoustic cavity, so that the gas-mixed solute absorbs electromagnetic radiation of the excitation light source under the irradiation of the excitation light source, the temperature of gas-mixed solute molecules is increased, the pressure in the photoacoustic cavity is changed, photoacoustic pressure waves are further formed, the concentration of the gas to be measured in the solution to be measured can be obtained after the photoacoustic pressure waves are detected and then processed, and the equipment does not need to be heated or calibrated in advance.

Compared with the prior art, the gas sensor disclosed by the application can achieve the purposes of simplifying preparation work and improving detection efficiency.

Drawings

The utility model is described in detail below with reference to examples and figures, wherein:

FIG. 1 is a schematic flow chart of a method for detecting gaseous solutes according to an embodiment of the present utility model;

FIG. 2 shows a graph of absorption coefficient of 420ppm CO ₂ and 2% H ₂ O as a function of wavelength at 1 atmosphere and 298K temperature;

FIG. 3 shows a schematic diagram of a gas sensor according to an embodiment of the present utility model;

FIG. 4 shows a schematic of the diffusion of dissolved gases in PDMS membrane separation water;

FIG. 5 shows a graph of the atomic diffusion rate of PDMS film versus CO ₂ over time;

FIG. 6 illustrates a schematic diagram of the operation of a gas sensor provided in accordance with an embodiment of the present utility model;

Fig. 7 shows a graph of photoacoustic signal and noise as a function of frequency;

FIG. 8 shows a graph of signal-to-noise ratio as a function of frequency;

FIG. 9 shows a graph of measured electrical signal as a function of concentration of CO ₂/N₂ gas mixture;

FIG. 10 is a schematic diagram showing the result of AllaN-Werle bias analysis based on the noise distribution of a gas sensor;

FIG. 11 shows a graph of measured electrical signals as a function of test time;

Fig. 12 shows a graph of the measured electrical signal as a function of test time for different flow rates.

Detailed Description

For the purpose of making the objects, technical solutions and advantages of the present utility model more apparent, the embodiments of the present utility model will be described in further detail with reference to the accompanying drawings. Examples of the embodiments are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to like or similar elements throughout, or elements having like or similar functionality. The embodiments described below by referring to the drawings are illustrative only and are not to be construed as limiting the utility model.

The utility model discloses a detection method of a gaseous solute, please refer to fig. 1, comprising the following steps:

Acquiring a gas mixed solute in a solution to be detected;

Selecting one point/section from infrared wave bands as an irradiation wave band;

Placing the gas mixed solute in the photoacoustic cavity, and irradiating the gas mixed solute by an excitation light source of an irradiation wave band;

Acquiring sound signals in the photoacoustic cavity;

and processing the sound signal to obtain the concentration of the gas to be detected in the solution to be detected.

Through the steps, the excitation light source can directly irradiate the gas-mixed solute in the photoacoustic cavity, so that the gas-mixed solute absorbs electromagnetic radiation of the excitation light source under the irradiation of the excitation light source, the temperature of gas-mixed solute molecules is increased, the pressure in the photoacoustic cavity is changed, photoacoustic pressure waves are further formed, the concentration of the gas to be measured in the solution to be measured can be obtained after the photoacoustic pressure waves are detected and then processed, and the equipment does not need to be heated or calibrated in advance.

Specifically, under irradiation of an excitation light source, gas molecules within the photoacoustic cavity will absorb energy, transition from a ground state to an excited state, and then return to the ground state through non-radiative impact relaxation. As the energy is released, it will be converted into translational kinetic energy of the molecule. When the excitation light source is modulated, either directly or indirectly, the light intensity will vary periodically, as will the energy absorbed by the gas within the photoacoustic cavity. An increase in the translational kinetic energy of the gas molecules means an increase in the gas temperature. In the photoacoustic cavity with good air tightness, the air pressure increases. The periodically varying thermal energy will produce a periodic pressure change, thereby forming photoacoustic pressure waves. The weak pressure wave signal generated by the photoacoustic can be detected by a high-sensitivity acoustic wave detector. After being converted into an electric signal or other types of signals, the sound signals are further processed by a signal processing circuit to obtain concentration information of the gas to be detected in the solution to be detected.

The amplitude of the photoacoustic signal can be expressed as:

In this formula, P represents the incident optical power, α is the absorption coefficient of the gas, γ is the heat capacity ratio, r is the radius of the photoacoustic cavity, a is the radius of the incident beam, ω is the frequency of the acoustic signal, τ ₀ is the damping time of the plane wave, determined by the heat conduction of the gas.

In the aspect of trace gas detection, the detection performance of the gas sensor can be directly determined through a proper absorption spectrum line. Different gases have different differences in the absorption intensity of radiant energy in different wave bands, so that one point/one section in the infrared wave band is required to be selected as an irradiation wave band according to the different gases.

For example, CO ₂ has a relatively weak absorption in the near infrared band, but a higher absorption in the mid-infrared band, which is advantageous for the selective and high sensitivity detection of CO ₂. The optical filter should be selected to have a passband that, firstly, should cover as much of the strong absorption peak of the absorption band of CO ₂ as possible and, secondly, should avoid overlapping with the absorption band of other gases and high concentrations of water in the environment as much as possible. Obviously, the application can detect the concentration of other gases in liquid besides the concentration of CO ₂ in liquid.

It should be noted that the light wave band emitted by the excitation light source should include a selected point/a section of the irradiation wave band to aim at the absorption intensity of the gas to be measured to different energies. In the actual operation process, the light wave band emitted by the excitation light source is not necessarily completely adapted to the selected one point/one section of the irradiation wave band, so that the light wave band emitted by the excitation light source can be processed in a filtering manner, and the required irradiation wave band is obtained.

It should be further noted that the photoacoustic cavity can be a resonant photoacoustic cavity or a non-resonant photoacoustic cavity, and when the resonant photoacoustic cavity is selected for testing, a resonance curve of the resonant photoacoustic cavity along with acoustic vibration needs to be obtained first, and noise brought by the resonant photoacoustic cavity is eliminated after the acoustic signal in the photoacoustic cavity is obtained. By selecting a non-resonant photoacoustic cavity that does not resonate with the photoacoustic pressure wave, the above-described steps are not required.

In detecting dissolved gases in seawater, common methods include Gas Chromatography (GC) techniques, mass Spectrometry (MS) techniques, and optical techniques. GC has the characteristics of low sample consumption, high precision, and linear response over a wide concentration range. However, the GC-based technique for detecting dissolved gases in seawater is complicated in operation, the analysis results cannot reflect the real-time concentration of CO ₂, and the chromatographic column is vulnerable to contamination and needs to be replaced periodically, thus being unsuitable for long-term continuous monitoring of dissolved CO ₂ in water. MS is a method for measuring dissolved gas in seawater with high sensitivity and wide dynamic range, but the structure is relatively complex, a strong electric field, a strong magnetic field and a high vacuum environment are needed, and the equipment is expensive, large in volume and complex in structure, so that the application scene is limited.

The utility model discloses a detection method of a gaseous solute, which is an optical method and simultaneously uses a photoacoustic spectrum technology in infrared absorption spectrum detection technology. The optical method has advantages of high sensitivity and rapid response in gas detection compared to the two methods described above. The optical method can specifically use a Raman spectrum technology and an infrared absorption spectrum detection technology to detect the gas, the Raman spectrum technology determines the existence and the concentration of the gas by measuring the frequency shift of Raman scattered light after the Raman spectrum technology interacts with the gas in water, and compared with the Raman spectrum technology, the infrared absorption spectrum detection technology has higher detection sensitivity.

The infrared absorption spectrum detection technology can be further specifically classified into non-dispersive infrared (NDIR), annular decease (CRDS), cavity Enhanced Absorption (CEAS), tunable Diode Laser Absorption (TDLAS) and Photoacoustic (PAS) technologies. NDIR uses a wide spectrum light source and an optical filter to perform spectrum analysis, light is focused on an infrared detector after passing through the gas, the intensity of an electric signal converted from the intensity of the light field is in direct proportion to the concentration of the measured gas, and the method is easy to be influenced by temperature and needs to calibrate zero drift and standard concentration periodically. CRDS and CEAS increase the equivalent absorption path by reflecting laser light multiple times between two high-reflectivity mirrors in the resonator, but ultra-high-reflectivity precision mirrors are costly and the instrument setup is complex. TDLAS uses a detector to measure the laser light intensity after passing through the gas under test, and analyzes the relationship between absorbance and concentration by fitting a baseline, but generally has a large gas chamber, which is not conducive to miniaturization of the system. Therefore, the application has the advantages of small volume, low cost and low power consumption by using the photoacoustic spectroscopy PAS technology, and simultaneously gives consideration to the sensitivity in the measuring process.

In some embodiments, the step of obtaining a gaseous mixed solute in a solution to be tested comprises:

and acquiring a gas mixed solute in the solution to be detected through a polymer membrane.

The membrane separation technique uses a polymeric material as an interface between the gas 9 and the liquid phase 10. When the two phases are not in direct contact, gas-liquid separation is achieved by a pressure difference or a concentration difference across the separation membrane 8. This process can be described as a "solution-diffusion" model, as shown in fig. 4. The concentration of a substance in a liquid phase or a gas in a membrane can be expressed as:

C=PS(2)

In this model, P is the equilibrium partial pressure of the gas and S is the dissolution coefficient. In this model, the solubility coefficient of the gas is only dependent on temperature and salinity.

The diffusion process of the gas in the membrane can be expressed by the simplified fick's first law:

Here J is the permeation flux of the gas in the membrane, D is the diffusion coefficient of the gas, C is the substance concentration of the gas in the membrane, and x is the distance.

The permeability coefficient of the membrane is determined by the solubility and diffusion coefficients of the gas and can be expressed as

H=SD(4)

In combination with equations (2), (3) and (4), equation (3) may be expressed as

This shows that when the permeability of the gas in the membrane is constant, the permeation flux of the gas is proportional to the concentration gradient.

The volume fraction of the target gas in the gas cell can be expressed as

Where C0 is the volume fraction of dissolved gas in seawater, k is the Henry coefficient, A is the effective area of the membrane, V is the volume of the chamber, d is the thickness of the membrane, cg0 and Cg are the volume fractions of the target gas in the chamber at the initial time and at any time, respectively. Assuming that the gas volume fraction ratio in the gas chamber is r=c _g/9.87kC₀, equation (6) can be expressed as

In order to shorten the time for water-gas separation, on the one hand, the separation membrane needs to be in direct contact with water, which requires the membrane to have pressure resistance, salt and alkali resistance and good air permeability. On the other hand, factors such as permeability, thickness, effective area and the like of the separation membrane material can influence the efficiency of water-gas separation, so that dissolved gas in the seawater can effectively pass through. Among these influencing factors, the permeability of the membrane material plays a critical role in gas diffusion.

In other embodiments, the step of obtaining a gaseous mixed solute in a solution to be measured includes:

the gas mixed solute in the solution to be tested is obtained by changing the pressure/passing through the filter medium/ionization.

In some embodiments, the step of selecting a point/segment from the infrared band as the illumination band includes:

and selecting one point/one section as an irradiation wave band according to the absorption intensity of the gas to be detected on different wavelengths in the infrared wave band and the absorption intensity of other gases on different wavelengths in the infrared wave band.

In some embodiments, the step of placing the gas-mixed solute within a photoacoustic cavity, and illuminating the gas-mixed solute by an excitation light source in an illumination wavelength band, comprises:

A gas-mixed solute is placed within the non-resonant photoacoustic cavity.

And (3) after filtering by an excitation light source of an irradiation wave band, irradiating the gas mixed solute.

In some embodiments, the gas-mixed solute is disposed within a photoacoustic cavity, the step of illuminating the gas-mixed solute by an excitation light source of an illumination wavelength band comprising:

the maximum length of the cross section of the photoacoustic cavity is not more than 6mm, and the minimum length of the cross section of the photoacoustic cavity is not less than 1.5mm.

The main parameters of the photoacoustic cavity are the radius and length of the cavity. As can be seen from equation (1), the length of the photoacoustic cavity does not affect the amplitude of the photoacoustic signal. However, as the radius of the photoacoustic cavity increases, the amplitude of the photoacoustic signal will decrease. From the standpoint of photoacoustic signal amplitude and miniaturization, the radius of the photoacoustic cavity should be as small as possible. But when the radius is too small, the diverging beam is absorbed when it impinges on the cavity wall, thereby introducing additional noise.

Based on the above analysis, a cylindrical non-resonant photoacoustic cell with an inner cavity radius of 1.5 mm and a length of 10 mm was selected to achieve a balance of miniaturization and inner wall polishing.

In some specific embodiments, the step of illuminating the gas-mixed solute with an excitation light source of an illumination band, disposed within the photoacoustic cavity, comprises:

The photoacoustic cavity is in the shape of a cylindrical cavity with a radius ranging from 0.75mm to 3mm.

The photoacoustic cavity has a volume in the range of 50 μl to 150 μl.

In some embodiments, comprising:

Acquiring a gas mixed solute in a solution to be detected;

Selecting one point/section from the middle infrared wave band as an irradiation wave band;

Acquiring sound signals in the photoacoustic cavity;

And processing the sound signal to obtain the concentration of the carbon dioxide gas in the solution to be detected.

The absorption intensity of CO ₂ in the near infrared band is relatively weak, but the absorption intensity in the middle infrared band is higher, which is beneficial to the selective and high-sensitivity detection of CO ₂.

In some embodiments, the step of selecting a point/segment from the mid-infrared band as the illumination band includes:

The wavelength range of the irradiation band is 4.06 μm to 4.46 μm.

FIG. 2 shows the absorption coefficients of 420ppm CO ₂ and 2% H2O in the 2.5 μm-11 μm band at 1 atmosphere and 298K temperature. The data is derived from the HITRAN2016 database. The water vapor has an absorption line almost in the entire spectral region, and it can be seen from FIG. 2 that there is a strong absorption peak of CO ₂ between 4.06 μm and 4.46 μm, while the H2O curve is located below the CO ₂ curve.

In some specific embodiments, the step of placing the gas-mixed solute within a photoacoustic cavity and illuminating the gas-mixed solute by an excitation light source in an illumination wavelength band comprises:

the excitation light source of the irradiation wave band irradiates the gas mixed solute after filtering, and a filter with the central wavelength of 4.27 mu m and the bandwidth of 0.05 mu m is selected for filtering.

CO ₂ has a strong absorption band around 4.26 μm and is less affected by water vapor. Thus, a narrow band optical filter with a center wavelength of 4.27 μm and a bandwidth of 0.05 μm was selected.

In some specific embodiments, the step of obtaining a gaseous mixed solute in a solution to be tested comprises:

and acquiring a gas mixed solute in the solution to be detected through a polymer membrane, wherein the polymer membrane is made of polydimethylsiloxane.

Polydimethylsiloxane (PDMS) is a polymeric material with excellent gas permeability, water repellency, good elasticity, and chemical stability. The gas dissolved in water passes through the PDMS membrane and separates into the gas phase driven by the concentration difference.

Fig. 5 shows the R value of PDMS film versus CO ₂ over time, indicating its excellent CO ₂ permeability.

The utility model also discloses a gas sensor, which comprises:

To achieve trace gas detection in the mid-infrared band, mid-infrared laser or thermal radiation sources are typically used as excitation units. Mid-infrared lasers, represented by Interband Cascade Lasers (ICL) and Quantum Cascade Lasers (QCL), have the advantage of narrow linewidth, but the high price increases the cost of the detection system. In addition, there are optical detection methods such as TDLAS, CRDS, and CEAS, which require precise adjustment of the optical path to obtain an optimal detection signal, adding to the complexity of the adjustment process. The scheme of combining the intermediate infrared band excitation unit with the optical filter and the chopper has the advantage of low cost, and can realize multi-component gas detection. In addition, the light source is close to the photoacoustic cell, and the light source divergence is small. But due to the presence of the chopper the system is relatively bulky. The medium infrared MEMS heat source can realize direct and rapid electric modulation without a chopper, and is suitable for portable and miniature application scenes. Accordingly, MEMS thermal light sources (EMIRS, AXETRIS) are selected as the excitation units.

In some embodiments, the excitation unit and the sound receiving unit are respectively located at two sides of the photoacoustic cavity, the separation unit is communicated with the photoacoustic cavity, and the processing unit is in signal connection with the excitation unit and the sound receiving unit.

In some specific embodiments, an optical filter is disposed between the excitation unit and the photoacoustic cell.

In some specific embodiments, a window lens is disposed on a side of the photoacoustic chamber adjacent to the excitation unit, and the window lens and the photoacoustic chamber are in sealing connection.

In some specific embodiments, a window lens is disposed on a side of the photoacoustic chamber adjacent to the excitation unit, the window lens being a barium fluoride window lens.

In some specific embodiments, the separation unit is a polymer membrane, one side of the polymer membrane is communicated with the photoacoustic chamber, and the other side of the polymer membrane is communicated with the solution to be tested.

In some specific embodiments, the separation unit communicates with the photoacoustic chamber through a diffusion channel having a radius in the range of 0.2mm to 0.8mm.

In some specific embodiments, the photoacoustic cell is made of brass and the inner wall is polished.

In a particularly specific embodiment, as shown in fig. 3, a gas sensor of the present disclosure includes a non-resonant photoacoustic cell 1, a diffusion channel 2, an electric microphone 3, a BaF2 window 4 (barium fluoride window lens), a mid-infrared band excitation unit 5, an optical filter 6, and a light source housing 7. The diffusivity of a gas into a non-resonant photoacoustic cell through a diffusion channel located on the side of the gas sensor is proportional to the size of the diffusion channel. However, increasing its size will cause the acoustic impedance to decrease, resulting in photoacoustic signal leakage. To accelerate the gas balance while maintaining a high acoustic impedance, a diffusion channel radius of 0.4 mm was chosen. The overall volume of the gas sensor was 2.43 cm by 1.25 cm by 1.8 cm and the gas cell volume was about 73 microliters. The miniaturization design goal is achieved by small volume and low gas consumption.

In another particular embodiment, fig. 6 shows a schematic diagram of the operation of a gas sensor according to an embodiment of the present utility model, taking as an example the detection of dissolved CO ₂ in seawater, mainly comprising a PDMS membrane 8, two stainless steel support plates 11, a membrane pump 12 (Kamoer, EDLP 600) and a simulated water chamber 13 of about 250 ml. With the help of the water pump, water passes through the surface of the PDMS membrane 8 at a certain flow rate to form dynamic water flow, and the continuous separation process is completed. The gas sensor 14 is directly connected to the analog water chamber.

The average power based on mid-infrared band excitation units is about 340 milliwatts. A narrow absorption line is obtained by an optical filter with a central wavelength of 4.27 microns and a bandwidth of 0.05 microns, corresponding to a higher absorption coefficient of CO ₂ in the mid-infrared spectrum. After modulating the excitation unit by the drive circuit 15, the light enters the photoacoustic cell directly through an optical filter and a BaF2 window (barium fluoride window lens Daheng Optics). The non-resonant photoacoustic cell is made of brass and the inner walls are polished to further reduce fundamental frequency signal interference caused by cavity wall absorption. The gas diffuses into the gas chamber through diffusion channels in the sidewalls of the photoacoustic chamber. The volume of the cavity of the gas sensor is about 73 microliters due to the compact structure. The MEMS electric microphone circuit board is arranged opposite to the excitation unit by adopting a gold deposition process, so that the dual-channel absorption is realized. A self-made Field Programmable Gate Array (FPGA) based digital lock-in amplifier is used that processes photoacoustic signals generated by intensity modulated light based on fundamental frequency intensity modulation (1 f-IM). Finally, a demodulated photoacoustic signal is obtained, which contains the concentration of the target gas.

The frequency response of the gas sensor is measured by sweeping the modulation frequency. The gas sensor was placed in a gas chamber filled with 500ppm CO ₂/N₂ mixed gas and the temperature was controlled at room temperature. The working frequency of the mid-infrared band excitation unit is 5 to 50 Hz. Therefore, the modulation frequency adjustment range in the experiment is within this range. As shown in fig. 7, the broken line 16 shows the change of the photoacoustic signal with the modulation frequency, and the photoacoustic signal reaches a maximum value at 10 hz. The fold line 17 shows the variation of noise with modulation frequency, and the noise level at each frequency was tested by injecting pure N ₂. At low frequencies, 1/f noise dominates. As shown in FIG. 8, the fold line 18 shows the variation of the signal-to-noise ratio with modulation frequency, based on which the signal-to-noise ratio (SNR) can be calculated, which is relatively high at 11 Hz. Thus, 11 hertz is selected as the modulation frequency.

The sensitivity of the gas sensor and the response of the gas sensor to the CO ₂/N₂ mixed gas with different concentrations are verified. The ratio of pure N ₂ to 1000ppm and 10000ppm CO ₂ was controlled by two Mass Flow Controllers (MFCs) to obtain CO ₂/N₂ gas mixtures of different concentrations. Experiments were completed at seven different concentrations of 2000ppm, 1500ppm, 1000ppm, 750ppm, 500ppm, 250ppm and 0 ppm. Line 19 of fig. 9 shows the photoacoustic signal values for CO ₂/N₂ gas mixtures of different concentrations. Since CO ₂ exhibits a high absorption coefficient within a specified optical filter range, its photoacoustic signal is nonlinear. The weak absorption linear approximation of the lambert-beer law is not applicable to the exponential decay of optical power caused by the absorption of high concentration CO ₂. By fitting a quadratic polynomial of the photoacoustic signal, the calculation equation of the CO ₂ gas concentration can be obtained as

Here, S represents the photoacoustic signal value of CO ₂. The coefficients of the quadratic polynomial fit are calculated as a= -0.0002, b = 2.5185, c = 93.634, and R squared is 0.9999.

To determine the Minimum Detection Limit (MDL) of the gas sensor, the gas cell is filled with pure N ₂. The integration time was set to 4 seconds and the measurement process lasted 4000 seconds. Fig. 10 shows the measured noise point 20 with a deviation (1 sigma) of 3.6 microvolts. As shown in fig. 10, an allin-Werle deviation analysis was performed, and the discrete points 21 were calculated results to evaluate the measurement sensitivity and stability of the gas sensor. The MDL of CO ₂ can be reduced from 3.88ppm for an average time of 4 seconds to 0.72ppm for 100 seconds.

The water gas balance time of a gas sensor is an important parameter of a dissolved gas detection device in water. Faster equilibration times mean shorter measurement times. This parameter is related to factors such as the structure of the photoacoustic cell, gas consumption, polymer film structure, liquid flow rate, etc. To simulate the salinity of seawater, 15 g of sodium chloride is added into 600 ml of pure water, and the mixture is left for more than 2 hours until the concentration of CO ₂ in the atmosphere and the dissolved CO ₂ in the simulated seawater reach dynamic balance. Then using a syringe to extract 270 ml of simulated seawater, injecting 30 ml of 10000ppm CO ₂ gas into the simulated seawater, shaking for 5 minutes to dynamically exchange gas phase and water phase, rapidly removing redundant gas in the syringe after standing to obtain simulated seawater, and adding the simulated seawater into a water chamber.

At normal temperature and pressure, the water pump circulates water at a flow rate of 100 ml/min on the membrane surface of the closed water chamber. The integration time was set to 10 seconds. After the water pump was turned on for 1 minute, it was turned off, and the gas sensor signal value was recorded. The CO ₂ molecules dissolved in the water diffuse through the PDMS membrane into the gas sensor chamber. Once the gas enters the gas phase, the gas sensor can immediately sense the concentration of CO ₂. The broken line 22 in fig. 11 shows the variation of experimental time with photoacoustic signal value.

At the beginning of the experiment, the gas sensor measured a concentration of CO ₂ gas of 450ppm, near the average concentration of CO ₂ in the atmosphere. After 14 water pump cycles, the concentration of CO ₂ in the gas chamber of the gas sensor reaches equilibrium. The equilibrium signal value is about 2500 microvolts, corresponding to a concentration of CO ₂ of 1042 ppm. After the 9 th water pump cycle, the equilibrium concentration reached a value of 90%, with a corresponding time constant t90 of about 11 minutes.

The boundary layer on the water side provides resistance to gas mass transfer. The thickness of the boundary layer may vary, as determined by the fluid dynamics near the membrane surface. The thicker the boundary layer, the longer the diffusion time of the gas through it. The still water will create the thickest boundary layer and the slowest diffusion time, resulting in a longer equilibration time. When a water flow creates shear forces at the water-air interface, the boundary conditions at the interface will change. Maximizing such shear forces may minimize the thickness of the boundary layer. The water pump circulation can effectively increase the shearing force of water, thereby shortening the balancing time. At the same time, the higher shearing effect can reduce the adhesion of aquatic organisms on the surface of the membrane

To further investigate the effect of membrane surface water flow rate on equilibration time, four different water pump flow rates were selected for the experiments, 100 ml/min, 140 ml/min, 300 ml/min and 600 ml/min. Other conditions were the same as the balance time experiment described above except for the difference in water pump flow rate. FIG. 12 shows the concentration of CO ₂ in the gas chamber of the gas sensor at different water pump flow rates as a function of experimental time. The flow rates of 140 ml/min, 300 ml/min and 600 ml/min reached equilibrium concentrations after the 12 th, 9 th and 5 th cycles, respectively. The corresponding time constants t90 are about 8.6 minutes, 7.2 minutes and 3.5 minutes, respectively. In these four flow rate experiments, t90 decreased with increasing flow rate. At a flow rate of 600 ml/min, t90 is less than 5 minutes.

In the description of the present specification, if the terms "embodiment" and "embodiment," "this embodiment," "in one embodiment," etc. are used, they refer to a particular feature, structure, material, or characteristic described in connection with the embodiment or example being included in at least one embodiment or example of the utility model. In this specification, a schematic representation of the above terms does not necessarily refer to the same embodiment or example, and the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

In the description of the present specification, the terms "connected", "mounted", "fixed", "arranged", "having", etc. are to be understood in a broad sense, e.g. the "connection" may be a fixed connection, a detachable connection, or an integral connection, may be a mechanical connection, may be an electrical connection, may be a direct connection, may be an indirect connection via an intermediary, or may be a communication between two elements. The specific meaning of the above terms in the present application can be understood by those of ordinary skill in the art according to the specific circumstances.

In the description of this specification, relational terms such as "first" and "second", and the like, are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises an element.

The embodiments have been described so as to facilitate a person of ordinary skill in the art in order to understand and apply the present technology, it will be apparent to those skilled in the art that various modifications may be made to these examples and that the general principles described herein may be applied to other embodiments without undue burden. Therefore, the present application is not limited to the above embodiments, and modifications in the following cases should be made within the scope of the present application, namely ① a new technical solution based on the technical solution of the present application and combined with the prior art, the technical effect produced by the new technical solution is not beyond the technical effect of the present application, ② an equivalent substitution of part of the features of the technical solution of the present application by the prior art is adopted, the produced technical effect is the same as the technical effect of the present application, ③ a development is performed based on the technical solution of the present application, the substantial content of the developed technical solution is not beyond the technical solution of the present application, and ④ an equivalent transformation made by the description and the accompanying drawings of the present application is directly or indirectly applied to other related technical fields.

Claims

1. A gas sensor, comprising:

A separation unit is used to obtain the gas mixed solute in the solution to be tested;

The excitation unit selects a point/segment from the infrared band as the irradiation band;

A photoacoustic chamber, wherein a gas mixture solute is placed in the photoacoustic cavity, and an excitation light source of an irradiation band in the excitation unit irradiates the gas mixture solute;

A sound receiving unit for acquiring sound signals in the photoacoustic cavity;

The processing unit processes the sound signal to obtain the concentration of the gas to be tested in the solution to be tested.

2. A gas sensor according to claim 1, characterized in that the excitation unit and the sound receiving unit are respectively located on both sides of the photoacoustic chamber, the separation unit is communicated with the photoacoustic chamber, and the processing unit is signal-connected with the excitation unit and the sound receiving unit.

3 . The gas sensor according to claim 2 , wherein an optical filter is provided between the excitation unit and the photoacoustic chamber.

4 . The gas sensor according to claim 2 , wherein a window lens is provided on a side of the photoacoustic chamber close to the excitation unit, and the window lens is sealed and connected to the photoacoustic chamber.

5 . The gas sensor according to claim 2 , wherein a window lens is provided on a side of the photoacoustic chamber close to the excitation unit, and the window lens is a barium fluoride window lens.

6 . The gas sensor according to claim 2 , wherein the separation unit is a polymer membrane, one side of the polymer membrane is connected to the photoacoustic chamber, and the other side of the polymer membrane is connected to the solution to be tested.

7 . The gas sensor according to claim 2 , wherein the separation unit is connected to the photoacoustic chamber through a diffusion channel, and a radius of the diffusion channel ranges from 0.2 mm to 0.8 mm.

8. A gas sensor according to claim 2, characterized in that the photoacoustic chamber is made of brass and the inner wall is polished.