Gas sensor

Abstract

The utility model relates to a gas sensor which comprises the following steps: acquiring a gas mixed solute in a solution to be detected; selecting one point / one section from the infrared wave band 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 a sound signal in the photoacoustic cavity; and processing the sound signal to obtain the concentration of the to-be-detected gas in the to-be-detected solution. 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 intensity in the photoacoustic cavity is changed, and the photoacoustic effect is improved. The concentration of the to-be-detected gas in the to-be-detected solution can be obtained after the photoacoustic pressure waves are detected and then treated, the equipment does not need to be heated or calibrated in advance, and the purposes of simplifying preparation work and improving detection efficiency can be achieved.

Description

Technical Field

[0001] The utility model relates to the technical field of gas detection, in particular to a gas sensor. Background Art

[0002] With the development of modern industrial society, human activities have led to the emission of large amounts of carbon dioxide (CO2) into the atmosphere. Over the years, the concentration of CO2 in the atmosphere has increased steadily, from 285 ppm before the 1860s to more than 400 ppm today. The ocean plays an irreplaceable role in the global carbon cycle, absorbing about a quarter of the total amount of CO2 emitted by humans into the atmosphere each year through sea-air exchange. However, as the concentration of CO2 in the atmosphere continues to increase, the rate at which the ocean absorbs CO2 is also increasing, causing the pH value of seawater to drop, causing ocean acidification, and affecting the marine ecosystem. In order to assess the impact of climate change on the ocean, it is of great significance to measure the concentration of CO2 in seawater.

[0003] Photoacoustic spectroscopy (PAS) is an indirect absorption spectroscopy technique different from tunable diode laser absorption spectroscopy (TDLAS). By detecting the amplitude of the photoacoustic pressure wave generated by the absorption of light energy by gas molecules, the concentration of the gas to be measured is inferred. This is an extremely sensitive, background-free, and low-drift detection method. In the prior art, a scheme for directly measuring the concentration of CO2 in a gas-liquid solution based on PZT piezoelectric-photoacoustic spectroscopy (PAS) is disclosed. Its advantages are that it does not require a water-gas separation component and has a fast response time, which can be completed within 60 seconds. However, its sensitivity is insufficient, and the device needs to be preheated for 30 minutes before normal use. In addition, a small QEPAS gas sensor for detecting dissolved gases in deep-sea environments is disclosed, equipped with a compact spectrometer. It has a low gas consumption of about 300 microliters, and the minimum detection limit of CH4 is 1.1ppm. However, the resonant frequency and responsiveness of the gas sensor are prone to drift, so it needs to be calibrated before use. In summary, although the existing technology of using infrared absorption spectroscopy to study dissolved gases in seawater shows high accuracy, it still faces the challenge of low detection efficiency due to complex preparation work when the equipment is running. Utility Model Content

[0004] In order to solve the defect of low detection efficiency caused by complicated preparation work when the equipment is running, the utility model proposes a gas sensor.

[0005] The technical solution adopted by the utility model is a gas sensor, comprising:

[0006] A separation unit is used to obtain the gas mixed solute in the solution to be tested;

[0007] The excitation unit selects a point / segment from the infrared band as the irradiation band;

[0008] 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;

[0009] A sound receiving unit for acquiring sound signals in the photoacoustic cavity;

[0010] The processing unit processes the sound signal to obtain the concentration of the gas to be tested in the solution to be tested.

[0011] Preferably, the excitation unit and the sound receiving unit are respectively located on two 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.

[0012] Preferably, an optical filter is provided between the excitation unit and the photoacoustic chamber.

[0013] Preferably, 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.

[0014] Preferably, 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.

[0015] Preferably, 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.

[0016] Preferably, the separation unit is connected to the photoacoustic chamber via a diffusion channel, and the radius of the diffusion channel ranges from 0.2 mm to 0.8 mm.

[0017] Preferably, the photoacoustic chamber is made of brass, and the inner wall is polished.

[0018] Compared with the prior art, the utility model has the following beneficial effects:

[0019] The present application also discloses a gas sensor, including: a separation unit, which obtains the gas mixture solute in the solution to be tested; an excitation unit, which selects a point / segment from the infrared band as the irradiation band; a photoacoustic chamber, which places the gas mixture solute in the photoacoustic cavity, and irradiates the gas mixture solute by the excitation light source of the irradiation band in the excitation unit; a sound receiving unit, which obtains the sound signal in the photoacoustic cavity; and a processing unit, which processes the sound signal to obtain the concentration of the gas to be tested in the solution to be tested. Through the above steps, the excitation light source can directly irradiate the gas mixture solute in the photoacoustic cavity, so that the gas mixture solute absorbs the electromagnetic radiation of the excitation light source under the irradiation of the excitation light source, so that the temperature of the gas mixture solute molecules increases, thereby changing the pressure in the photoacoustic cavity, and then forming a photoacoustic pressure wave. After detecting the photoacoustic pressure wave and processing it, the concentration of the gas to be tested in the solution to be tested can be obtained, without the need to heat or calibrate the equipment in advance.

[0020] Compared with the prior art, the gas sensor disclosed in the present application can simplify preparation work and improve detection efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention is described in detail below with reference to the embodiments and drawings, wherein:

[0022] Figure 1 A schematic diagram of a flow chart of a gas solute detection method provided according to an embodiment of the utility model is shown;

[0023] Figure 2 A graph showing the absorption coefficient of 420 ppm CO2 and 2% H2O as a function of wavelength at 1 atm and 298K is shown;

[0024] Figure 3 A schematic diagram of the structure of a gas sensor provided according to an embodiment of the utility model is shown;

[0025] Figure 4 A schematic diagram showing the diffusion of dissolved gases in water separated by PDMS membrane;

[0026] Figure 5 A graph showing the atomic diffusion rate of CO2 through the PDMS membrane as a function of time;

[0027] Figure 6 A schematic diagram showing the operation of a gas sensor provided according to an embodiment of the utility model is shown;

[0028] Figure 7 shows a graph of photoacoustic signal and noise as a function of frequency;

[0029] Figure 8shows a graph of signal-to-noise ratio versus frequency;

[0030] Fig. 9 A graph showing the change in the measured electrical signal with the concentration of the CO2 / N2 mixed gas;

[0031] Fig.10 A schematic diagram of AllaN-Werle deviation analysis results based on noise distribution of a gas sensor is shown;

[0032] Fig.11 A graph showing the change of the measured electrical signal with the test time;

[0033] Fig.12 A curve diagram showing the change of the measured electrical signal with the test time at different flow rates. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the utility model clearer, the embodiments of the utility model will be further described in detail below in conjunction with the accompanying drawings. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar components or components with the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the utility model, and cannot be interpreted as limiting the utility model.

[0035] The utility model discloses a method for detecting gas solutes, please refer to Figure 1 ,include:

[0036] Obtaining gas mixed solutes in the solution to be tested;

[0037] Select a point / segment from the infrared band as the irradiation band;

[0038] The gas mixture solute is placed in the photoacoustic cavity, and the gas mixture solute is irradiated by an excitation light source in an irradiation band;

[0039] Acquiring a sound signal in the photoacoustic cavity;

[0040] The sound signal is processed to obtain the concentration of the gas to be tested in the solution to be tested.

[0041] Through the above steps, the excitation light source can directly irradiate the gas mixture solute in the photoacoustic cavity, so that the gas mixture solute absorbs the electromagnetic radiation of the excitation light source under the irradiation of the excitation light source, and the temperature of the gas mixture solute molecules increases, thereby changing the pressure in the photoacoustic cavity, and then forming a photoacoustic pressure wave. After detecting the photoacoustic pressure wave and processing it, the concentration of the gas to be tested in the solution to be tested can be obtained without heating or calibrating the equipment in advance.

[0042] Specifically, under the irradiation of the excitation light source, the gas molecules in the photoacoustic cavity will absorb energy, transition from the ground state to the excited state, and then return to the ground state through non-radiative collision relaxation. As the energy is released, this energy will be converted into the translational kinetic energy of the molecules. When the excitation light source is modulated directly or indirectly, the light intensity will change periodically, and the energy absorbed by the gas in the photoacoustic cavity will also change periodically. The increase in the translational kinetic energy of the gas molecules means an increase in the temperature of the gas. In a photoacoustic cavity with good airtightness, the gas pressure will increase. The periodically changing thermal energy will produce periodic pressure changes, thereby forming photoacoustic pressure waves. The weak pressure wave signal generated by photoacoustics can be detected by a highly sensitive sound wave detector. After the sound signal is converted into an electrical signal or other types of signals, it is further processed by the signal processing circuit to obtain the concentration information of the gas to be tested in the solution to be tested.

[0043] The amplitude of the photoacoustic signal can be expressed as:

[0044]

[0045] In this formula, P represents the incident light 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 light beam, ω is the frequency of the sound signal, and τ0 is the damping time of the plane wave, which is determined by the thermal conduction of the gas.

[0046] In terms of trace gas detection, the detection performance of the gas sensor can be directly determined by the appropriate absorption spectrum line. Different gases have different absorption strengths of radiation energy in different bands, so it is necessary to select a point / segment in the infrared band as the irradiation band according to different gases.

[0047] For example, the absorption intensity of CO2 in the near-infrared band is relatively weak, but the absorption intensity in the mid-infrared band is higher, which is conducive to the selective and highly sensitive detection of CO2. The selection of optical filters should meet two conditions: first, the passband of the optical filter should cover the strong absorption peak of the CO2 absorption band as much as possible; second, it should avoid overlapping with the absorption bands of other gases and high-concentration water in the environment as much as possible. Obviously, in addition to detecting the concentration of CO2 in liquid, the present application can also detect the concentration of other gases in liquid.

[0048] It should be noted that the light wave band emitted by the excitation light source should include a selected point / segment of irradiation band to target the absorption intensity of the gas to be tested for different energies. However, in actual operation, the light wave band emitted by the excitation light source may not be completely adapted to the selected point / segment of irradiation band. Therefore, the light wave band emitted by the excitation light source can be processed by filtering to obtain the required irradiation band.

[0049] It should also be noted that the photoacoustic cavity can be a resonant photoacoustic cavity or a non-resonant photoacoustic cavity. When a resonant photoacoustic cavity is used for the test, it is necessary to first obtain the resonance curve of the resonant photoacoustic cavity vibrating with the sound wave, and then eliminate the noise caused by the resonant photoacoustic cavity after obtaining the sound signal in the photoacoustic cavity. By selecting a non-resonant photoacoustic cavity that does not resonate with the photoacoustic pressure wave, the above steps are not required.

[0050] In the detection of dissolved gases in seawater, common methods include gas chromatography (GC), mass spectrometry (MS) and optical technology. GC has the characteristics of low sample consumption, high precision and linear response in a wide concentration range. However, the operation process of GC-based dissolved gas detection technology in seawater is complicated, the analysis results cannot reflect the real-time concentration of CO2, and the chromatographic column is easily contaminated and needs to be replaced regularly. Therefore, it is not suitable for long-term continuous monitoring of dissolved CO2 in water. MS is a method for measuring dissolved gases in seawater with high sensitivity and wide dynamic range, but its structure is relatively complex and requires strong electric fields, strong magnetic fields and high vacuum environments. In addition, the equipment is expensive, large in size and complex in structure, which limits its application scenarios.

[0051] The detection method of a gas solute disclosed in the utility model is an optical method, and the photoacoustic spectroscopy technology in the infrared absorption spectrum detection technology is used at the same time. Compared with the above two methods, the optical method has the advantages of high sensitivity and rapid response in gas detection. The optical method can specifically use Raman spectroscopy technology and infrared absorption spectrum detection technology for gas detection. Raman spectroscopy technology determines the presence and concentration of gas by measuring the frequency shift of Raman scattered light after interaction with gas in water. Compared with Raman spectroscopy technology, infrared absorption spectrum detection technology has a higher degree of detection sensitivity.

[0052] The infrared absorption spectrum detection technology can be specifically subdivided into non-dispersive infrared spectroscopy (NDIR), circular decay spectroscopy (CRDS), cavity enhanced absorption spectroscopy (CEAS), tunable diode laser absorption spectroscopy (TDLAS) and photoacoustic spectroscopy (PAS) technology. NDIR uses a wide spectrum light source and an optical filter for spectral analysis. After the light passes through the gas, it is focused on the infrared detector. The intensity of the electrical signal converted from the light field intensity is proportional to the concentration of the measured gas. This method is susceptible to temperature and requires regular calibration of zero drift and standard concentration. CRDS and CEAS reflect the laser light multiple times between two high-reflection mirrors in the resonator to increase the equivalent absorption light path, but the ultra-high reflectivity precision mirror is expensive and the instrument setting is complicated. TDLAS uses a detector to measure the intensity of the laser light after passing through the measured gas, and analyzes the relationship between absorbance and concentration by fitting the baseline, but it usually has a larger gas chamber, which is not conducive to system miniaturization. Therefore, the present application uses the photoacoustic spectroscopy PAS technology to have the advantages of small size, low cost and low power consumption, while taking into account the sensitivity during the measurement process.

[0053] In some embodiments, the step of obtaining the gas mixed solute in the solution to be tested includes:

[0054] The gas mixed solute in the solution to be tested is obtained through the polymer membrane.

[0055] Membrane separation technology uses polymer materials as the interface between the gas 9 and liquid phases 10. When the two phases are not in direct contact, the gas-liquid separation is achieved through the pressure difference or concentration difference on both sides of the separation membrane 8. This process can be described as a "solution-diffusion" model, such as Figure 4 The substance concentration of the gas in the liquid phase or film can be expressed as:

[0056] C=PS(2)

[0057] In this model, P is the equilibrium partial pressure of the gas and S is the solubility coefficient. In this model, the solubility coefficient of the gas is only related to temperature and salinity.

[0058] The diffusion process of gas in the membrane can be expressed by the simplified Fick's first law:

[0059]

[0060] Here, J is the permeation flux of the gas in the membrane, D is the diffusion coefficient of the gas, C is the species concentration of the gas in the membrane, and x is the distance.

[0061] The permeability coefficient of the membrane is determined by the solubility coefficient and diffusion coefficient of the gas and can be expressed as

[0062] H=SD(4)

[0063] Combining equations (2), (3) and (4), equation (3) can be expressed as

[0064]

[0065] 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.

[0066] The volume fraction of the target gas in the gas chamber can be expressed as

[0067]

[0068] Where C0 is the volume fraction of gas dissolved in seawater, k is Henry's coefficient, A is the effective area of ​​the membrane, V is the volume of the gas chamber, d is the thickness of the membrane, Cg0 and Cg are the volume fractions of the target gas in the gas chamber at the initial moment and at any moment, respectively. Assume that the gas volume fraction ratio in the gas chamber is R = C g / 9.87kC0, then equation (6) can be expressed as

[0069]

[0070] In order to shorten the time of water-gas separation, on the one hand, the separation membrane needs to be in direct contact with water, which requires the membrane to be pressure-resistant, salt-resistant and have good air permeability. On the other hand, factors such as the permeability, thickness and effective area of ​​the separation membrane material will affect the efficiency of water-gas separation, allowing the dissolved gas in seawater to pass effectively. Among these influencing factors, the permeability of the membrane material plays a key role in gas diffusion.

[0071] In other embodiments, the step of obtaining the gas mixed solute in the solution to be tested includes:

[0072] The gas mixed solute in the solution to be tested is obtained by changing the pressure / passing through the filter medium / ionization.

[0073] In some embodiments, the step of selecting a point / segment from the infrared band as the irradiation band includes:

[0074] According to the absorption intensity of the gas to be tested at different wavelengths in the infrared band and the absorption intensity of other gases at different wavelengths in the infrared band, a point / segment is selected as the irradiation band.

[0075] In some embodiments, the step of placing a gas mixture solute in a photoacoustic cavity and irradiating the gas mixture solute with an excitation light source in an irradiation band includes:

[0076] A gas mixed solute is placed in a non-resonant photoacoustic cavity.

[0077] In some embodiments, the step of placing a gas mixture solute in a photoacoustic cavity and irradiating the gas mixture solute with an excitation light source in an irradiation band includes:

[0078] The gaseous mixed solute is irradiated by the excitation light source of the irradiation band after being filtered.

[0079] In some embodiments, the gas mixture solute is placed in a photoacoustic cavity, and the step of irradiating the gas mixture solute with an excitation light source in an irradiation band includes:

[0080] The maximum length of the cross section of the photoacoustic cavity is not greater than 6 mm, and the minimum length of the cross section of the photoacoustic cavity is not less than 1.5 mm.

[0081] 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 decreases. From the perspective of photoacoustic signal amplitude and miniaturization, the radius of the photoacoustic cavity should be as small as possible. However, when the radius is too small, the divergent light beam will be absorbed when it hits the cavity wall, thereby introducing additional noise.

[0082] Based on the above analysis, a cylindrical non-resonant photoacoustic chamber with an inner radius of 1.5 mm and a length of 10 mm was selected to achieve a balance between miniaturization and inner wall polishing.

[0083] In some specific embodiments, the gas mixture solute is placed in a photoacoustic cavity, and the step of irradiating the gas mixture solute with an excitation light source in an irradiation band includes:

[0084] The shape of the photoacoustic cavity is a cylindrical cavity with a radius ranging from 0.75 mm to 3 mm.

[0085] In some specific embodiments, the gas mixture solute is placed in a photoacoustic cavity, and the step of irradiating the gas mixture solute with an excitation light source in an irradiation band includes:

[0086] The volume of the photoacoustic cavity ranges from 50 μL to 150 μL.

[0087] In some embodiments, including:

[0088] Obtaining gas mixed solutes in the solution to be tested;

[0089] Select a point / segment from the mid-infrared band as the irradiation band;

[0090] The gas mixture solute is placed in the photoacoustic cavity, and the gas mixture solute is irradiated by an excitation light source in an irradiation band;

[0091] Acquiring a sound signal in the photoacoustic cavity;

[0092] The sound signal is processed to obtain the concentration of carbon dioxide gas in the solution to be tested.

[0093] The absorption intensity of CO2 in the near-infrared band is relatively weak, but the absorption intensity in the mid-infrared band is higher, which is conducive to the selective and highly sensitive detection of CO2.

[0094] In some specific embodiments, the step of selecting a point / segment from the mid-infrared band as the irradiation band includes:

[0095] The wavelength range of the irradiation band is 4.06 μm to 4.46 μm.

[0096] Figure 2 The absorption coefficients of 420ppm CO2 and 2% H2O in the 2.5μm-11μm band are shown under 1 atm and 298K. The data is from the HITRAN2016 database. Water vapor has absorption lines in almost the entire spectral region, which are Figure 2It can be seen that there is a strong absorption peak of CO2 between 4.06μm and 4.46μm, and the H2O curve is below the CO2 curve.

[0097] In some specific embodiments, the step of placing a gas mixture solute in a photoacoustic cavity and irradiating the gas mixture solute with an excitation light source in an irradiation band includes:

[0098] The gaseous mixed solute is irradiated after the excitation light source of the irradiation band is filtered, and a filter with a central wavelength of 4.27 μm and a bandwidth of 0.05 μm is selected for filtering.

[0099] CO2 has a strong absorption band near 4.26μm and is less affected by water vapor. Therefore, a narrow-band optical filter with a central wavelength of 4.27μm and a bandwidth of 0.05μm was selected.

[0100] In some specific embodiments, the step of obtaining the gas mixed solute in the solution to be tested includes:

[0101] The gas mixed solute in the solution to be tested is obtained through a polymer membrane, and the polymer membrane is made of polydimethylsiloxane.

[0102] Polydimethylsiloxane (PDMS) is a polymer material with excellent gas permeability, water resistance, good elasticity and chemical stability. Gases dissolved in water pass through the PDMS membrane and separate into the gas phase driven by concentration differences.

[0103] Figure 5 The R value of the PDMS membrane to CO2 changes over time, indicating its excellent CO2 permeability.

[0104] The utility model also discloses a gas sensor, comprising:

[0105] A separation unit is used to obtain the gas mixed solute in the solution to be tested;

[0106] The excitation unit selects a point / segment from the infrared band as the irradiation band;

[0107] 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;

[0108] A sound receiving unit for acquiring sound signals in the photoacoustic cavity;

[0109] The processing unit processes the sound signal to obtain the concentration of the gas to be tested in the solution to be tested.

[0110] In order to realize the detection of trace gases in the mid-infrared band, mid-infrared lasers or thermal radiation sources are usually used as excitation units. Mid-infrared lasers represented by interband cascade lasers (ICL) and quantum cascade lasers (QCL) have the advantages 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 the best detection signal, increasing the complexity of the adjustment process. The solution of combining the mid-infrared band excitation unit with an optical filter and a chopper has the advantage of low cost and can realize multi-component gas detection. In addition, the light source is close to the photoacoustic chamber and the divergence of the light source is small. However, due to the presence of the chopper, the system volume is relatively large. The mid-infrared MEMS heat source can realize direct and fast electrical modulation without the need for a chopper, which is suitable for portable and miniature application scenarios. Therefore, a MEMS thermal light source (EMIRS200, AXETRIS) was selected as the excitation unit.

[0111] In some embodiments, the excitation unit and the sound receiving unit are respectively located on two 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.

[0112] In some specific embodiments, an optical filter is provided between the excitation unit and the photoacoustic chamber.

[0113] In some specific embodiments, 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.

[0114] In some specific embodiments, 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.

[0115] In some specific embodiments, 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.

[0116] In some specific embodiments, the separation unit is connected to the photoacoustic chamber via a diffusion channel, and the radius of the diffusion channel ranges from 0.2 mm to 0.8 mm.

[0117] In some specific embodiments, the photoacoustic chamber is made of brass, and the inner wall is polished.

[0118] In a particularly specific embodiment, Figure 3As shown, a gas sensor disclosed in the utility model includes a non-resonant photoacoustic chamber 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 shell 7. The diffusion rate of the gas entering the non-resonant photoacoustic chamber through the diffusion channel located on the side of the gas sensor is proportional to the size of the diffusion channel. However, increasing its size will lead to a decrease in acoustic impedance, thereby causing leakage of photoacoustic signals. In order to maintain high acoustic impedance while accelerating gas equilibrium, a diffusion channel radius of 0.4 mm was selected. The overall volume of the gas sensor is 2.43 cm × 1.25 cm × 1.8 cm, and the gas chamber volume is about 73 microliters. The miniaturization design goal is achieved through small volume and low gas consumption.

[0119] In another particularly specific embodiment, Figure 6 The schematic diagram of the working of a gas sensor provided according to an embodiment of the utility model is shown. Taking the detection of dissolved CO2 in seawater as an example, it mainly includes a PDMS membrane 8, two stainless steel support plates 11, a diaphragm pump 12 (Kamoer, EDLP600) 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 a dynamic water flow and complete the continuous separation process. The gas sensor 14 is directly connected to the simulated water chamber.

[0120] The average power of the excitation unit based on the mid-infrared band is about 340 milliwatts. A narrow absorption spectral line is obtained through an optical filter with a central wavelength of 4.27 microns and a bandwidth of 0.05 microns, corresponding to the higher absorption coefficient of CO2 in the mid-infrared spectrum. After the excitation unit is modulated by the driving circuit 15, the light directly enters the photoacoustic chamber through the optical filter and the BaF2 window (barium fluoride window lens, Daheng Optics). The non-resonant photoacoustic chamber is made of brass, and the inner wall is polished to further reduce the fundamental frequency signal interference caused by the absorption of the cavity wall. The gas diffuses into the gas chamber through the diffusion channel on the side wall of the photoacoustic chamber. Due to the compact structure, the cavity volume of the gas sensor is about 73 microliters. The MEMS electric microphone circuit board adopts the gold immersion process and is installed opposite the excitation unit to achieve dual-channel absorption. A homemade digital phase-locked amplifier based on a field programmable gate array (FPGA) is used, which processes the photoacoustic signal generated by the intensity modulated light based on the fundamental frequency intensity modulation (1f-IM). Finally, a demodulated photoacoustic signal is obtained, which contains the concentration of the target gas.

[0121] The frequency response of the gas sensor is measured by scanning the modulation frequency. The gas sensor is placed in a gas chamber filled with 500ppm CO2 / N2 mixed gas and the temperature is controlled at room temperature. The operating 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. Figure 7 As shown, the broken line 16 shows the change of the photoacoustic signal with the modulation frequency, and the photoacoustic signal reaches the maximum value at 10 Hz. The broken line 17 shows the change of the noise with the modulation frequency. The noise level at each frequency is tested by injecting pure N2. At low frequencies, 1 / f noise is dominant. Figure 8 As shown, the line 18 shows how the signal-to-noise ratio varies with the modulation frequency. Based on the measured signal and noise, the signal-to-noise ratio (SNR) can be calculated, which is relatively high at 11 Hz. Therefore, 11 Hz is selected as the modulation frequency.

[0122] The sensitivity of the gas sensor and its response to CO2 / N2 mixed gas of different concentrations were verified. The ratio of pure N2 to 1000ppm and 10000ppm CO2 was controlled by two mass flow controllers (MFCs) to obtain CO2 / N2 gas mixtures of different concentrations. The experiments were completed at seven different concentrations of 2000ppm, 1500ppm, 1000ppm, 750ppm, 500ppm, 250ppm and 0ppm. Fig. 9 Line 19 shows the photoacoustic signal values ​​of CO2 / N2 mixed gas with different concentrations. Since CO2 exhibits a high absorption coefficient within the specified optical filter range, its photoacoustic signal is nonlinear. The weak absorption linear approximation condition of the Lambert-Beer law is not applicable to the exponential attenuation of optical power caused by high concentration CO2 absorption. By fitting the quadratic polynomial of the photoacoustic signal, the calculation equation for CO2 gas concentration can be obtained as

[0123]

[0124] Here, S represents the photoacoustic signal value of CO 2. The coefficients of the quadratic polynomial fitting were calculated to be a=-0.0002, b=2.5185, c=93.634, and the R-squared value was 0.9999.

[0125] To determine the minimum detection limit (MDL) of the gas sensor, the gas chamber was filled with pure N2. The integration time was set to 4 seconds and the measurement process lasted for 4000 seconds. Fig.10 The measured noise point 20 shows a deviation (1σ) of 3.6 microvolts. Fig.10As shown, Allan-Werle deviation analysis was performed and discrete point 21 is the calculation result to evaluate the measurement sensitivity and stability of the gas sensor. The MDL of CO2 can be reduced from 3.88ppm at 4 seconds averaging time to 0.72ppm at 100 seconds.

[0126] The water-gas equilibrium time of the gas sensor is an important parameter of the dissolved gas detection equipment in water. Faster equilibrium time means shorter measurement time. This parameter is related to factors such as the structure of the photoacoustic chamber, gas consumption, polymer membrane structure, and liquid flow rate. To simulate the salinity of seawater, 15 grams of sodium chloride was added to 600 ml of pure water and left for more than 2 hours until the CO2 concentration in the atmosphere reached a dynamic equilibrium with the dissolved CO2 in the simulated seawater. Then use a syringe to extract 270 ml of simulated seawater, and inject 30 ml of 10,000 ppm CO2 gas into it. The gas phase and the water phase were dynamically exchanged by shaking for 5 minutes. After standing, the excess gas in the syringe was quickly removed to obtain simulated seawater and added to the water chamber.

[0127] At room temperature and pressure, the water pump circulates water on the membrane surface of the closed water chamber at a flow rate of 100 ml / min. The integration time is set to 10 seconds. The water pump is turned on for 1 minute and then turned off, and the gas sensor signal value is recorded. The CO2 molecules dissolved in the water diffuse into the gas sensor chamber through the PDMS membrane. Once the gas enters the gas phase, the gas sensor can immediately sense the concentration of CO2. Fig.11 The broken line 22 in FIG. 2 shows the change of the photoacoustic signal value with the experimental time.

[0128] At the beginning of the experiment, the CO2 gas concentration measured by the gas sensor was 450ppm, which is close to the average concentration of CO2 in the atmosphere. After 14 water pump cycles, the CO2 concentration in the gas sensor chamber reached equilibrium. The equilibrium signal value was about 2500 microvolts, corresponding to a CO2 concentration of 1042ppm. After the 9th water pump cycle, the equilibrium concentration reached 90% of the value, and the corresponding time constant t90 was about 11 minutes.

[0129] The boundary layer on the water side provides resistance to gas mass transfer. The thickness of the boundary layer can vary and is determined by the fluid dynamics near the membrane surface. The thicker the boundary layer, the longer it takes for gases to diffuse through it. Still water produces the thickest boundary layer and the slowest diffusion time, resulting in longer equilibrium times. When water flow creates shear forces at the water-air interface, it changes the boundary conditions at the interface. Maximizing this shear force minimizes the thickness of the boundary layer. Pumping water can effectively increase the shear force on the water, thereby shortening the equilibrium time. At the same time, the higher shear effect can reduce the attachment of aquatic organisms to the membrane surface.

[0130] In order to further study the effect of water flow rate on the membrane surface on the equilibrium time, four different water pump flow rates of 100 ml / min, 140 ml / min, 300 ml / min and 600 ml / min were selected for the experiment. Except for the different water pump flow rates, other conditions were the same as the above equilibrium time experiment. Fig.12 The figure shows the variation of CO2 concentration in the gas sensor chamber with experimental time at different water pump flow rates. The flow rates of 140 ml / min, 300 ml / min, and 600 ml / min reach equilibrium concentration after the 12th, 9th, and 5th cycles, respectively. The corresponding time constants t90 are approximately 8.6 minutes, 7.2 minutes, and 3.5 minutes, respectively. In these four flow rate experiments, t90 decreases with increasing flow rate. At a flow rate of 600 ml / min, t90 is less than 5 minutes.

[0131] In the description of this specification, if the terms "embodiment one", "this embodiment", "in an embodiment" and the like appear, it means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the utility model or the utility model. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example; moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in an appropriate manner.

[0132] In the description of this specification, the terms "connect", "install", "fix", "set", "have", etc. are all understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0133] In the description of this specification, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprises", "comprising" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the statement "comprising a ..." does not exclude the presence of other identical elements in the process, method, article or device including the element.

[0134] The above description of the embodiments is to facilitate ordinary technicians in the technical field to understand and apply the technology of this case. People familiar with the technology in this field can obviously make various modifications to these examples easily and apply the general principles described here to other embodiments without creative work. Therefore, this case is not limited to the above embodiments. Modifications to the following situations should all be within the scope of protection of this case: ① A new technical solution based on the technical solution of the utility model and combined with existing common knowledge, the technical effect produced by the new technical solution does not exceed the technical effect of the utility model; ② The equivalent replacement of some features of the technical solution of the utility model by using known technology, the technical effect produced is the same as the technical effect of the utility model; ③ The technical solution of the utility model can be expanded based on the technical solution of the utility model, and the substantive content of the expanded technical solution does not exceed the technical solution of the utility model; ④ The equivalent transformation made by using the contents of the specification and drawings of the utility model is directly or indirectly applied to other related technical fields.

Claims

1. A gas sensor, characterized in that: include: 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 two 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. A gas sensor according to claim 2, characterized in that: An optical filter is provided between the excitation unit and the photoacoustic chamber.

4. A gas sensor according to claim 2, characterized in that: A window lens is provided on one side of the photoacoustic chamber close to the excitation unit, and the window lens is sealed and connected to the photoacoustic chamber.

5. A gas sensor according to claim 2, characterized in that: A window lens is provided on one side of the photoacoustic chamber close to the excitation unit, and the window lens is a barium fluoride window lens.

6. A gas sensor according to claim 2, characterized in that: 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. A gas sensor according to claim 2, characterized in that: The separation unit is connected to the photoacoustic chamber through a diffusion channel, and the 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.

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