A fast measuring, dynamic range enhanced in-situ ocean gas sensor and its application

Abstract

The application relates to a fast measuring and dynamic range enhanced marine in-situ gas sensor and application thereof, and belongs to the technical field of marine measurement. The sensor comprises a shell, a watertight plug is arranged on one side of the shell, the watertight plug is connected with a communication module, a laser is fixed in the shell, the laser is connected with a temperature control module and a constant current driving module respectively, an optical fiber collimator is connected to an output end of the laser, a gas absorption cell is connected to the optical fiber collimator, a first photoelectric detector and a second photoelectric detector are arranged on the gas absorption cell, the first photoelectric detector and the second photoelectric detector are sequentially connected with a transimpedance amplifier and a signal demodulation module, and the gas absorption cell is connected with a seawater flow system. The application realizes the splicing of an optical path of two orders of magnitude, increases the linear dynamic range of the gas sensor, reduces the volume of the gas absorption cell while increasing the area of a degassing film, thereby improving the degassing efficiency and reducing the response time of the sensor.

Description

Technical Field

[0001] This invention relates to a rapid measurement, dynamic range-enhanced in-situ marine gas sensor and its applications, belonging to the field of marine measurement technology. Background Technology

[0002] The ocean covers approximately 71% of the Earth's surface and is a major component of the hydrosphere, one of the Earth's five major spheres, playing a crucial role in climate change, the carbon cycle, and biological evolution. In recent years, with the discovery and exploitation of methane hydrate (combustible ice), the ocean has gradually gained global attention in the energy sector. Regarding climate change, phenomena associated with the greenhouse effect, such as global warming, sea-level rise, and frequent extreme weather events, have attracted considerable attention. CH4 and CO2 are two major greenhouse gases, widely distributed in ocean waters. 27% of CO2 emitted by human activities is absorbed by the ocean, playing a regulatory role in atmospheric CO2 concentration and climate change through air-sea interactions. Furthermore, the amount of dissolved carbon dioxide is closely related to phytoplankton photosynthesis, seawater pH, carbonate levels, and other biochemical phenomena, and is influenced by seawater temperature, salinity, pressure, the Kuroshio Current, warm and cold water masses, and small- and medium-scale eddies. Therefore, the concentration of dissolved CO2 in the ocean has far-reaching implications and is an essential parameter in marine fisheries, ecological protection, and primary productivity estimation. As a product of biochemical processes in the ocean, CH4, in addition to its role in energy exploration, has become an important marker for studying the evolution of special areas such as deep-sea hydrothermal vents and cold seeps. Therefore, marine dissolved gas detection technology, which measures CO2 and CH4, plays a crucial supporting and guiding role in marine resource exploration and marine ecological change studies.

[0003] Commonly used in-situ detection technologies for dissolved gases in water include underwater Raman spectroscopy, semiconductor gas sensing, mass spectrometry, and infrared absorption spectroscopy. Raman spectroscopy, in particular, detects the intensity of Raman scattered light to reflect information about the analyte, allowing for direct detection of dissolved gases underwater. However, due to the relatively weak intensity of Raman scattered light, underwater Raman spectroscopy has limited sensitivity and detection range, and is mostly used for qualitative detection. With the advent of polymer degassing membranes, semiconductor gas sensing, mass spectrometry, and infrared absorption spectroscopy have been developed in the field of seawater dissolved gas detection. Infrared absorption spectroscopy, in particular, has seen rapid development due to its high measurement accuracy and ease of encapsulation. However, the degassing efficiency of the degassing membrane results in a long overall sensor response time. For example, the PMEL MAPCO2 system, operated and maintained by the National Oceanic and Atmospheric Administration (NOAA), has a response time of up to 20 minutes for detecting CO2 in water.

[0004] Furthermore, the concentration of dissolved gases in seawater can vary greatly. For example, the concentrations of dissolved CO2 and CH4 are high in areas rich in natural gas hydrates at the sea surface and on the seabed, while the concentrations are very low in other areas. Therefore, in addition to measurement sensitivity, the dynamic range provided by the sensor is also very important. The CO2-Pro sensor developed by Pro-Oceanus, a Canadian company, and... Their measurement ranges are 0-600ppm and 0-10000ppm, with accuracies of ±0.5% and ±3% of the maximum range, respectively. This indicates that while increasing the measurement range, the instrument cannot adequately guarantee measurement sensitivity. In their paper "A Prototype of High-Precision Carbon Isotopic Ratio Sensing System for CO2 Dissolved in Water," Chen Chen's team at Jilin University described their developed marine dissolved CO2 detection system. This system utilizes an improved Herriott multi-pass cell, achieving an effective optical path of 24m while maintaining a gas chamber volume of 275mL, thus improving the detection range and sensitivity. However, the excessively large multi-pass cell volume results in a long response time when combined with a water-gas separation device.

[0005] Currently available commercial marine gas sensors cannot meet the measurement requirements of certain scenarios in terms of dynamic measurement range and response time. Published research also struggles to achieve both long optical path lengths and compact size with rapid degassing. Therefore, this invention addresses this need by designing a rapid measurement and enhanced dynamic range in-situ marine gas sensor. The sensor's gas absorption cell employs a ring-shaped reflective cavity integrated with a beam splitter, achieving optical path length splicing across two orders of magnitude, thus increasing the sensor's linear dynamic range. The ring-shaped reflective cavity within the gas absorption cell is designed as a flattened structure with bidirectional openings for degassing, reducing the volume of the gas absorption cell while increasing the degassing membrane area, thereby improving degassing efficiency and reducing the sensor's response time. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a marine in-situ gas sensor with rapid measurement and enhanced dynamic range. The gas absorption cell adopts a design with an integrated beam splitter using a ring-shaped reflective cavity, achieving optical path splicing across two orders of magnitude and increasing the linear dynamic range of the gas sensor. Furthermore, the ring-shaped reflective cavity within the gas absorption cell is designed as a flat structure with bidirectional opening for degassing, reducing the volume of the gas absorption cell while increasing the degassing membrane area, thereby improving degassing efficiency and reducing the sensor's response time.

[0007] The present invention also provides applications of the above-mentioned rapid measurement and dynamic range enhanced marine in-situ gas sensor.

[0008] The technical solution of the present invention is as follows:

[0009] A rapid measurement and enhanced dynamic range marine in-situ gas sensor includes a housing, within which are housed a watertight plug, a communication module, a microcontroller, a temperature control module, a constant current drive module, a transimpedance amplifier, a signal demodulation module, an optical fiber collimator, a gas absorption cell, and a seawater circulation system.

[0010] A watertight plug is provided on one side of the housing, and a communication module is connected to the watertight plug. The communication module receives and transmits command signals and measurement signals. A laser is fixed inside the housing. The laser is connected to a temperature control module and a constant current drive module. The laser output is connected to an optical fiber collimator. The optical fiber collimator is connected to a gas absorption cell. A first photodetector and a second photodetector are provided on the gas absorption cell. The first photodetector and the second photodetector are connected to a transimpedance amplifier and a signal demodulation module in sequence. The gas absorption cell is connected to a seawater circulation system. The communication module, signal demodulation module, temperature control module, constant current drive module, and seawater circulation system are all connected to a microcontroller.

[0011] According to a preferred embodiment of the present invention, the communication module, transimpedance amplifier, signal demodulation module, temperature control module, constant current drive module, and seawater circulation system are all connected to a power supply module, which provides power to the entire system.

[0012] According to a preferred embodiment of the present invention, the seawater circulation system includes a seawater circulation pool, a flow limiting valve, a solenoid valve, a pressure reducing valve, a filter, and a booster pump. The seawater circulation pool is fixedly installed inside the shell. One end of the seawater circulation pool is sequentially connected to the flow limiting valve, the solenoid valve, and the pressure reducing valve. The pressure reducing valve is connected to a filter fixed to the outside of the shell. The solenoid valve is connected to a microcontroller. The other end of the seawater circulation pool is connected to the booster pump. The booster pump is connected to the microcontroller through a motor driver and a motor control module. The output end of the booster pump is connected to the drain outlet on the shell through a pipe.

[0013] According to a preferred embodiment of the present invention, a one-way valve is provided on the pipeline between the booster pump and the drain outlet to prevent seawater backflow.

[0014] According to a preferred embodiment of the present invention, the gas absorption cell includes a fixed block, a degassing membrane, a sintered metal block, drying paper, a sealing ring, an annular reflective cavity, and a beam splitter. The annular reflective cavity is disposed inside the seawater flow tank. Sealing rings are respectively disposed on both sides of the annular reflective cavity. Drying paper is disposed inside the sealing rings. The sintered metal block, the degassing membrane, and the fixed block are sequentially disposed outside the drying paper. An inlet is disposed on one side of the annular reflective cavity. A beam splitter is disposed inside the inlet. An optical fiber collimator, a first photodetector, and a second photodetector are disposed outside the beam splitter. The laser beam exits from the optical fiber collimator and passes through the beam splitter. 10% of the laser beam's light energy... The light is reflected by the beam splitter and enters the second photodetector. 90% of the laser beam's energy passes through the beam splitter and enters the annular reflection cavity. After multiple reflections within the annular reflection cavity, it is received by the first photodetector. The sintered metal block acts as a support to prevent water flow from damaging the degassing membrane and disrupting the entire gas detection system. The drying paper filters out water vapor to prevent water molecules from interfering with gas detection. The sealing ring and degassing membrane provide a watertight seal to prevent seawater from entering the gas absorption pool. Simultaneously, the degassing membrane degasses the gas, and the degassed gas enters the annular reflection cavity for detection.

[0015] According to a preferred embodiment of the present invention, the inner wall of the annular reflective cavity is composed of multiple closely fitted spherical surfaces. The spherical surfaces are equivalent to having curvature in both the vertical and horizontal directions, which can suppress the divergence of the light source.

[0016] According to a preferred embodiment of the present invention, after the laser beam enters the annular reflecting cavity, the reflection propagation path within the annular reflecting cavity is a star-shaped polygon. After reflection, it exits from the entrance port. The star-shaped polygon is defined by two parameters: the number p of spherical surfaces (i.e., reflecting surfaces), and the number q of line segments within the short arc divided by the vertices of the star-shaped polygon, where:

[0017]

[0018] The length of the entire optical path is expressed by the following formula:

[0019] l=p*D*cos(θ) (2)

[0020] D is the diameter of the annular reflector. By fixing the diameter of the annular reflector and changing the angle of the incident light, different optical paths can be achieved.

[0021] According to a preferred embodiment of the present invention, the annular reflective cavity between the two dried papers is a gas chamber. When the laser beam spot diameter is 0.85 mm, the gas chamber thickness is compressed to 1.5 mm, and the gas chamber volume is 11.9 mL. Furthermore, the annular reflective cavity inside the gas absorption cell adopts a bidirectional opening design, and the total area of ​​the two degassing membranes is 157.08 cm². 2By increasing the degassing membrane area while reducing the gas chamber volume, the sensor's response time is reduced.

[0022] According to a preferred embodiment of the present invention, a first watertight partition and a second watertight partition are arranged side by side in the middle of the shell, a seawater circulation pool is arranged on one side of the second watertight partition, and a gas absorption pool is fixed to the second watertight partition.

[0023] The application of the aforementioned rapid measurement and enhanced dynamic range marine in-situ gas sensor follows these steps:

[0024] (1) Connect the watertight plug to the host computer, then place the gas sensor in the ocean setting position. Seawater enters the pressure reducing valve through the filter. The filter removes large molecular impurities and seabed sediments to prevent the circulation system from being blocked. The pressure reducing valve reduces the high pressure seawater to normal pressure. Normal pressure seawater enters the solenoid valve.

[0025] (2) During operation, seawater enters the flow limiting valve through the solenoid valve. The flow limiting valve regulates the flow rate and velocity of the seawater, and then enters the seawater circulation pool. The microcontroller sends the temperature control command to the temperature control module. The temperature control module controls the laser to work at a constant temperature. At the same time, the microcontroller emits a high-frequency sine wave and a low-frequency scanning sawtooth wave. The sine wave and sawtooth wave are transmitted to the constant current drive module, which converts them into current signals to drive the laser and realize the wavelength scanning and modulation output of the laser.

[0026] (3) The seawater in the seawater circulation pool passes through the gas absorption pool, and after being degassed by the degassing membrane, it enters the annular reflection cavity. The laser beam is emitted from the fiber collimator and passes through the beam splitter. 10% of the light energy of the laser beam is reflected by the beam splitter, and the reflected light enters the second photodetector. 90% of the light energy of the laser beam passes through the beam splitter and enters the annular reflection cavity. After multiple reflections in the annular reflection cavity, it is received by the first photodetector.

[0027] (4) After receiving the signal, the first photodetector and the second photodetector convert photons into electrons to form a photocurrent. The photocurrent enters the transimpedance amplifier and is converted into a voltage. At the same time, the signal is amplified. The voltage signal is further amplified by the signal demodulation module and the demodulated output fundamental harmonic signal and second harmonic signal are transmitted to the microcontroller. The microcontroller performs calculations to obtain the concentration of the gas to be measured. Then, the concentration of the gas to be measured is uploaded to the host computer for display through the watertight plug.

[0028] (5) The seawater flows through the seawater circulation pool to the booster pump. The booster pump is connected to the motor driver. The motor driver controls the booster pump to complete the cycle of forward rotation-stop-reverse rotation-stop, and discharges the normal pressure seawater into the external high pressure environment.

[0029] (6) When the machine stops working, the solenoid valve closes, seawater no longer enters the seawater circulation pool, and the laser stops working.

[0030] According to a further preferred embodiment of the present invention, in step (4), after the first photodetector and the second photodetector receive the signal, the emitted light intensity is I, the incident light intensity is I0, and the relationship between the emitted light intensity and the incident light intensity is:

[0031] I = I0e -k(λ)cl (3)

[0032] Where k is the absorption coefficient of the gas molecules to be measured, which is directly determined by the wavelength and is also affected by temperature, pressure and humidity; c is the concentration of the gas to be measured; and l is the optical path length to be measured. Theoretically, if the optical path length is known, the concentration of the gas can be obtained by measuring the ratio of the incident light intensity I0 to the outgoing light intensity.

[0033] Utilizing the harmonic detection principle, in addition to being tuned with a sawtooth wave, the driving current is also modulated by a high-frequency sine wave. Then, after passing through a transimpedance amplifier and a signal demodulation module, the fundamental and higher harmonics are demodulated for analysis of the target gas concentration, as shown in the following equation:

[0034] I0=I(1+nsin(2πft)) (4)

[0035] υ=υ0+υ f sin(2πft) (5)

[0036] I is the initial light intensity, I0 is the emitted light intensity, n is the modulation coefficient of the light intensity, f is the frequency of the modulation current, υ is the output wavelength after modulation, and υ0 is the wavelength of the initial light intensity. f Substituting equations (4) and (5) into equation (3) to obtain the amplitude of the frequency modulation, we get:

[0037] I1=I(1+n sin(2πft))exp(―k(υ0+sin(2πft))cl) (6)

[0038] The absorption coefficient k is generally of Lorentz linear form. Taking a Fourier transform of the above equation yields the fundamental and second harmonic values ​​of the gas absorption, as shown in the following formulas:

[0039] I f =nI0 (7)

[0040]

[0041] In the formula, Δυ is the full width at half maximum (FWHM) of the absorption spectrum of the gas to be measured, and k0 is the section coefficient of the absorption spectrum of the gas to be measured.

[0042] The fundamental and second harmonics demodulated by the signal demodulation module are fed into the microcontroller for calculation to obtain the concentration of the gas to be measured.

[0043]

[0044] The beneficial effects of this invention are as follows:

[0045] This invention provides a marine in-situ gas sensor with rapid measurement and enhanced dynamic range. The gas absorption cell adopts a design of annular reflective cavity integrated with a beam splitter, which realizes optical path splicing across two orders of magnitude, increasing the linear dynamic range of the gas sensor. Moreover, the annular reflective cavity in the gas absorption cell is designed as a flat structure with bidirectional opening degassing, which reduces the volume of the gas absorption cell while increasing the degassing membrane area, thereby improving degassing efficiency and reducing the sensor response time. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of the structure of the present invention;

[0047] Figure 2 This is a schematic diagram of the gas absorption cell in this invention;

[0048] Figure 3 This is a schematic diagram of the seawater circulation pool in this invention;

[0049] Figure 4 This is a schematic diagram of the adjustable optical path in the gas absorption cell of the present invention.

[0050] 1. Watertight plug; 2. Communication module; 3. Microcontroller; 4. Power supply module; 5. Temperature control module; 6. Constant current drive module; 7. Laser; 8. Transimpedance amplifier; 9. Signal demodulation module; 10. Motor driver; 11. Motor control module; 12. First watertight partition; 13. Second watertight partition; 14. Gas absorption tank; 15. Seawater circulation tank; 16. Flow limiting valve; 17. Solenoid valve; 18. Pressure reducing valve; 19. Filter; 20. Drain outlet; 21. Check valve; 22. Booster pump; 23. First photodetector; 24. Second photodetector; 25. Fiber optic collimator.

[0051] 14-1, Fixing block; 14-2, Degassing membrane; 14-3, Metal sintered block; 14-4, Drying paper; 14-5, Sealing ring; 14-6, Annular reflective cavity; 14-7, Beam splitter.

[0052] 15-1, Fixing screw; 15-2, Water inlet; 15-3, Sealing ring; 15-4, Water outlet. Detailed Implementation

[0053] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0054] Example 1:

[0055] like Figure 1-4 As shown, this embodiment provides a marine in-situ gas sensor with rapid measurement and enhanced dynamic range, including a housing. Inside the housing are a watertight plug 1, a communication module 2, a microcontroller 3, a temperature control module 5, a constant current drive module 6, a transimpedance amplifier 8, a signal demodulation module 9, an optical fiber collimator 25, a gas absorption cell 14, and a seawater circulation system.

[0056] A watertight plug 1 is provided on one side of the housing. The watertight plug 1 is connected to a communication module 2. The communication module 2 receives and transmits command signals and measurement signals. A laser 7 is fixed inside the housing. The laser 7 is connected to a temperature control module 5 and a constant current drive module 6. The output end of the laser 7 is connected to an optical fiber collimator 25. The optical fiber collimator 25 is connected to a gas absorption cell 14. A first photodetector 23 and a second photodetector 24 are provided on the gas absorption cell 14. The first photodetector 23 and the second photodetector 24 are connected to a transimpedance amplifier 8 and a signal demodulation module 9 in sequence. The gas absorption cell 14 is connected to a seawater circulation system. The communication module 2, the signal demodulation module 9, the temperature control module 5, the constant current drive module 6, and the seawater circulation system are all connected to a microcontroller 3.

[0057] The communication module 2, transimpedance amplifier 8, signal demodulation module 9, temperature control module 5, constant current drive module 6, and seawater circulation system are all connected to the power supply module 4, which provides power to the entire system.

[0058] The seawater circulation system includes a seawater circulation pool 15, a flow limiting valve 16, a solenoid valve 17, a pressure reducing valve 18, a filter 19, and a booster pump 22. The seawater circulation pool 15 is fixed inside the shell by fixing screws 15-1. The inlet 15-2 at one end of the seawater circulation pool 15 is connected in sequence to the flow limiting valve 16, the solenoid valve 17, and the pressure reducing valve 18. The pressure reducing valve 18 is connected to the filter 19 fixed on the outside of the shell. The solenoid valve 17 is connected to a microcontroller 3. The outlet 15-4 at the other end of the seawater circulation pool 15 is connected to the booster pump 22. The booster pump 22 is connected to the microcontroller 3 through a motor driver 10 and a motor control module 11. The output end of the booster pump 22 is connected to the drain outlet 20 on the shell through a pipe.

[0059] The above-mentioned filter uses a 1250-mesh stainless steel filter core, which can effectively filter out large molecular particles in seawater and prevent the entire pipeline from clogging; the communication module uses RS-485 communication; the temperature control module uses a temperature control system based on the integrated chip MAX1978 to ensure that the laser operates precisely at a certain temperature; the constant current module driver uses ATLS1.5A104, which has high output current accuracy and high stability; the power supply module is powered by an external 24V DC power supply. The power supply module has a DC / DC converter that outputs 12V DC power, and a conversion module based on the BL9342 chip that can convert 12V to 5V, and then the 5V power supply is stabilized at 3.3V by an AMS1117-3.3 voltage regulator.

[0060] The gas absorption cell includes a fixed block 14-1, a degassing membrane 14-2, a sintered metal block 14-3, a drying paper 14-4, a sealing ring 14-5, an annular reflective cavity 14-6, and a beam splitter 14-7. The annular reflective cavity 14-6 is located inside the seawater circulation tank 15. Sealing rings 14-5 are respectively arranged on both sides of the annular reflective cavity 14-6. Drying paper 14-4 is placed inside the sealing rings 14-5. The sintered metal block 14-3, the degassing membrane 14-2, and the fixed block 14-1 are arranged sequentially outside the drying paper 14-4. An inlet is provided on one side of the annular reflective cavity 14-6. A beam splitter 14-7 is arranged inside the inlet. An optical fiber collimator 25, a first photodetector 23, and a second photodetector 24 are arranged outside the beam splitter 14-7. The laser beam originates from the optical fiber... After exiting the collimator, the laser beam passes through a beam splitter. 10% of the laser beam's energy is reflected by the beam splitter, and the reflected light enters the second photodetector. 90% of the laser beam's energy passes through the beam splitter and enters the annular reflection cavity. After undergoing multiple reflections within the annular reflection cavity, it is received by the first photodetector. The ratio of reflected light to transmitted light is 1:9, thus achieving the sensor's wide dynamic range advantage. The sintered metal block acts as a support to prevent water flow from damaging the degassing membrane and disrupting the entire gas detection system. The drying paper filters out water vapor, preventing water molecules from interfering with gas detection. The sealing ring and degassing membrane provide a watertight seal, preventing seawater from entering the gas absorption pool. Simultaneously, the degassing membrane degasses the gas, and the degassed gas enters the annular reflection cavity for detection.

[0061] The inner wall of the annular reflective cavity 14-6 is composed of multiple closely fitted spherical surfaces. The spherical surfaces are essentially curved in both the vertical and horizontal directions, which can suppress the divergence of the light source.

[0062] After the laser beam enters the annular reflecting cavity, its reflection propagation path within the cavity is a star-shaped polygon. After reflection, it exits from the entrance. The star-shaped polygon is defined by two parameters: the number of spherical surfaces (i.e., reflecting surfaces), p; and the number of line segments (q) within the short arc, divided by the vertices of the star-shaped polygon. Where:

[0063]

[0064] The length of the entire optical path is expressed by the following formula:

[0065] l=p*D*cos(θ) (2)

[0066] D is the diameter of the annular reflector. By fixing the diameter of the annular reflector and changing the angle θ of the incident light, different optical paths can be achieved.

[0067] The annular reflective cavity between the two drying sheets serves as a gas chamber. When the laser beam spot diameter is 0.85 mm, the gas chamber thickness is compressed to 1.5 mm, and the gas chamber volume is 11.9 mL. Furthermore, the annular reflective cavity inside the gas absorption cell employs a bidirectional opening design, and the total area of ​​the two degassing membranes is 157.08 cm². 2 By increasing the degassing membrane area while reducing the gas chamber volume, the sensor's response time is reduced.

[0068] The application of the aforementioned rapid measurement and enhanced dynamic range marine in-situ gas sensor involves the following steps:

[0069] (1) Connect the watertight plug to the host computer, then place the gas sensor in the ocean setting position. Seawater enters the pressure reducing valve through the filter. The filter removes large molecular impurities and seabed sediments to prevent the circulation system from being blocked. The pressure reducing valve reduces the high pressure seawater to normal pressure. Normal pressure seawater enters the solenoid valve.

[0070] (2) During operation, seawater enters the flow limiting valve through the solenoid valve. The flow limiting valve regulates the flow rate and velocity of the seawater, and then enters the seawater circulation pool. The microcontroller sends the temperature control command to the temperature control module. The temperature control module controls the laser to work at a constant temperature. At the same time, the microcontroller emits a high-frequency sine wave and a low-frequency scanning sawtooth wave. The sine wave and sawtooth wave are transmitted to the constant current drive module and converted into a current signal by the constant current drive module to drive the laser, realize the wavelength scanning and modulation output of the laser, and emit a laser with a center wavelength of 1653.87nm for the detection of CH4 gas.

[0071] (3) The seawater in the seawater circulation pool passes through the gas absorption pool, and after being degassed by the degassing membrane, it enters the annular reflection cavity. The laser beam is emitted from the fiber collimator and passes through the beam splitter. The diameter of the collimated spot is less than 1 mm and the divergence angle is 1.5 mrad. 10% of the light energy of the laser beam is reflected by the beam splitter. The reflected light enters the second photodetector, achieving a short absorption optical path of 3 cm, which solves the problem of nonlinearity in high-concentration gas measurement. 90% of the light energy of the laser beam passes through the beam splitter and enters the annular reflection cavity. After 35 reflections in the annular reflection cavity, it is received by the first photodetector, achieving a long absorption optical path of 3.5 m, which solves the problem of sensitivity in low-concentration gas measurement.

[0072] (4) After receiving the signal, the first photodetector and the second photodetector convert photons into electrons to form a photocurrent. The photocurrent enters the transimpedance amplifier and is converted into a voltage. At the same time, the signal is amplified. The voltage signal is further amplified by the signal demodulation module and the demodulated output fundamental harmonic signal and second harmonic signal are transmitted to the microcontroller. The microcontroller performs calculations to obtain the concentration of the gas to be measured. Then, the concentration of the gas to be measured is uploaded to the host computer for display through the watertight plug.

[0073] (5) The seawater flows through the seawater circulation pool to the booster pump. The booster pump is connected to the motor driver. The motor driver controls the booster pump to complete the cycle of forward rotation-stop-reverse rotation-stop, and discharges the normal pressure seawater into the external high pressure environment.

[0074] (6) When the machine stops working, the solenoid valve closes, seawater no longer enters the seawater circulation pool, and the laser stops working.

[0075] In step (4), after the first photodetector and the second photodetector receive the signal, the outgoing light intensity is I and the incident light intensity is I0. The relationship between the outgoing light intensity and the incident light intensity is:

[0076] I = I0e ―k(λ)cl (3)

[0077] Where k is the absorption coefficient of the gas molecules to be measured, which is directly determined by the wavelength and is also affected by temperature, pressure and humidity; c is the concentration of the gas to be measured; and l is the optical path length to be measured. Theoretically, if the optical path length is known, the concentration of the gas can be obtained by measuring the ratio of the incident light intensity I0 to the outgoing light intensity.

[0078] Utilizing the harmonic detection principle, in addition to being tuned with a sawtooth wave, the driving current is also modulated by a high-frequency sine wave. Then, after passing through a transimpedance amplifier and a signal demodulation module, the fundamental and higher harmonics are demodulated for analysis of the target gas concentration, as shown in the following equation:

[0079] I0=I(1+nsin(2πft)) (4)

[0080] υ=υ0+υ f sin(2πft) (5)

[0081] I is the initial light intensity, I0 is the emitted light intensity, n is the modulation coefficient of the light intensity, f is the frequency of the modulation current, υ is the output wavelength after modulation, and υ0 is the wavelength of the initial light intensity. f Substituting equations (4) and (5) into equation (3) to obtain the amplitude of the frequency modulation, we get:

[0082] I1=I(1+n sin(2πft))exp(―k(υ0+sin(2πft))cl) (6)

[0083] The absorption coefficient k is generally of Lorentz linear form. Taking a Fourier transform of the above equation yields the fundamental and second harmonic values ​​of the gas absorption, as shown in the following formulas:

[0084] I f =nI0 (7)

[0085]

[0086] In the formula, Δυ is the full width at half maximum (FWHM) of the absorption spectrum of the gas to be measured, and k0 is the section coefficient of the absorption spectrum of the gas to be measured.

[0087] The fundamental and second harmonics demodulated by the signal demodulation module are fed into the microcontroller for calculation to obtain the concentration of the gas to be measured.

[0088]

[0089] Example 2

[0090] A rapid measurement and dynamic range-enhanced marine in-situ gas sensor is provided, with the structure described in Example 1, except that a one-way valve 21 is provided on the pipe between the booster pump 22 and the drain outlet 20 to prevent seawater backflow.

[0091] Example 3

[0092] A rapid measurement and dynamic range-enhanced marine in-situ gas sensor has the structure described in Example 1, except that a first watertight partition 12 and a second watertight partition 13 are arranged side by side in the middle of the housing, a seawater flow pool is arranged on one side of the second watertight partition, a sealing ring 15-3 is arranged between the seawater flow pool and the second watertight partition, and a gas absorption pool is fixed to the second watertight partition.

[0093] Example 4

[0094] A rapid measurement and dynamic range-enhanced marine in-situ gas sensor is provided, with the structure described in Example 1. The difference is that the size of the gas chamber can be changed according to the laser spot. When the laser spot is less than 0.5 mm, the thickness of the gas chamber is compressed to 1 mm, and the gas chamber volume reaches 7.85 mL.

[0095] Example 5

[0096] An application of a rapid measurement, dynamic range-enhanced marine in-situ gas sensor as described in Example 1, differing in that, when detecting CO2 gas, the laser is selected as a laser with a center wavelength of 2.0 μm, and the first and second photodetectors are selected as detectors capable of detecting a peak wavelength of 2.0 μm.

Claims

1. A fast measuring, dynamic range enhanced in-situ ocean gas sensor, characterized in that, The shell is internally provided with a watertight plug, a communication module, a single-chip microcomputer, a temperature control module, a constant current driving module, a transimpedance amplifier, a signal demodulation module, a fiber collimator, a gas absorption cell and a seawater circulation system, wherein The shell is internally provided with a watertight plug, a communication module, a single-chip microcomputer, a temperature control module, a constant current driving module, a transimpedance amplifier, a signal demodulation module, a fiber collimator, a gas absorption cell and a seawater circulation system, wherein The gas absorption cell comprises a fixing block, a degassing film, a metal sintered block, a drying paper, a sealing ring, an annular reflection cavity and a light splitting sheet, the annular reflection cavity is arranged inside the seawater circulation cell, the sealing rings are arranged on the two sides of the annular reflection cavity, the drying paper is arranged in the sealing rings, the metal sintered block, the degassing film and the fixing block are sequentially arranged outside the drying paper, the annular reflection cavity is provided with an entrance, the light splitting sheet is arranged inside the entrance, and the fiber collimator, the first photodetector and the second photodetector are arranged outside the light splitting sheet.

2. The fast measuring, dynamic range enhanced, in situ ocean gas sensor of claim 1, wherein, The communication module, the transimpedance amplifier, the signal demodulation module, the temperature control module, the constant current driving module and the seawater circulation system are connected with a power supply module.

3. The fast measuring, dynamic range enhanced, in situ ocean gas sensor of claim 1, wherein, The seawater circulation system comprises a seawater circulation cell, a flow limiting valve, an electromagnetic valve, a pressure reducing valve, a filter and a booster water pump, the seawater circulation cell is fixedly arranged inside the shell, the seawater circulation cell is sequentially connected with the flow limiting valve, the electromagnetic valve and the pressure reducing valve at one end, the pressure reducing valve is connected with the filter fixed to the outside of the shell, the electromagnetic valve is connected with the single-chip microcomputer, the seawater circulation cell is connected with the booster water pump at the other end, the booster water pump is connected with the single-chip microcomputer through a motor driver and a motor control module, and the output end of the booster water pump is connected with a drain on the shell through a pipeline.

4. The fast measuring, dynamic range enhanced, in situ ocean gas sensor of claim 3, wherein, A one-way valve is arranged on the pipeline between the booster water pump and the drain.

5. The fast measuring, dynamic range enhanced, in situ ocean gas sensor of claim 4, wherein, The inner wall of the annular reflection cavity is composed of closely fitted spherical surfaces.

6. The fast measuring, dynamic range enhanced, in situ ocean gas sensor of claim 5, wherein, After the laser beam enters the annular reflection cavity, the reflection propagation route in the annular reflection cavity is a star-shaped polygon, and the laser beam is emitted from the entrance after reflection, the star-shaped polygon is defined by two parameters, the number p of the spherical surfaces, and the number q of the line segments in the short arc divided by the star-shaped polygon vertex, wherein （1） The length of the entire optical path is represented by the following formula: （2） In the formula, D is the diameter of the annular reflection cavity, the diameter of the fixed annular reflection cavity is changed, and the angle θ of the incident light is changed to realize different optical paths.

7. The fast measuring, dynamic range enhanced, in situ ocean gas sensor of claim 6, wherein, The first and second watertight partitions are arranged in parallel in the middle of the shell, the seawater circulation cell is arranged on one side of the second watertight partition, and the gas absorption cell is fixed to the second watertight partition.

8. Use of a fast measuring, dynamic range enhanced in-situ ocean gas sensor according to claim 7, characterized in that, The steps are as follows: (1) The water-tight plug is connected to the host computer, and then the gas sensor is placed in the set position in the sea. The seawater enters the pressure reducing valve through the filter, which filters out macromolecular impurities and seabed sediments to prevent the circulation system from being blocked. The pressure reducing valve reduces the high-pressure seawater to normal pressure, and the normal-pressure seawater enters the electromagnetic valve; (2) When working, the seawater enters the flow limiting valve through the electromagnetic valve, and the flow limiting valve adjusts the flow and flow rate of the seawater, and then the seawater enters the seawater flow tank. The single-chip microcomputer sends a temperature control instruction to the temperature control module, and the temperature control module controls the laser to work at a constant temperature. At the same time, the single-chip microcomputer sends a high-frequency sine wave and a low-frequency sawtooth wave. The sine wave and the sawtooth wave are transmitted to the constant current driving module and are converted into current signals by the constant current driving module to drive the laser, realizing wavelength scanning and modulated output of the laser; (3) The seawater in the seawater flow tank enters the annular reflection cavity after passing through the gas absorption cell and being degassed by the degassing film. The laser beam is emitted from the optical fiber collimator and passes through the light splitting sheet. 10% of the light energy of the laser beam is reflected by the light splitting sheet, and the reflected light enters the second photodetector. 90% of the light energy of the laser beam transmits through the light splitting sheet and enters the annular reflection cavity. After multiple reflections in the annular reflection cavity, the laser beam is received by the first photodetector; (4) After receiving the signals, the first photodetector and the second photodetector convert the photons into electrons to form a photocurrent. The photocurrent enters the transimpedance amplifier and is converted into a voltage. At the same time, signal amplification processing is performed. The voltage signal is further amplified by the signal demodulation module, and the demodulated output fundamental harmonic signal and second harmonic signal are transmitted to the single-chip microcomputer. The single-chip microcomputer calculates and processes to obtain the concentration of the measured gas, and then transmits the concentration of the measured gas to the host computer through the water-tight plug for display; (5) The seawater passing through the seawater flow tank flows to the booster water pump. The booster water pump is connected to the motor driver, and the motor driver controls the booster water pump to complete the cycle operation of forward rotation-stop-reverse rotation-stop to discharge the normal-pressure seawater to the outside high-pressure environment; (6) When stopping working, the electromagnetic valve is closed, and the seawater no longer enters the seawater flow tank. At the same time, the laser stops working.

9. Use of a fast measuring, dynamic range enhanced in-situ ocean gas sensor according to claim 8, characterized in that, In step (4), after receiving the signals, the first photodetector and the second photodetector emit light with an intensity of I, and the incident light has an intensity of I0. The relationship between the emitted light intensity and the incident light intensity is as follows: （3） where k is the absorption coefficient of the measured gas molecules, c is the concentration of the measured gas, and l is the measured optical path; Using the harmonic detection principle, the driving current is superimposed with a high-frequency sine wave for current modulation in addition to the sawtooth wave. Then, after passing through the transimpedance amplifier and the signal demodulation module, the fundamental harmonic and the high harmonic are demodulated for target gas concentration analysis, as shown in the following formula: （4） （5） n is the modulation coefficient of light intensity, f is the frequency of the modulation current, υ is the wavelength of the output after modulation, υ0is the wavelength of the initial light intensity, υ f is the amplitude of frequency modulation, and substituting equations (4) and (5) into equation (3) gives: （6） The absorption coefficient k is a Lorentz line type. Fourier transform is performed on the above formula to obtain the fundamental harmonic value and the second harmonic value of gas absorption, as shown in the following formula: （7） （8） where Δυ is the full width at half maximum of the absorption spectrum line of the measured gas, and k0 is the cross-section coefficient of the absorption spectrum line of the measured gas; The fundamental harmonic and the second harmonic demodulated by the signal demodulation module are transmitted to the single-chip microcomputer for calculation to obtain the concentration of the measured gas: （9）。

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