An in-situ calibration system for an underwater dissolved gas sensor with an integrated self-calibration module

Abstract

This invention discloses an in-situ calibration system for an underwater dissolved gas sensor with an integrated self-calibration module. A 2004nm butterfly laser module emits laser light, and a Herriott absorption cell module allows the laser to be reflected multiple times by the detector gas, resulting in selective absorption of the laser at a characteristic wavelength to form an absorption spectrum. A synchronization control module handles power supply, communication, environmental parameter control, and calibration timing control, enabling synchronized switching between the system's measurement and calibration modes. A data processing module inverts the spectral data collected by the system to obtain dissolved gas concentration data. A self-calibration gas supply unit introduces standard gas into the Herriott absorption cell module when the calibration program starts. This invention integrates the self-calibration function with the sensor detection unit, effectively simplifying the operation and maintenance process of underwater gas monitoring, improving the long-term stability of the sensor, and making it suitable for in-situ dissolved gas monitoring in various underwater environments such as oceans, lakes, and reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of underwater gas detection technology, specifically relating to in-situ calibration of an underwater dissolved gas sensor with an integrated self-calibration module. Background Technology

[0002] The concentration distribution characteristics of dissolved gases (carbon dioxide) underwater are core data supports for research on marine environmental evolution, seabed resource exploration, and aquatic ecological monitoring. Accurate monitoring of these gases is crucial for understanding the marine carbon cycle, assessing the potential of seabed oil and gas resources, and providing early warnings of aquatic ecological disasters. With the development of detection technology, underwater dissolved gas sensors based on laser absorption spectroscopy have become the mainstream technology for underwater dissolved gas monitoring due to their outstanding advantages such as high sensitivity, high selectivity, fast response speed, and strong anti-interference ability. They are widely used in various underwater environmental monitoring scenarios, including oceans, lakes, and reservoirs.

[0003] However, during long-term underwater operation, the detection accuracy of this type of laser absorption spectroscopy (TDLAS) underwater sensor is easily affected by the continuous influence of complex underwater environmental factors, which can cause significant drift. Specifically, changes in underwater water pressure with depth can cause deformation of the sensor's optical components, fluctuations in water temperature can affect the stability of the output wavelength of the laser source, and changes in water vapor can alter the Henry's law coefficient of dissolved gases and the light transmission characteristics. All of these factors can cause the sensor's detection reference to shift. Therefore, the sensor must be calibrated regularly to ensure the reliability of the monitoring data.

[0004] Currently, the mainstream sensor calibration method in the industry is the "recovery-laboratory calibration" model. This requires recovering the entire sensor deployed underwater to a land-based laboratory, manually preparing a standard gas of known concentration, and setting up a calibration experimental platform to complete the calibration operation. This traditional calibration method has several insurmountable technical drawbacks: First, the operation process is cumbersome, requiring multiple steps such as sensor recovery, transportation, laboratory calibration, and redeployment, resulting in a long maintenance cycle and a large workload. Second, the maintenance cost is high, especially for deep-sea exploration scenarios, where sensor recovery and redeployment rely on specialized research vessels and deep-sea operation equipment, significantly increasing monitoring costs. Third, it cannot achieve in-situ real-time calibration. During the interval between sensor recovery and redeployment, the sensor is in an uncalibrated state, and the accuracy of its monitoring data cannot be guaranteed, making it difficult to meet the application requirements of long-term continuous in-situ monitoring.

[0005] To address these issues, some studies have attempted to develop improved calibration systems integrating simplified calibration components, aiming to simplify the calibration process. However, existing improved solutions still have significant technical shortcomings and are difficult to adapt to complex underwater working environments: Firstly, the standard gas supply stability is poor. Existing simplified calibration components mostly adopt passive gas release structures, which cannot accurately control the release rate and concentration of the standard gas, leading to unstable calibration benchmarks. Secondly, the calibration sequence and measurement sequence are not synchronized, lacking a high-precision timing control mechanism, which easily generates signal interference during the switching between measurement and calibration modes, affecting calibration accuracy. More importantly, existing solutions generally lack environmental parameter compensation mechanisms, failing to consider the impact of underwater water pressure, water temperature, water vapor, and other environmental parameters on the calibration process, resulting in deviations between calibration results and actual underwater working conditions, and failing to guarantee the detection accuracy of the calibrated sensor.

[0006] In summary, existing underwater dissolved gas sensor calibration technologies suffer from several drawbacks, including cumbersome calibration procedures, high maintenance costs, inability to achieve in-situ real-time calibration, and significant susceptibility to environmental interference with calibration accuracy. These shortcomings severely restrict the long-term, accurate monitoring of underwater dissolved gases. Therefore, developing an underwater dissolved gas sensor calibration system that requires no retrieval, enables automatic in-situ calibration, and possesses excellent environmental adaptability has become a pressing technical challenge in this field. Summary of the Invention

[0007] The purpose of this invention is to provide an in-situ calibration system for underwater dissolved gas sensors with an integrated self-calibration module, which solves the problems of traditional underwater gas sensors requiring retrieval for calibration, low calibration efficiency, and calibration accuracy being affected by environmental factors. This system enables in-situ automatic calibration of the sensor, improving the stability and reliability of long-term monitoring.

[0008] To achieve the above objectives, the present invention provides an in-situ calibration system for an underwater dissolved gas sensor with an integrated self-calibration module. The system includes: a 2004nm butterfly laser module, a Herriott absorption cell module, a synchronization control module, a data processing module, a self-calibration gas supply unit, and a pressure-resistant sealed housing.

[0009] The 2004nm butterfly laser module is used to emit laser light and includes a butterfly laser with a working wavelength of 2004nm, a detector, and a modulation / demodulation and temperature control module.

[0010] The Herriott absorption cell module is used to cause the laser to reflect multiple times onto the probe gas, so that the probe gas selectively absorbs the laser of a characteristic wavelength to form an absorption spectrum, which is received and analyzed by the detector.

[0011] The synchronization control module is used for power supply communication, environmental parameter control, and calibration timing control, so as to realize the timing synchronization switching between the system measurement mode and calibration mode.

[0012] The data processing module is used to invert the spectral data collected by the system to obtain dissolved gas concentration data, and has the function of calibrating the concentration data.

[0013] The self-calibrating gas supply unit includes a standard built-in gas cylinder, a pressure reducing valve, and a gas flow controller, which are used to introduce standard gas into the Herriott absorption cell module when the calibration program is started.

[0014] The pressure-resistant sealed housing is used to house and protect the aforementioned modules.

[0015] Preferably, the modulation / demodulation and temperature control module of the 2004nm butterfly laser module adopts a PID closed-loop control algorithm, with a temperature control accuracy of ±0.01℃ and a current adjustment accuracy of ±0.1mA; the modulation / demodulation unit in the modulation / demodulation and temperature control module adopts a digital lock-in amplifier circuit to extract nA-level weak photocurrent signals, with a detection signal-to-noise ratio of not less than 100:1.

[0016] Preferably, the cavity mirror of the Herriott absorption cell module has a reflectivity of not less than 99.5% at a wavelength of 2004nm, the number of laser reflections in the absorption cell is adjustable, and the optical path adjustment range is 0.5~5m.

[0017] Preferably, the synchronization control module has a built-in high-precision clock signal source with a clock frequency error of less than ±1ppm; it supports dual communication modes of RS485 and CAN bus; it integrates an environmental parameter acquisition submodule for acquiring underwater water temperature, water pressure, and salinity parameters, with an acquisition frequency of 1~10Hz and acquisition accuracies of ±0.1℃, ±0.01MPa, and ±0.1‰, respectively; the calibration timing is initiated by a preset time interval or an external trigger signal.

[0018] Preferably, the data processing module has a built-in environmental parameter compensation model and a standard spectral database. The spectral data inversion adopts a nonlinear least squares fitting algorithm. By comparing the difference between the measured concentration and the standard gas concentration, and combining the influence law of environmental parameters on the absorption coefficient, the sensor detection parameters are calibrated and corrected, eliminating the interference of environmental factors on the calibration results.

[0019] Preferably, the standard built-in gas cylinder of the self-calibrating gas supply unit has a capacity of 0.22L and is filled with the target gas at a standard concentration with a concentration error of less than ±0.5%; the pressure reducing valve outputs pressure in the range of 0.1~0.3MPa; and the gas flow controller has a flow control accuracy of ±1%FS and a flow adjustment range of 0~100sccm.

[0020] Preferably, the pressure resistance rating of the pressure-resistant sealing shell is not less than 20MPa, and the protection rating reaches IP68; the inner wall of the pressure-resistant sealing shell is provided with a heat insulation buffer layer to protect each module from the effects of underwater high pressure, low temperature and water flow impact.

[0021] Preferably, in measurement mode, the 2004nm butterfly laser module emits a modulated laser into the Herriott absorption cell module. After the laser interacts with the dissolved gas underwater, an absorption spectrum is formed. The detector receives the absorption spectrum signal and transmits it to the data processing module for inversion to obtain dissolved gas concentration data.

[0022] In calibration mode, when the preset calibration time interval is reached or the measurement data drift exceeds the preset threshold, the synchronization control module controls the system to switch to calibration mode, the self-calibration gas supply unit introduces standard gas into the Herriott absorption cell module, and the data processing module compares the difference between the measured spectrum and the standard spectrum, and completes the calibration correction in combination with environmental parameters; after calibration is completed, the system switches back to measurement mode.

[0023] Preferably, the preset calibration time interval can be set to 12 hours or 24 hours; the preset threshold is the measurement data drift amount ±2%; the calibration mode also supports manual triggering via remote host computer commands.

[0024] Preferably, the data processing module further includes a storage unit for storing measurement data, calibration data, and calibration timestamps, with a storage capacity of not less than 100,000 sets of data; the data processing module transmits data to a remote host computer through the communication interface of the synchronization control module, supporting real-time data monitoring and historical data backtracking.

[0025] Compared with the prior art, the beneficial effects of the present invention are: This invention integrates a self-calibrating gas supply unit and a calibration timing control mechanism, eliminating the need to return the sensor to the laboratory. It precisely supplies standard gas through a built-in 0.22L standard gas cylinder and can automatically trigger the calibration process according to a preset time or data drift threshold to complete in-situ calibration. This greatly simplifies the operation and maintenance process, shortens the operation and maintenance cycle, and reduces the operation and maintenance cost. It is especially suitable for monitoring scenarios that are difficult to recover, such as deep sea, and meets the needs of long-term continuous in-situ monitoring. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0027] Figure 1 This is a block diagram of the overall structure of an in-situ calibration system for an underwater dissolved gas sensor with an integrated self-calibration module, according to the present invention. Figure 2 This is a schematic diagram of the internal structure of the 2004nm butterfly laser module of the present invention; Figure 3 This is a schematic diagram of the Herriott absorption cell module of the present invention; Figure 4 This is a schematic diagram of the self-calibrating gas supply unit of the present invention; Figure 5 This is a schematic diagram of the overall appearance and sealed outer shell structure of the system of the present invention.

[0028] Figure labeling: 21-2004nm butterfly laser module, 22-modulation and demodulation unit, 23-temperature control module; 36-high reflectivity cavity mirror, 34-gas inlet; 31, 32, 33-fixing holes, 35-pressure controller interface, 61-overall structure. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention are described clearly and completely below. Obviously, the described embodiments are only a part of the embodiments of this invention, not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0030] like Figure 1 The diagram shown is an overall structural block diagram of an underwater dissolved gas sensor in-situ calibration system with an integrated self-calibration module according to the present invention. It includes a 2004nm butterfly laser module, a Herriott absorption cell module, a synchronization control module, a data processing module, a self-calibration gas supply unit, and a pressure-resistant sealed shell. The modules work together to achieve high-precision detection and in-situ automatic calibration of underwater dissolved gases.

[0031] The system operates as follows: a 2004nm butterfly laser module emits a modulated laser of a specific wavelength. After entering the Herriott absorption cell module, the laser undergoes multiple reflections and interacts fully with the dissolved gas. The gas selectively absorbs the characteristic wavelength laser, forming an absorption spectrum. The detector receives the absorption spectrum signal and transmits it to the data processing module for spectral inversion, obtaining dissolved gas concentration data. The synchronization control module coordinates the working sequence of each module and collects underwater environmental parameters. When the system enters calibration mode, the self-calibration gas supply unit introduces a standard gas of known concentration into the Herriott absorption cell module. The data processing module performs calibration correction by comparing the measured spectrum with the standard spectrum. All functional modules are installed inside a pressure-resistant, sealed housing, ensuring stable operation of the system in a high-pressure underwater environment.

[0032] The 2004nm butterfly laser module is used to emit laser light and includes a butterfly laser with a working wavelength of 2004nm, a detector, and a modulation / demodulation and temperature control module.

[0033] The modulation / demodulation and temperature control module of the 2004nm butterfly laser module adopts a PID closed-loop control algorithm, with a temperature control accuracy of ±0.01℃ and a current regulation accuracy of ±0.1mA. The modulation / demodulation unit in the modulation / demodulation and temperature control module adopts a digital lock-in amplifier circuit to extract nA-level weak photocurrent signals, with a detection signal-to-noise ratio of not less than 100:1.

[0034] like Figure 2 The diagram shown is a schematic of the internal structure of the 2004nm butterfly laser module of the present invention. The 2004nm butterfly laser module is used to emit laser light and is the core of the system's light source. It includes a butterfly laser 21 with a working wavelength of 2004nm, a detector, a modulation and demodulation unit 22, and a temperature control module 23.

[0035] The butterfly laser 21 adopts a distributed feedback (DFB) laser structure with a working wavelength of 2004nm. This wavelength precisely matches the characteristic absorption peak of carbon dioxide gas, enabling highly selective detection of dissolved carbon dioxide underwater. The butterfly laser 21 is packaged in a standard 14-pin butterfly package, integrating a thermoelectric cooler (TEC) and a thermistor for precise temperature control.

[0036] The modulation / demodulation unit 22 and temperature control module 23 employ a PID closed-loop control algorithm to precisely regulate the laser's operating temperature and drive current. Specifically, the temperature control module 23 reads the resistance value of the thermistor inside the butterfly laser 21, calculates the deviation between the current temperature and the set temperature, and generates a control signal using a proportional-integral-derivative (PID) algorithm to drive the thermoelectric cooler to regulate the laser temperature. The temperature control accuracy reaches ±0.01℃, ensuring the long-term stability of the laser wavelength. Simultaneously, the drive current is supplied by a high-precision constant current source, with a current regulation accuracy of ±0.1mA, further guaranteeing the stability of the laser output power and wavelength.

[0037] The modulation and demodulation unit 22 employs a digital lock-in amplifier circuit to extract the weak optical signal after gas absorption. This digital lock-in amplifier circuit uses the modulation signal of the butterfly laser 21 as a reference signal to perform correlation demodulation on the photocurrent signal output by the detector, effectively filtering out noise interference. It can extract nA-level weak photocurrent signals with a detection signal-to-noise ratio of not less than 100:1. In this embodiment, the modulation frequency of the modulation and demodulation unit 22 is set to 10kHz, and the filtering bandwidth of the lock-in amplifier circuit is set to 1Hz.

[0038] The detector uses a mercury cadmium telluride (MCT) infrared detector or an extended indium gallium arsenide (InGaAs) detector with a response wavelength covering 2004nm. It is used to receive the laser signal reflected by the Herriott absorption cell module, convert the optical signal into an electrical signal, and output it to the modulation and demodulation unit 22 for processing.

[0039] The Herriott absorption cell module is used to cause the laser to be reflected multiple times by the probe gas, so that the probe gas selectively absorbs the laser at a characteristic wavelength to form an absorption spectrum, which is received and analyzed by the detector. The cavity mirror of the Herriott absorption cell module has a reflectivity of not less than 99.5% at a wavelength of 2004nm, the number of laser reflections in the absorption cell is adjustable, and the optical path adjustment range is 0.5~5m.

[0040] like Figure 3 The diagram shown is a structural schematic of the Herriott absorption cell module of the present invention. The Herriott absorption cell module is used to make the laser reflect multiple times on the probe gas, thereby extending the optical path between the laser and the gas and enhancing the absorption signal intensity. The Herriott absorption cell module includes two high reflectivity cavity mirrors 36 arranged opposite each other, a gas flow cavity, a gas inlet 34, a gas outlet, a pressure controller interface 35, and fixing holes 31, 32, and 33 for fixed installation.

[0041] The high-reflectivity cavity mirror 36 is fabricated using a dielectric film coating process, achieving a reflectivity of no less than 99.5% at a wavelength of 2004nm, ensuring efficient multiple reflections of the laser within the cavity. Two cavity mirrors 36 are positioned opposite each other, forming a Herriott optical cavity structure. After entering the cavity through the entrance aperture of one cavity mirror, the laser is reflected repeatedly between the two mirrors before finally exiting through the exit aperture and being received by the detector.

[0042] By adjusting the distance between the two cavity mirrors 36 and the laser incident angle, the number of laser reflections within the absorption cell can be changed, thereby adjusting the optical path. In this embodiment, the number of laser reflections within the absorption cell is adjustable, with an optical path adjustment range of 0.5~5m, adaptable to the detection and calibration requirements of gases of different concentrations: for low-concentration gas detection, the number of reflections can be increased to extend the optical path and enhance the absorption signal; for high-concentration gas detection, the number of reflections can be reduced to avoid signal saturation.

[0043] The gas inlet 34 is connected to the outlet of the gas flow controller of the self-calibrating gas supply unit via a gas pipeline, and is used to receive standard gas or dissolved gas to be tested. The gas outlet is equipped with a one-way valve to discharge the calibrated gas or dissolved gas. The pressure controller interface 35 is used to connect a pressure sensor to monitor the gas pressure in the chamber in real time.

[0044] The fixing holes 31, 32, and 33 are used to fix the Herriott absorption tank module inside the pressure-resistant and sealed housing. The fixing holes 31 and 32 are tightened with bolts to ensure the installation stability and sealing effect of the absorption tank module.

[0045] The synchronization control module is used for power supply communication, environmental parameter control, and calibration timing control, so as to realize the timing synchronization switching between the system measurement mode and calibration mode.

[0046] The synchronization control module has a built-in high-precision clock signal source with a clock frequency error of less than ±1ppm; it supports dual communication modes of RS485 and CAN bus; it integrates an environmental parameter acquisition submodule for acquiring underwater water temperature, water pressure, and salinity parameters, with an acquisition frequency of 1~10Hz and acquisition accuracies of ±0.1℃, ±0.01MPa, and ±0.1‰, respectively; the calibration timing is initiated by a preset time interval or an external trigger signal.

[0047] The synchronization control module serves as the core of the system's control, used for power supply communication, environmental parameter control, and calibration timing control, enabling the timing-synchronous switching between the system's measurement mode and calibration mode.

[0048] The synchronization control module incorporates a high-precision clock signal source, which employs a temperature-compensated crystal oscillator (TCXO). The parameters of this clock signal source are programmable, and the clock frequency error is less than ±1ppm, ensuring the accuracy of timing control and preventing signal interference during the switching between measurement and calibration modes. In this embodiment, the clock signal source frequency is set to 10MHz.

[0049] The synchronization control module supports both RS485 and CAN bus communication modes, allowing users to select the appropriate communication method based on the specific application scenario. RS485 communication mode is suitable for long-distance underwater communication and offers strong anti-interference capabilities; CAN bus communication mode is suitable for multi-node networking applications, facilitating the construction of underwater sensor networks. In this embodiment, RS485 bus communication mode is used, with a baud rate set to 9600bps, enabling stable communication with the remote host computer and various functional modules.

[0050] The synchronization control module integrates an environmental parameter acquisition submodule, used to collect underwater water temperature, water pressure, and salinity parameters in real time, providing accurate environmental data support for calibration accuracy compensation. This environmental parameter acquisition submodule includes a water temperature sensor, a water pressure sensor, and a salinity sensor. The sensor probes are fixed to the outside of a pressure-resistant sealed housing, and the signal cable passes through the sealed interface of the housing and connects to the inside of the synchronization control module. The environmental parameter acquisition frequency is 1~10Hz, set to 5Hz in this embodiment; the acquisition accuracies are: water temperature ±0.1℃, water pressure ±0.01MPa, and salinity ±0.1‰.

[0051] The calibration timing control function of the synchronization control module supports two triggering methods: one is automatic triggering through a preset time interval, which can be set to 12 hours, 24 hours or other custom intervals; the other is activation through an external trigger signal, including a manual trigger command sent by a remote host computer, or a trigger signal automatically issued by the data processing module when it detects that the measurement data drift exceeds the threshold. In this embodiment, the preset calibration time interval is 24 hours.

[0052] The data processing module is used to invert the spectral data collected by the system to obtain dissolved gas concentration data, and has the function of calibrating the concentration data.

[0053] The data processing module is used to invert the spectral data collected by the system to obtain dissolved gas concentration data, and has the function of calibrating the concentration data. It is the data processing core of the system. The data processing module includes a microprocessor, a storage unit, and an algorithm module. The microprocessor adopts a high-performance ARM processor or DSP digital signal processor, which has powerful data computing capabilities and can complete the inversion calculation of spectral data in real time.

[0054] The data processing module incorporates a standard spectral database and an environmental parameter compensation model. The standard spectral database stores standard absorption spectra of target gases at different concentrations at a wavelength of 2004 nm. In this embodiment, standard absorption spectra of carbon dioxide concentrations of 100 ppm, 500 ppm, 1000 ppm, 5000 ppm, and 10000 ppm are pre-stored. The environmental parameter compensation model is constructed based on the correlation between water temperature, water pressure, salinity, and the gas absorption coefficient. In this embodiment, correlation functions between water temperature (0~40℃), water pressure (0.1~20MPa), salinity (0~40‰), and the carbon dioxide absorption coefficient are loaded.

[0055] The data processing module employs a nonlinear least squares fitting algorithm for spectral data inversion. This algorithm calculates the dissolved gas concentration by matching measured absorption spectra with a standard spectral database. Specifically, based on Lambert-Beer's law, the algorithm establishes a mathematical model of absorbance and gas concentration, iteratively optimizes the model to find the optimal fitting parameters, and ultimately obtains the accurate gas concentration value.

[0056] The calibration function of the data processing module compares the difference between the measured concentration and the standard gas concentration, and combines the influence of environmental parameters on the absorption coefficient to complete the calibration correction of the sensor detection parameters, eliminating the interference of environmental factors on the calibration results. Specifically, in calibration mode, the data processing module compares the measured standard gas absorption spectrum with the standard spectrum of the corresponding concentration in the built-in standard spectrum database, calculates the deviation of parameters such as spectral intensity and absorption peak position, and combines environmental parameters such as water temperature, water pressure, and salinity collected by the synchronous control module. The environmental parameter compensation model is used to correct the inverted concentration data, and the corrected detection parameters (such as signal gain, wavelength calibration coefficient, etc.) are updated in the system configuration for subsequent measurements.

[0057] The data processing module also includes a storage unit for storing measurement data, calibration data, and calibration timestamps, with a storage capacity of no less than 100,000 data sets to facilitate historical data tracing and analysis. The data processing module transmits data to a remote host computer through the communication interface of the synchronization control module, supporting real-time data monitoring and historical data backtracking.

[0058] like Figure 4 The diagram shown is a structural schematic of the self-calibrating gas supply unit of the present invention. The self-calibrating gas supply unit is used to introduce standard gas into the Herriott absorption cell module when the calibration program is started, and includes a standard built-in gas cylinder, a pressure reducing valve, a gas flow controller, and a gas pipeline.

[0059] The standard internal gas cylinder has a capacity of 0.22L and is made of high-strength, corrosion-resistant materials (such as titanium alloy or stainless steel), possessing excellent pressure resistance and corrosion resistance, and is suitable for underwater high-pressure, high-salinity environments. The cylinder is filled with a standard concentration of the target gas, with a concentration error of less than ±0.5%, ensuring the accuracy of the calibration benchmark. In this embodiment, the cylinder is filled with a 1000ppm concentration of carbon dioxide standard gas. The cylinder has excellent sealing performance, ensuring the stability of the standard gas during long-term storage; under normal operating conditions, the standard gas has a shelf life of no less than two years.

[0060] The pressure reducing valve is used to reduce the pressure of the high-pressure gas in the gas cylinder to a pressure range suitable for the operation of the absorption tank, with an output pressure range of 0.1~0.3MPa. The pressure reducing valve adopts a diaphragm structure, possessing good pressure stability and adjustment accuracy, ensuring a constant standard gas pressure supplied to the absorption tank. In this embodiment, the output pressure of the pressure reducing valve is set to 0.2MPa.

[0061] The gas flow controller employs a high-precision mass flow controller (MFC), achieving a flow control accuracy of ±1%FS and a flow adjustment range of 0~100 sccm. It is used to precisely control the introduction rate and volume of the standard gas, ensuring the stability of the standard gas concentration within the absorption cell and establishing a repeatable calibration environment. In this embodiment, the calibration flow rate of the gas flow controller is set to 50 sccm, and the standard gas introduction time is set to 30 seconds.

[0062] The self-calibrating gas supply unit also includes a solenoid valve assembly for controlling the opening and closing of the gas path and switching. In this embodiment, the solenoid valve assembly includes two normally closed solenoid valves and one normally open solenoid valve: normally closed solenoid valve 1 controls the introduction of zero gas (pure nitrogen), normally closed solenoid valve 2 controls the introduction of standard gas, and normally open solenoid valve 3 controls the opening and closing of the measurement gas path. In measurement mode, normally closed solenoid valves 1 and 2 remain closed when not energized, and normally open solenoid valve 3 remains open when not energized, ensuring unobstructed measurement gas path; in calibration mode, the opening and closing of the corresponding solenoid valves are controlled according to the calibration procedure to achieve accurate supply of zero gas or standard gas.

[0063] like Figure 5 The diagram shows the overall appearance and sealed housing structure of the system of the present invention. The pressure-resistant sealed housing 61 is used to accommodate and protect the above-mentioned modules. The pressure-resistant sealed housing is made of titanium alloy or high-strength engineering plastic, with a pressure resistance of not less than 20MPa, and can adapt to underwater environments with water depths of 0 to 2000 meters. The protection level reaches IP68, which has excellent waterproof and dustproof performance, ensuring the reliability of the system in long-term underwater operation.

[0064] The inner wall of the pressure-resistant sealed outer shell is equipped with a heat-insulating buffer layer, made of polyurethane foam or other heat-insulating buffer materials, to protect each module from the effects of underwater high pressure, low temperature, and water flow impact. Specifically, the heat-insulating buffer layer can reduce the impact of underwater temperature changes on internal optical components and electronic components, maintaining the relative stability of the system's internal temperature; at the same time, the buffer layer can absorb water flow impact and mechanical vibration during deployment, protecting precision optical components from damage.

[0065] The sealing interface of the pressure-resistant sealing shell adopts an O-ring sealing structure, including an electrical interface, a communication interface, and an environmental sensor interface. Each interface uses a special underwater sealing connector to ensure sealing reliability in high-pressure underwater environments.

[0066] All functional modules are fixed inside a pressure-resistant, sealed housing, with shock-absorbing pads placed between the modules to further reduce the impact of mechanical vibration on system performance.

[0067] In measurement mode, the 2004nm butterfly laser module emits a modulated laser into the Herriott absorption cell module. The laser interacts with the dissolved gas underwater to form an absorption spectrum. The detector receives the absorption spectrum signal and transmits it to the data processing module for inversion to obtain dissolved gas concentration data.

[0068] In calibration mode, when the preset calibration time interval is reached or the measurement data drift exceeds the preset threshold, the synchronization control module controls the system to switch to calibration mode, the self-calibration gas supply unit introduces standard gas into the Herriott absorption cell module, and the data processing module compares the difference between the measured spectrum and the standard spectrum, and completes the calibration correction in combination with environmental parameters; after calibration is completed, the system switches back to measurement mode.

[0069] The preset calibration time interval can be set to 12 hours or 24 hours; the preset threshold is the measurement data drift amount ±2%; the calibration mode also supports manual triggering via remote host computer commands.

[0070] The data processing module also includes a storage unit for storing measurement data, calibration data, and calibration timestamps, with a storage capacity of no less than 100,000 sets of data. The data processing module transmits data to a remote host computer through the communication interface of the synchronization control module, supporting real-time data monitoring and historical data backtracking.

[0071] The system of this invention has two working states: measurement mode and calibration mode. The workflow of the system in measurement mode and calibration mode is as follows: In measurement mode, the system performs the following workflow: Step 1: After the system is powered on, the synchronous control module initializes each module, and the temperature control module 23 starts PID closed-loop control to stabilize the temperature of the butterfly laser 21 to the set value (25℃ in this embodiment).

[0072] Step 2: Modulation and demodulation unit 22 starts laser modulation, and butterfly laser 21 emits modulated laser with a wavelength of 2004nm. The laser enters the Herriott absorption cell module.

[0073] Step 3: The laser is reflected multiple times between the high-reflectivity cavity mirrors 36, and fully interacts with the underwater dissolved gas passing through the gas flow cavity. The dissolved gas (such as carbon dioxide) selectively absorbs the characteristic wavelength laser, forming an absorption spectrum.

[0074] Step 4: The detector receives the absorption spectrum signal, converts the optical signal into an electrical signal, extracts the weak signal through the lock-in amplifier circuit of the modulation and demodulation unit 22, and then transmits it to the data processing module.

[0075] Step 5: The environmental parameter acquisition submodule of the synchronous control module collects water temperature, water pressure, and salinity data in real time and transmits them synchronously to the data processing module.

[0076] Step 6: The microprocessor of the data processing module calls the nonlinear least squares fitting algorithm to invert the spectral data and obtain preliminary dissolved gas concentration data; then it calls the environmental parameter compensation model and combines the synchronously acquired environmental parameters to correct the concentration data and obtain the final measurement results.

[0077] Step 7: The measurement results are stored in the storage unit on one hand, and transmitted to the remote host computer on the other hand through the communication interface of the synchronous control module.

[0078] In calibration mode, when the preset calibration time interval is reached or the measurement data drift exceeds the preset threshold, the system executes the following calibration procedure: When the system operates continuously for a preset calibration time interval (24 hours in this embodiment), or when the data processing module determines that the drift of 10 consecutive sets of measurement data exceeds a preset threshold (±2% in this embodiment), the calibration program is automatically triggered; the calibration mode also supports manual triggering by sending instructions via a remote host computer.

[0079] Step 1 (Mode Switching): After receiving the calibration trigger signal, the synchronous control module switches the control system from measurement mode to calibration mode, pauses the output of measurement data, and simultaneously controls the self-calibration gas supply unit to start.

[0080] Step 2 (Standard Gas Supply): The pressure reducing valve of the self-calibrating gas supply unit is opened to reduce the pressure of the high-pressure standard gas in the built-in standard gas cylinder to the set pressure (0.2MPa in this embodiment). The gas flow controller introduces standard concentration gas (1000ppm carbon dioxide standard gas in this embodiment) into the gas flow chamber of the Herriott absorption cell module at the set flow rate (50sccm in this embodiment). The introduction time is set to 30s to ensure that the absorption cell is filled with a stable concentration of standard gas.

[0081] Step 3 (Spectral Acquisition): The butterfly laser 21 maintains stable light output. The laser passes through the standard gas in the absorption cell. The detector collects the absorption spectrum signal of the standard gas, which is then processed by the modulation and demodulation unit 22 and transmitted to the data processing module. The synchronous control module synchronously collects the environmental parameters at this time and transmits them to the data processing module.

[0082] Step 4 (Calibration Correction): The data processing module compares the measured standard gas absorption spectrum with the standard spectrum of the corresponding concentration in the built-in standard spectrum database, calculates the deviation of parameters such as spectral intensity and absorption peak position, and corrects the sensor's detection parameters (such as signal gain, wavelength calibration coefficient, etc.) in conjunction with the environmental parameter compensation model, thus completing the calibration.

[0083] Step 5 (Mode Recovery): After calibration, the self-calibration gas supply unit is shut down, the calibration gas in the absorption tank is discharged through the one-way valve of the gas outlet, the synchronous control module controls the system to switch back to the measurement mode, and the system continues to monitor the dissolved gas concentration; the calibrated measurement data is transmitted to the remote host computer in real time, and the calibration parameters and calibration timestamp are stored in the storage unit.

[0084] In this embodiment, the complete automatic calibration process of the system is as follows: Communicate with the sensor, collect concentration and pressure data every 15 seconds (the acquisition frequency can be adjusted as needed), and store them in the storage unit; perform the following calibration operations every 7 days (the time point can be adjusted as needed): turn off the gas pump, close the normally open solenoid valve 3, open the normally closed solenoid valve 1, introduce zero gas, send a zero-point calibration command, and close the normally closed solenoid valve 1 after the zero-point calibration is completed; then open the normally closed solenoid valve 2, introduce standard gas, collect the equilibrium concentration data after 30 seconds, calculate the range calibration coefficient, write the calibration coefficient to the sensor, and the range calibration is completed; finally, turn on the gas pump and the normally open solenoid valve to continue collecting the gas concentration and pressure data after the membrane, and at the same time obtain the standard time for data timestamp marking.

[0085] In this embodiment, the specific parameter configurations of each module are as follows: (I) 2004nm butterfly laser module parameters: laser operating temperature is 25℃, temperature control accuracy is ±0.01℃; driving current is 80mA, current adjustment accuracy is ±0.1mA; modulation frequency of modulation and demodulation unit 22 is 10kHz, and the filtering bandwidth of lock-in amplifier circuit is 1Hz; detection signal-to-noise ratio is not less than 100:1. (II) Herriott absorption cell module parameters: the reflectivity of cavity mirror 36 at a wavelength of 2004nm is 99.8% (not less than 99.5%); optical path is adjusted to 3m (adjustment range 0.5~5m). (III) Synchronization control module parameters: calibration time interval is 24 hours (can be set to 12 hours or other intervals); environmental parameter acquisition frequency is 5Hz (range 1~10Hz); clock signal source frequency is 10MHz, frequency error is less than ±1ppm; communication mode adopts RS485 bus, baud rate is 9600bps. (iv) Data processing module parameters: The standard spectral database pre-stores standard absorption spectral data with carbon dioxide concentrations of 100ppm, 500ppm, 1000ppm, 5000ppm, and 10000ppm; the environmental parameter compensation model loads the correlation function between water temperature (0~40℃), water pressure (0.1~20MPa), salinity (0~40‰), and carbon dioxide absorption coefficient; the data storage capacity is no less than 100,000 sets. (v) Self-calibrating gas supply unit parameters: The standard built-in gas cylinder capacity is 0.22L, filled with carbon dioxide standard gas at a concentration of 1000ppm, with a concentration error of less than ±0.5%; the pressure reducing valve output pressure is set to 0.2MPa (range 0.1~0.3MPa); the calibrated flow rate of the gas flow controller is set to 50sccm (range 0~100sccm), with a flow control accuracy of ±1%FS; the standard gas introduction time is set to 30s. (vi) Pressure-resistant sealing shell parameters: pressure resistance rating not less than 20MPa, suitable for water depths of 0~2000 meters; protection rating reaches IP68.

[0086] In this embodiment, the connection methods of each module are as follows: Internal connection of the 2004nm butterfly laser module: The modulation / demodulation unit 22 and the temperature control module 23 are electrically connected to the butterfly laser 21, respectively. The signal output terminal of the detector is connected to the microprocessor signal of the data processing module via a shielded cable. Connection of the Herriott absorption cell module: The gas inlet 34 is connected to the outlet of the gas flow controller of the self-calibration gas supply unit via a gas pipeline. A one-way valve is installed at the gas outlet to discharge the calibrated gas. The laser incident end of the absorption cell is aligned with the light output port of the butterfly laser 21, and the laser emitting end is aligned with the light-receiving surface of the detector. Connection of the synchronization control module: The timing control unit is connected to the 2004nm butterfly laser module, ... The self-calibrating gas supply unit and data processing module are electrically connected to achieve power supply control and timing signal transmission. The sensor probes (water temperature sensor, water pressure sensor, and salinity sensor) of the environmental parameter acquisition submodule are fixed to the outside of the pressure-resistant sealed shell. The signal cable passes through the sealed interface of the shell and connects to the inside of the synchronous control module. The communication interface is connected to the remote host computer through an underwater communication cable. Overall installation: All functional modules are fixed inside the pressure-resistant sealed shell 61. Shock-absorbing pads are set between the modules. The inner wall of the shell is fitted with a heat insulation buffer layer. The sealing interface of the shell is sealed with O-rings. Align each fixing hole in turn and tighten the fixing holes 31 and 32 with bolts to ensure the stability and sealing effect of the Herriott absorption tank module.

[0087] To verify the performance of the system of this invention, performance tests were conducted in a simulated underwater environment and a real marine environment in the laboratory. The test results are as follows: (I) Calibration accuracy test: In the laboratory environment simulating 0~10MPa water pressure, 5~30℃ water temperature, and 0~35‰ salinity, standard gases of different concentrations (100ppm, 500ppm, 1000ppm, 5000ppm) were used for calibration. The test results showed that the calibration error of the system was less than ±1%, which was significantly better than the traditional calibration method (error ±3~5%).

[0088] (ii) Long-term stability test: The system was deployed in a marine environment at a depth of 100 meters and operated continuously for 30 days, with automatic calibration once a day. The test results showed that the drift of the system's measurement data was less than ±2%, which was much lower than the drift in the uncalibrated state (±5~8%), proving that the system has good long-term stability.

[0089] (III) Environmental adaptability test: Under the conditions of 20MPa water pressure (simulated water depth of 2000 meters), 0℃ low temperature and 35‰ high salinity, the system can still work normally, the calibration accuracy error is less than ±1.5%, the pressure-resistant sealed shell has no water leakage or deformation, and each module works stably, which proves that the system has excellent environmental adaptability.

[0090] (iv) Comparison of operation and maintenance efficiency: Compared with the traditional recycling-laboratory calibration method, this system does not require the recycling of sensors, the calibration process is completed automatically, the calibration time is only 40 seconds, the operation and maintenance cycle is shortened from the traditional 7-10 days to 24 hours (which can be adjusted as needed), the operation and maintenance cost is reduced by more than 80%, and the operation and maintenance efficiency is greatly improved.

[0091] The core innovation of this invention lies in integrating the self-calibrating gas supply unit and the sensor detection unit into one unit. Through precise timing control and environmental parameter compensation, underwater in-situ automatic calibration is achieved. This design concept can be extended to calibration scenarios of laser gas sensors of other wavelengths.

[0092] The 0.22L standard built-in gas cylinder of the self-calibrating gas supply unit can be filled with standard gases of different concentrations and types (such as methane, nitrogen, etc.) according to actual monitoring needs. Only the standard spectral database and environmental parameter compensation model of the data processing module need to be updated accordingly to achieve calibration for different dissolved gases, which has strong adaptability.

[0093] The system's pressure-resistant sealing shell can be customized in terms of materials and structural design based on the water depth requirements of the actual application scenario. For example, a high-strength engineering plastic shell can be used in shallow sea environments (water depth less than 500 meters), which is less expensive; while a titanium alloy shell can be used in deep sea environments (water depth 500~2000 meters), which offers superior pressure resistance.

[0094] The data processing module's algorithm can be optimized through firmware upgrades, supporting the addition of more accurate spectral inversion algorithms and environmental parameter compensation models to continuously improve the system's detection and calibration accuracy.

[0095] This invention can be widely applied to in-situ monitoring of dissolved gases in various underwater environments such as oceans, lakes, and reservoirs, and is particularly suitable for marine environmental research, seabed resource exploration, and aquatic ecological monitoring.

[0096] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An in-situ calibration system for an underwater dissolved gas sensor with an integrated self-calibration module, characterized in that, The system includes: a 2004nm butterfly laser module, a Herriott absorption cell module, a synchronization control module, a data processing module, a self-calibrating gas supply unit, and a pressure-resistant sealed housing. The 2004nm butterfly laser module is used to emit laser light and includes a butterfly laser with a working wavelength of 2004nm, a detector, and a modulation / demodulation and temperature control module. The Herriott absorption cell module is used to make the laser reflect multiple times onto the probe gas, so that the probe gas selectively absorbs the laser of a characteristic wavelength to form an absorption spectrum, which is received and analyzed by the detector. The synchronization control module is used for power supply communication, environmental parameter control, and calibration timing control, so as to realize the timing synchronization switching between the system measurement mode and calibration mode. The data processing module is used to invert the spectral data collected by the system to obtain dissolved gas concentration data, and has the function of calibrating the concentration data. The self-calibrating gas supply unit includes a standard built-in gas cylinder, a pressure reducing valve, and a gas flow controller, which are used to introduce standard gas into the Herriott absorption cell module when the calibration program is started. The pressure-resistant sealed housing is used to house and protect the aforementioned modules.

2. The system according to claim 1, characterized in that, The modulation / demodulation and temperature control module of the 2004nm butterfly laser module adopts a PID closed-loop control algorithm, with a temperature control accuracy of ±0.01℃ and a current adjustment accuracy of ±0.1mA. The modulation / demodulation unit in the modulation / demodulation and temperature control module adopts a digital phase-locked loop amplifier circuit to extract nA-level weak photocurrent signals, with a detection signal-to-noise ratio of not less than 100:

1.

3. The system according to claim 2, characterized in that, The cavity mirror of the Herriott absorption cell module has a reflectivity of no less than 99.5% at a wavelength of 2004nm, the number of laser reflections in the absorption cell is adjustable, and the optical path adjustment range is 0.5~5m.

4. The system according to claim 3, characterized in that, The synchronization control module has a built-in high-precision clock signal source with a clock frequency error of less than ±1ppm; it supports dual communication modes of RS485 and CAN bus; it integrates an environmental parameter acquisition submodule for acquiring underwater water temperature, water pressure, and salinity parameters, with an acquisition frequency of 1~10Hz and acquisition accuracies of ±0.1℃, ±0.01MPa, and ±0.1‰, respectively; the calibration timing is initiated by a preset time interval or an external trigger signal.

5. The system according to claim 4, characterized in that, The data processing module has a built-in environmental parameter compensation model and a standard spectral database. The spectral data inversion adopts a nonlinear least squares fitting algorithm. By comparing the difference between the measured concentration and the standard gas concentration, and combining the influence law of environmental parameters on the absorption coefficient, the sensor detection parameters are calibrated and corrected, eliminating the interference of environmental factors on the calibration results.

6. The system according to claim 5, characterized in that, The self-calibrating gas supply unit has a standard built-in gas cylinder capacity of 0.22L, filled with the target gas at a standard concentration, with a concentration error of less than ±0.5%; the pressure reducing valve outputs pressure ranging from 0.1 to 0.3 MPa; the gas flow controller has a flow control accuracy of ±1%FS and a flow adjustment range of 0 to 100 sccm.

7. The system according to any one of claims 1 to 6, characterized in that, The pressure-resistant sealing shell has a pressure resistance rating of not less than 20MPa and a protection rating of IP68. The inner wall of the pressure-resistant sealing shell is provided with a heat insulation buffer layer to protect each module from the effects of underwater high pressure, low temperature and water flow impact.

8. The system according to any one of claims 1 to 6, characterized in that, In measurement mode, the 2004nm butterfly laser module emits modulated laser light into the Herriott absorption cell module. The laser light interacts with the dissolved gas underwater to form an absorption spectrum. The detector receives the absorption spectrum signal and transmits it to the data processing module for inversion to obtain dissolved gas concentration data. In calibration mode, when the preset calibration time interval is reached or the measurement data drift exceeds the preset threshold, the synchronization control module controls the system to switch to calibration mode, the self-calibration gas supply unit introduces standard gas into the Herriott absorption cell module, and the data processing module compares the difference between the measured spectrum and the standard spectrum, and completes the calibration correction in combination with environmental parameters; after calibration is completed, the system switches back to measurement mode.

9. The system according to claim 8, characterized in that, The preset calibration time interval can be set to 12 hours or 24 hours; the preset threshold is the measurement data drift amount ±2%; the calibration mode also supports manual triggering via remote host computer commands.

10. The system according to claim 9, characterized in that, The data processing module also includes a storage unit for storing measurement data, calibration data, and calibration timestamps, with a storage capacity of no less than 100,000 sets of data. The data processing module transmits data to a remote host computer through the communication interface of the synchronization control module, supporting real-time data monitoring and historical data backtracking.

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