CN103698287B - For detecting the dim light signal detection device of seawater pH value - Google Patents

Abstract

The invention discloses a low-light signal detection device for detecting the pH value of seawater, which is provided with a flow path system for circulating seawater samples and indicators, and is used for mixing seawater samples or seawater samples and indicators in the flow path system The optical path detection system for light detection of the solution and the main control system for receiving the detection signal output by the optical path detection system and completing the calculation of the pH value of the seawater sample. The present invention designs a low-light signal detection device for the special field of seawater pH value, adopts a precise optical path detection system and a small-sized flow path system, and realizes the operation of a fully automatic process from collecting low-light signals to controlling water sample mixing and distribution control, It not only avoids the clumsy and complicated manual operation of various laboratory pH testing instruments, saves energy and cost, but also, the formed professional advantages and high-performance equipment can ensure that information collection, processing and control are more systematic and efficient.

Description

Low-light signal detection device for detecting the pH value of seawater

technical field

The invention belongs to the technical field of marine environment monitoring, and in particular relates to a device for detecting the pH value of seawater.

Background technique

The pH value is one of the important parameters of the carbonate system of seawater, and it is also the direct evidence of ocean acidification. Ocean carbon cycle and ocean acidification studies require continuous and accurate monitoring of seawater pH. my country is a large ocean country. The research on ocean acidification and carbon cycle system is an important task related to marine environment, marine disaster warning, marine safety and marine resource development. The pH value of seawater is one of the important indicators reflecting the marine carbon cycle system. Therefore, Detecting the pH value of seawater is crucial to the safety monitoring of the marine environment.

At present, there are two main methods for online detection of pH value in the world: one is the electrode method; the other is the spectrophotometric method. The detection accuracy limit of the electrode method is ±0.005, while that of the spectrophotometric method is ±0.0007. The detection device designed by the electrode method has the advantage of low price, but low precision, which requires frequent correction and replacement of the probe. The detection device designed by spectrophotometry has high detection accuracy, good precision, and does not need to be corrected. Therefore, it has become the standard method for measuring the pH value of seawater in the research of seawater carbonate system. Although foreign researchers have developed a series of pH measurement systems based on spectrophotometry, most of them are bulky, clumsy, and require complex manual operations, which cannot fully meet the needs of ocean carbon cycle, seawater acidification, and even global climate change research. need.

Contents of the invention

Aiming at the application field of seawater pH value detection, the invention designs a low-light signal detection device to simplify manual operation and improve the detection accuracy of seawater pH value.

In order to solve the above-mentioned technical problems, the present invention adopts the following technical solutions to achieve:

A low-light signal detection device for detecting the pH value of seawater, which is provided with

Flow path system for the flow of seawater samples and indicators, including flow cells;

The optical path detection system is used for light detection of the seawater sample or the mixed solution of seawater sample and indicator in the flow path system, including LED light source, one-in and three-out fiber coupler, three optical filters and three photodiodes; The LED light source irradiates the seawater sample in the flow cell or the mixed solution of the seawater sample and the indicator, and the light that passes through the seawater sample or the mixed solution is coupled into the one-input and three-outlet optical fiber coupler, and passes through the one-input and three-outlet optical fiber coupler. The coupler divides the single-path light into three paths, which are directed to three optical filters respectively, and three paths of light with specific wavelengths are filtered out by the three optical filters, respectively directed to the three photodiodes, and then respectively transmitted through the three photodiodes. After photoelectric conversion, output three detection signals;

The main control system receives the detection signals converted and output by the three photodiodes, and calculates the pH value of the seawater sample.

In order to realize quantitative sampling of seawater samples and indicators, a three-way valve group, a water sample pump, an indicator pump and two quantitative loops are also arranged in the flow path system, and the flow cell is connected to the first quantitative loop ; When performing light detection on seawater samples, the three-way valve group is controlled to form a double open-loop flow path, and under the drive of the water sample pump, the seawater sample is quantitatively sampled through the first quantitative loop; under the drive of the indicator pump, through The second quantitative loop performs quantitative sampling on the indicator;

When performing light detection on the mixed solution of seawater samples and indicators, the three-way valve group is controlled to form a single closed-loop flow path, so that the two quantitative loops are connected to form a closed loop, and the water sample pump or indicator Driven by the pump, the seawater sample and the indicator flow in a closed loop, and are fully mixed before light detection.

In order to increase the volume of the flow cell, reduce the retention volume of the solution as much as possible, and avoid the retention of air bubbles, the flow cell is preferably designed as a conical flow cell, and the apex and bottom surface of the conical flow cell are connected in the first quantitative loop; Interfaces are designed on the side of the tapered flow cell and on opposite sides, and are respectively coupled and connected with the LED light source and the one-in-three-out fiber coupler through optical fibers to shorten the length of the optical path and improve the detection accuracy.

As a preferred circuit construction structure of the main control system, the main control system is provided with a main processor, an LED light source driver module, a low-light signal acquisition module and a pump valve driver module; the main processor is testing After starting, execute the following control process:

a. Control the pump valve drive module to adjust the communication path of the three-way valve group, so that the first quantitative loop is connected between the seawater sample container and the waste liquid pool, and the second quantitative loop is connected between the indicator container and the waste liquid pool. ;

b. Control the pump valve drive module to start the water sample pump and the indicator pump respectively, flush the pipelines of the two quantitative loops and complete the quantitative sampling;

c. Control the LED light source drive module to start, drive the LED light source to emit light, irradiate the seawater sample in the flow cell, and control the low-light signal acquisition module to collect the seawater sample detection signal output by the three-way photodiode, and record it;

d. Control the LED light source drive module to turn off;

e. Control the pump valve drive module to adjust the communication path of the three-way valve group, so that the two quantitative loops are connected to form a closed loop; then, start the water sample pump to drive the seawater sample and indicator for quantitative sampling in the closed loop Flow in the road, after fully mixed, control the water sample pump to stop;

f. Control the LED light source drive module to start, drive the LED light source to emit light, irradiate the mixed solution in the flow cell, and control the low-light signal acquisition module to collect the mixed solution detection signal output by the three photodiodes, and record it;

g. Calculate the pH value of the ocean sample according to the recorded detection signal of the seawater sample and the detection signal of the mixed solution.

In order to improve the stability of LED light source irradiation, a constant current reference source is provided in the LED light source drive module. After receiving the start signal output by the main processor, the constant current reference source outputs a constant current to the LED light source to drive the LED The light source emits white light.

Still further, a shaping filter isolation circuit, a programmable gain amplifier and an A/D converter are arranged in the low-light signal acquisition module; the shaping filter isolation circuit receives the detection signals output by three photodiodes, and After isolation and shaping and filtering processing, the detection signal of the road is sent to the programmable gain amplifier for amplification, and then sent to the A/D converter for analog-to-digital conversion, and then output to the main processor; the main processor output gain Select the control signal to the programmable gain amplifier to adjust the amplification factor of the programmable gain amplifier.

Furthermore, a power supply module, a storage module and a communication module are also provided in the low-light signal detection device; the power supply module receives battery power and converts it into different DC power supplies to supply power to different electrical loads in the device; The main processor writes the received detection signal and calculation results into the storage module for storage, and uploads the calculation results to the upper computer through the communication module.

Compared with the prior art, the advantages and positive effects of the present invention are: the present invention designs a low-light signal detection device for the special field of seawater pH value, adopts a precise optical path detection system and a small and exquisite flow path system, and realizes the collection of microscopic signals. The operation of the optical signal to control the fully automated process of water sample mixing and distribution control not only avoids the clumsy and complicated manual operation of various laboratory pH detection instruments, saves energy and cost, but also, the formed professional advantages and high-performance equipment can ensure Information collection, processing and control are more systematic and efficient.

Other features and advantages of the present invention will become more apparent after reading the detailed description of the embodiments of the present invention in conjunction with the accompanying drawings.

Description of drawings

Fig. 1 is a flow path system connection diagram when the low-light signal detection device proposed by the present invention performs light detection on seawater samples;

Fig. 2 is a flow path system connection diagram when the low-light signal detection device proposed by the present invention performs light detection on the mixed solution of seawater sample and indicator;

Fig. 3 is a functional block diagram of an embodiment of the main control system in the low-light signal detection device proposed by the present invention.

Detailed ways

Specific embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings.

According to the pH parameter of seawater, this embodiment designs a low-light signal detection device, which mainly includes a flow path system, an optical path detection system and a main control system. Among them, the flow path system is mainly responsible for injecting the seawater sample or the mixed solution of the seawater sample and the indicator into the flow cell for subsequent light detection. In order to realize quantitative sampling and automatic mixing of seawater samples and indicators, this embodiment designs a three-way valve group 1, a water sample pump 2, an indicator pump 3, and a first quantitative loop 5 in the flow path system. , the second quantitative loop 4 and the flow cell 6, as shown in Fig. 1 and Fig. 2 . In this embodiment, the three-way valve group 1 is preferably formed by connecting four three-way valves and four three-way valves, and by switching the connecting passages of the four three-way valves, a double open-loop flow path is formed (as shown in FIG. 1) or a single closed-loop flow path (as shown in Figure 2).

Specifically, the flow cell 6 is connected to the first quantitative loop 5, and the first and last ends of the two quantitative loops 4 and 5 are respectively connected to different interfaces of the three-way valve group 1, and are connected through the three-way valve group 1. Containers for seawater samples, containers for indicators and waste tanks. A water sample pump 2 is set in the first quantitative loop 5, and the seawater sample is driven to flow in the first quantitative loop 5 and the flow cell 6 by the water sample pump 2; an indicator pump 3 is set in the second quantitative loop 4, and the The indicator pump 3 drives the indicator to flow in the second quantitative loop 4 . According to the mixing ratio of the seawater sample and the indicator, the lengths of the two quantitative loops 4 and 5 are adjusted to realize quantitative sampling of the seawater sample and the indicator.

In this embodiment, the water sample pump 2 and the indicator pump 3 are preferably peristaltic pumps to push the solutions in the quantitative loops 4 and 5 .

In order to reduce the retention volume (dead volume) of the reaction solution on the premise of increasing the volume of the flow cell as much as possible, and realize no reagent residue, the flow cell 6 is designed in a cone shape in this embodiment, as shown in Fig. 1 and Fig. 2 . Compared with the traditional Z-shaped flow cell, this cone-shaped flow cell 6 basically has no stagnation volume and bubbles are not easy to retain because there is no right angle, and the LED light source and the one-in and three-out fiber couplers are respectively coupled and connected to the cone. On the two opposite interfaces on the side of the type flow cell 6, the length of the incident light passing through the seawater sample or mixed solution can be greatly shortened, which is very beneficial to improve the detection accuracy. Interfaces are respectively designed on the apex and the bottom surface of the conical flow cell 6, and are connected to the first quantitative loop 5 to ensure that the seawater sample or mixed solution can fill the entire flow cell 6 during the flow, to further improve detection precision.

An LED light source 10, a fiber coupler 7 with one input and three outputs, an optical filter group 8 and a photodiode group 9 are arranged in the optical path detection system, as shown in Fig. 1 and Fig. 2 . In this embodiment, the LED light source 10 is preferably a white high-brightness LED lamp, which emits high-brightness white light to irradiate the solution in the flow cell 6 . Three optical filters are arranged in the optical filter group 8, and the filter wavelengths of the three optical filters are different, and they are respectively arranged at the three-way exit ports of the one-in and three-out fiber coupler 7 in one-to-one correspondence, Filter the three paths of light emitted by the one-in and three-out fiber coupler 7 , extract the light of the required wavelength, and project it to the photodiode group 9 . Three photodiodes are arranged in the photodiode group 9, respectively receive the light of three paths of specific wavelengths filtered by three filters, and perform photoelectric conversion on the three paths of light to generate a voltage or current detection signal and output to the main control system.

The main control system is provided with main components such as a main processor CPU, an LED light source drive module, a low-light signal acquisition module, and a pump valve drive module, as shown in FIG. 3 . Among them, the main processor CPU preferably adopts a 32-bit ARM chip as the main control unit, controls the LED light source, the water sample pump 2, the indicator pump 3 and the three-way valve group 1 through the LED light source drive module and the pump valve drive module according to the set The process runs sequentially, and the detection signals converted and output by three photodiodes are collected through the low-light signal acquisition module to complete the calculation of the pH value of the seawater sample.

In this embodiment, in order to make the light emitted by the LED light source more stable, so as to improve the accuracy of seawater sample detection, for the LED light source drive module, this embodiment preferably uses a constant current reference source to achieve stable control of the LED light source. As shown in Figure 3. The constant current reference source is connected to the main processor CPU, and after receiving the control voltage output by the main processor CPU, it outputs a constant current source to drive the LED light source to emit light stably. The main processor CPU can automatically adjust the output current of the constant current reference source by adjusting the control voltage output to the constant current reference source, and then realize the on-demand adjustment of the luminance of the LED light source.

The light from the LED light source to the flow cell passes through the solution in the flow cell to the one-in and three-out fiber coupler, and the one-in and three-out fiber coupler divides the incident light into three lines of light, which pass through three filters respectively. The light sheet filters out three paths of light with different wavelengths and shoots them to three photodiodes respectively, and is converted by the three photodiodes to generate three paths of detection signals, which are output to the low-light signal acquisition module. In order to improve the accuracy of detection signal collection, in this embodiment, a shaping filter isolation circuit, a programmable gain amplifier and an A/D converter are preferably designed in the low-light signal collection module, as shown in FIG. 3 . The three-way detection signals converted and output by three photodiodes pass through the shaping filter isolation circuit for signal isolation, waveform shaping, and filtering processing, and then output to the programmable gain amplifier for signal amplification processing, and then transmit to the A/D converter to convert the analog signal After being converted into a digital signal, it is sent to the main processor CPU. In order to improve conversion precision, the A/D converter preferably adopts a 16-bit A/D converter to convert and generate a 16-bit digital signal and output it to the main processor CPU for further data smoothing and filtering processing. For the processed data, on the one hand, the main processor CPU can store it in the storage module in the low-light signal detection device according to a fixed encrypted format, for example, it can be written into the FLASH memory through the latch circuit through the SPI interface of the main processor CPU , so that the main processor CPU can subsequently calculate the pH value of the seawater sample; on the other hand, it can be uploaded to the host computer through the communication module for monitoring. In this embodiment, the communication module is preferably connected to the main processor CPU through a universal serial asynchronous bus UART, receives processed data, converts it into serial data in RS232 format, and uploads it to the host computer. The host computer performs data communication with the low-light signal detection device through the RS232 serial bus to realize real-time monitoring of the low-light signal detection device.

The magnification of the programmable gain amplifier can be adjusted by the main processor CPU, as shown in Figure 3. The main processor CPU outputs a gain selection control signal to the programmable gain amplifier through one I/O port, and adjusts the amplitude of the detection signal output by the programmable gain amplifier by adjusting the amplification factor of the programmable gain amplifier.

Another latch circuit and a control module are provided in the pump valve drive module. After the main processor CPU starts the seawater sample detection process, it outputs a control signal according to the set time sequence flow, and transmits it to the control module through another latch circuit. Module, through the control module, the load capacity of the control signal is improved to realize the control of the three-way valve group, the water sample pump or the indicator pump.

In order to ensure that the low-light signal detection device can work underwater for a long time, a 12V battery is preferably used in this embodiment to provide power supply for the entire low-light signal detection device. The 12V DC power supply output by the battery is converted into 5V or 3.3V DC power supply through the regulated power supply module, and output to each electrical load in the main control system to supply power for each electrical load.

The specific workflow of the low-light signal detection device proposed in this embodiment will be described in detail below in conjunction with the system architecture shown in FIGS. 1-3 .

(1) Inject a sufficient amount of seawater sample to be tested into the sample container, and inject a sufficient amount of indicator into the indicator solution.

In this embodiment, m-cresyl violet is preferably used as the indicator to be mixed with the seawater sample to be detected. Since m-cresyl violet has a dissociation constant at a temperature of 25°C and a salinity of 35 The value of is 8.006, and the change range is consistent with the pH value of seawater, so it is a more commonly used indicator in the study of measuring the pH value of seawater.

Of course, the indicator can also be phenol red, cresol red, thymol blue, etc., and this embodiment is not limited to the above examples.

(2) The main processor CPU outputs a control signal to the three-way valve group 1 to control the connection between the inlet and outlet ports ① and ② of the four three-way valves in the three-way valve group 1 to form a double open-loop flow path, as shown in Figure 1.

(3) The main processor CPU outputs control signals to the water sample pump 2 and the indicator pump 3, starts the water sample pump 2 to extract seawater samples to the first quantitative loop 5, and discharges them into the sample via the flow cell 6 and the three-way valve group 1 The waste liquid pool flushes the pipeline of the first quantitative loop 5 and the flow cell 6, and completes the quantitative sampling of seawater samples. At the same time, start the indicator pump 3 to extract the indicator to the second quantitative loop 4, so that the indicator is filled with the entire second quantitative loop 4, and is discharged into the waste liquid pool by the three-way valve group 1, and the second quantitative loop 4 The pipeline is flushed and the quantitative sampling of the indicator is completed.

(4) The main processor CPU starts the LED driver module to output a constant current source, drives the LED light source 10 to emit light, and irradiates the seawater sample in the flow cell 6 . The light passing through the seawater sample enters the one-in and three-out fiber coupler 7, and the coupler 7 divides one line of light into three lines of light and shoots them to three filters respectively, and outputs three channels of different wavelengths through the three filters The light is directed to three photodiodes respectively.

In this embodiment, it is preferable to set the filter wavelengths of the three optical filters as λ1, λ2, and λ3 respectively, wherein λ1 is the maximum absorption wavelength of the alkali state of the indicator, λ2 is the maximum absorption wavelength of the acid state of the indicator, and λ3 As the reference wavelength, at the reference wavelength λ3, neither the acidic state nor the basic state of the indicator absorbs, so λ3 can be used as a correction wavelength to eliminate the unstable influence of incident light.

As a preferred design solution of this embodiment, for the m-cresyl violet indicator, this embodiment preferably sets λ1=578nm, λ2=432nm, and λ3=730nm. For other types of indicators, the corresponding λ1, λ2, and λ3 can be selected according to their characteristics.

(5) The main processor CPU collects the detection signals converted and output by three photodiodes through the low-light signal acquisition module, that is, the seawater sample detection signals, which are respectively recorded as: I _{λ1 seawater} , I _{λ2 seawater} , and I _{λ3 seawater} , and write them into the storage The module is saved and uploaded to the host computer.

(6) The main processor CPU controls the LED light source 10 to extinguish, and switches the communication pipeline of the three-way valve group 1, so that the liquid inlet and outlet ports ① and ③ of the four three-way valves in the three-way valve group 1 are communicated, and then The two quantitative loops 4 and 5 are connected to form a closed loop, that is, a single closed-loop flow path, as shown in FIG. 2 .

(7) Start the water sample pump 2 to run, promote the seawater sample and the indicator in the two quantitative loops 4, 5 to flow in the closed loop for a long enough time (preferably greater than 5 minutes), so that the seawater sample and the indicator can Mix well. After the mixture is uniform, the water sample pump 2 is controlled to stop, and the proportioning and mixing of the seawater sample and the indicator is completed.

(8) Start the LED light source 10 again to emit light, irradiate the mixed solution of seawater sample and indicator in the flow cell 6, and start three photodiodes at the same time to receive light of three different wavelengths, and after photoelectric conversion, output three mixed solutions The detection signals are respectively recorded as: I _{λ1 mixed solution} , I _{λ2 mixed solution} , and I _{λ3 mixed solution} , which are written into the storage module for storage and uploaded to the host computer.

Since the determination process of the mixed solution is carried out in a closed loop system, the influence of factors such as gas exchange on the calculation of the pH value of the seawater sample can be avoided.

(9) The main processor CPU utilizes the received three-way seawater sample detection signal I _{λ1 seawater} , I _{λ2 seawater} , I _{λ3 seawater} and the three-way mixed solution detection signal I _{λ1 mixed solution} , I _{λ2 mixed solution} , and I _{λ3 mixed solution} , Calculate the pH value of the seawater sample to be tested. The specific calculation process is as follows:

use the formula Calculate the pH of a seawater sample. in, is the dissociation constant of the indicator; [I ^2- ] and [HI ^- ] are the equilibrium concentrations of the indicator in the alkaline state and acid state, respectively.

In this example, for m-cresyl violet indicator, its dissociation constant The value of can be directly determined according to the temperature and salinity of the test, that is, under the conditions of temperature 20℃≤T≤30℃ and salinity 30≤S≤37, ${\mathrm{pK}}_{{\mathrm{HI}}^{-}}=1245.69/T+3.8275+0.00211\×(35-S).$

Set the absorbance of the indicator at the wavelengths λ1 and λ2 as A _λ1 and A _λ2 , then the pH value of the seawater sample can be calculated by the following formula:

$\mathrm{<span\; class="notranslate">\; p</span>}\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; pK</span>}}_{{\mathrm{<span\; class="notranslate">\; HI</span>}}^{<span\; class="notranslate">\; -</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; g</span>}\frac{\mathrm{<span\; class="notranslate">\; R</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; e</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; e</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; Re</span>}}_{\mathrm{<span\; class="notranslate">\; 3</span>}}}<span\; class="notranslate">\; ,</span>$

in, $R=\frac{A_{\mathrm{\&lambda;}2}}{A_{\mathrm{\&lambda;}1}},$ e ₁ , e ₂ , e ₃ are the molar absorption coefficient ratios of the indicator at the maximum absorption wavelength λ1, λ2, and $e_1=\left({\mathrm{\&epsiv;}}_{{\mathrm{HL}}^{-}\mathrm{\&lambda;}1}\right)/\left({\mathrm{\&epsiv;}}_{{\mathrm{HL}}^{-}\mathrm{\&lambda;}2}\right),e_2=\left({\mathrm{\&epsiv;}}_{{L^2}^{-}\mathrm{\&lambda;}2}\right)/\left({\mathrm{\&epsiv;}}_{{\mathrm{HL}}^{-}\mathrm{\&lambda;}1}\right),e_3=\left({\mathrm{\&epsiv;}}_{L^{2-}\mathrm{\&lambda;}1}\right)/\left({\mathrm{\&epsiv;}}_{{\mathrm{HL}}^{-}\mathrm{\&lambda;}1}\right);$ in, It is the molar absorptivity of the acid state and basic state of the indicator at their maximum absorption wavelengths λ1 and λ2. Using the reference wavelength λ3 as the calibration wavelength and introducing the calculation of the absorbance as A _λ1 and A _λ2 can ensure the consistency of the sample measurement conditions and further improve the precision and accuracy of the measurement.

For other types of indicators, the relevant thermodynamic constants of the indicators can be determined spectrophotometrically Then, the pH value of the seawater sample was calculated using the above formula. At present, researchers have determined the relevant thermodynamic constants of some indicators, such as: phenol red, cresol red, thymol blue, etc., and these data can be directly quoted.

(10) The main processor CPU writes the calculation result into the storage module, and uploads it to the host computer through the communication module to complete the determination of the seawater sample.

The low-light signal detection device of this embodiment has high precision, good accuracy, and is easy to operate. The pH value of the seawater sample can be calculated by measuring the absorbance of the seawater sample combined with the physical and chemical parameters (temperature, salinity) of the measurement process. Use pH standard buffer solution for calibration, suitable for on-site determination. In addition, the low-light signal detection device of this embodiment has strong stability, can adapt to various marine monitoring working environments, even harsh environments, and meets the requirements of seawater pH value monitoring with strong anti-interference, high reliability and low power consumption. , which can well adapt to the common framework of various in-situ seawater pH monitoring systems.

The low-light signal detection device of this embodiment can be mounted on marine monitoring systems such as buoys, automatic stations, and coast-based/platform-based, and can undertake various types of marine monitoring projects and scientific research projects that are currently extensive and long-term in my country. .

Of course, the above description is only a preferred embodiment of the present invention, and it should be pointed out that for those skilled in the art, some improvements and modifications can be made without departing from the principle of the present invention. Improvements and retouches should also be considered within the protection scope of the present invention.

Claims (6)

1. A low-light signal detection device for detecting the pH value of seawater, which is provided with Flow path system for the flow of seawater samples and indicators, including flow cells; The optical path detection system is used for light detection of the seawater sample or the mixed solution of seawater sample and indicator in the flow path system, including LED light source, one-in and three-out fiber coupler, three optical filters and three photodiodes; The LED light source irradiates the seawater sample in the flow cell or the mixed solution of the seawater sample and the indicator, and the light that passes through the seawater sample or the mixed solution is coupled into the one-input and three-outlet optical fiber coupler, and passes through the one-input and three-outlet optical fiber coupler. The coupler divides the single-path light into three paths, which are directed to three optical filters respectively, and three paths of light with specific wavelengths are filtered out by the three optical filters, respectively directed to the three photodiodes, and then respectively transmitted through the three photodiodes. After photoelectric conversion, output three detection signals; The main control system receives the detection signals converted and output by three photodiodes, and calculates the pH value of the seawater sample; A three-way valve group, a water sample pump, an indicator pump, and two quantitative loops are also arranged in the flow path system, and the flow cell is connected to the first quantitative loop; when performing light detection on seawater samples, the control The three-way valve group forms a double open-loop flow path. Under the drive of the water sample pump, the seawater sample is quantitatively sampled through the first quantitative loop; under the drive of the indicator pump, the indicator is quantitatively sampled through the second quantitative loop. ; When performing light detection on the mixed solution of seawater samples and indicators, the three-way valve group is controlled to form a single closed-loop flow path, so that the two quantitative loops are connected to form a closed loop, and the water sample pump or indicator Driven by the pump, the seawater sample and the indicator flow in a closed loop, and are fully mixed before light detection. 2. The low-light signal detection device for detecting the pH value of seawater according to claim 1, characterized in that: the flow cell is a conical flow cell, and the apex and the bottom surface of the conical flow cell are connected in the first path In the quantification loop; the interface is designed on the side of the tapered flow cell and on the opposite sides, and the optical fiber is respectively coupled with the LED light source and the one-in and three-out fiber coupler. 3. the low-light signal detection device for detecting seawater pH value according to claim 1, characterized in that: in the main control system, a main processor, an LED light source driver module, a low-light signal acquisition module and A pump valve drive module; after the test starts, the main processor executes the following control process: a. Control the pump valve drive module to adjust the communication path of the three-way valve group, so that the first quantitative loop is connected between the seawater sample container and the waste liquid pool, and the second quantitative loop is connected between the indicator container and the waste liquid pool. ; b. Control the pump valve drive module to start the water sample pump and the indicator pump respectively, flush the pipelines of the two quantitative loops and complete the quantitative sampling; c. Control the LED light source drive module to start, drive the LED light source to emit light, irradiate the seawater sample in the flow cell, and control the low-light signal acquisition module to collect the seawater sample detection signal output by the three-way photodiode, and record it; d. Control the LED light source drive module to turn off; e. Control the pump valve drive module to adjust the communication path of the three-way valve group, so that the two quantitative loops are connected to form a closed loop; then, start the water sample pump to drive the seawater sample and indicator for quantitative sampling in the closed loop Flow in the road, after fully mixed, control the water sample pump to stop; f. Control the LED light source drive module to start, drive the LED light source to emit light, irradiate the mixed solution in the flow cell, and control the low light signal acquisition module to collect the mixed solution detection signal output by the three photodiodes, and record it; g. Calculate the pH value of the ocean sample according to the recorded detection signal of the seawater sample and the detection signal of the mixed solution. 4. The low-light signal detection device for detecting the pH value of seawater according to claim 3, characterized in that: a constant current reference source is arranged in the LED light source driving module, and the constant current reference source receives the main After the start signal output by the processor, a constant current is output to the LED light source to drive the LED light source to emit white light. 5. The low-light signal detection device for detecting the pH value of seawater according to claim 3, characterized in that: the low-light signal acquisition module is provided with a shaping filter isolation circuit, a programmable gain amplifier and an A/D Converter; the shaping filter isolation circuit receives the detection signals output by the three photodiodes, and after the three detection signals are isolated and shaped and filtered, they are sent to the programmable gain amplifier for amplification, and then sent to the A/D converter After analog-to-digital conversion, it is output to the main processor; the main processor outputs a gain selection control signal to the programmable gain amplifier to adjust the amplification factor of the programmable gain amplifier. 6. the low-light signal detection device for detecting the pH value of seawater according to claim 3, characterized in that: a power module, a storage module and a communication module are also provided in the low-light signal detection device; the power supply The module receives battery power and converts it into different DC power sources to supply power to different electrical loads in the device; the main processor writes the received detection signals and calculation results into the storage module for storage, and transmits the calculation results through the communication module Upload to the host computer.