A measuring system and method for measuring the pH value of low-conductivity water samples

Abstract

The application relates to the technical field of water quality analysis, and discloses a pH value measuring system and measuring method for low-conductivity water samples, which comprises a cooler, an electric regulating valve, a thermometer, a first online conductivity meter, a mixing pipe, a variable-frequency dosing pump, a second online conductivity meter, a pH meter and a PLC controller; the PLC controller is electrically connected with the thermometer, the electric regulating valve, the first online conductivity meter, the second online conductivity meter and the variable-frequency dosing pump respectively. The water sample is cooled by the cooler and temperature closed-loop control is adopted, so that the interference of temperature change on pH measurement is eliminated; KCl standard solution is added into the low-conductivity water sample and conductivity closed-loop control is adopted, so that the influence of liquid junction potential is reduced; the value drift problem caused by temperature fluctuation and liquid junction potential in the pH measurement of the low-conductivity water sample is effectively solved; and the accuracy and stability of the pH measurement are improved.

Description

Technical Field

[0001] This invention relates to the field of water quality analysis technology, specifically to a measurement system and method for measuring the pH value of water samples with low electrical conductivity. Background Technology

[0002] Currently, the pH value of low-conductivity water samples is mainly measured directly using the electrode method. This involves immersing a pH measuring electrode and a reference electrode in the water sample to form a measuring cell, and then determining the pH value. However, due to the extremely low ion concentration in low-conductivity water samples (such as pure water and ultrapure water in thermal power generating units), direct measurement faces two inherent interferences: firstly, temperature changes alter the ionization equilibrium constant of water, causing the H+ ion concentration to rise. + Changes in concentration cause pH measurement results to deviate from the true value; secondly, a large concentration gradient exists between the water sample and the high-concentration potassium chloride (KCl) solution filling the reference electrode, causing the salt bridge solution to continuously diffuse and permeate into the water sample, triggering changes and significant fluctuations in the liquid junction potential, resulting in continuous drift in the measured value. The combination of these two factors has long resulted in poor accuracy and stability in the direct measurement of pH values ​​from low-conductivity water samples. To overcome these shortcomings, some methods employ indirect estimation, using mathematical models to infer pH values ​​by measuring parameters such as the conductivity and hydrogen conductivity of the water sample. However, this method is an indirect measurement, dependent on the accuracy and applicability of the model, and still fails to fundamentally address the direct interference of temperature changes and liquid junction potential on the measurement process. Summary of the Invention

[0003] In view of this, the present invention provides a measurement system and method for pH value of low conductivity water samples, so as to solve the problem of poor measurement accuracy caused by temperature changes and liquid junction potential interference when directly measuring pH of low conductivity water samples in the prior art.

[0004] In a first aspect, the present invention provides a system for measuring the pH value of a low conductivity water sample, comprising: a cooler, an electric regulating valve, a thermometer, a first online conductivity meter, a mixing tube, a variable frequency dosing pump, a second online conductivity meter, a pH meter, and a PLC controller. The cooling water inlet of the cooler is connected to the electric regulating valve; the water sample outlet of the cooler is connected to the inlet of the mixing tube via a pipeline; the thermometer and the first online conductivity meter are disposed on the pipeline between the cooler and the mixing tube; the inlet of the variable frequency dosing pump is injected with KCl standard solution, and the outlet of the variable frequency dosing pump is connected to the dosing port of the mixing tube; the outlet of the mixing tube is connected to the second online conductivity meter via a pipeline, and the outlet of the second online conductivity meter is connected to the pH meter via a pipeline; the PLC controller is electrically connected to the thermometer, the electric regulating valve, the first online conductivity meter, the second online conductivity meter, and the variable frequency dosing pump.

[0005] The pH measurement system for low-conductivity water samples provided by this invention uses a cooler to cool the water sample and an electric regulating valve to adjust the cooling water flow, effectively reducing the interference of temperature changes on pH measurement. A variable frequency dosing pump adds KCl standard solution to the water sample, increasing its ionic strength and conductivity, effectively reducing the influence of liquid junction potential. A first online conductivity meter and a second online conductivity meter detect the conductivity of the water sample before and after dosing, respectively. A PLC controller automatically regulates the electric regulating valve and the variable frequency dosing pump, ensuring the stability and accuracy of pH measurement without manual intervention. The system has a compact structure, a reasonable layout, and integrates automatic and intelligent measurement functions.

[0006] In one alternative embodiment, the cooler is connected to a cooling water inlet pipe and a cooling water outlet pipe, and an electric regulating valve is installed on the cooling water inlet pipe.

[0007] In one alternative embodiment, a flow meter is also provided on the pipeline between the cooler and the mixing tube, the flow meter being positioned between the first online conductivity meter and the mixing tube.

[0008] In one optional implementation, a check valve and a flow meter are installed on the pipeline between the variable frequency dosing pump and the mixing pipe, and the check valve and the flow meter are electrically connected to the PLC controller.

[0009] In one alternative implementation, a three-way valve is provided on the pipeline between the mixing pipe and the pH meter, and the third port of the three-way valve is connected to a drain pipe.

[0010] In one alternative implementation, an electrically controlled regulating valve is installed on the inlet pipe of the cooler, and the electrically controlled regulating valve is electrically connected to the PLC controller.

[0011] Secondly, the present invention provides a method for measuring the pH value of a low-conductivity water sample, applied to a PLC controller in a measurement system of the first aspect or any corresponding embodiment described above. The method includes: adjusting the opening of an electric regulating valve according to the temperature signal of the cooled water sample fed back by a thermometer, so as to maintain the temperature of the water sample cooled by the cooler within a preset temperature range; controlling a variable frequency dosing pump to pump KCl standard solution into a mixing tube to mix with the cooled water sample according to the conductivity signal of the water sample before dosing fed back by a first online conductivity meter; adjusting the pumping volume of KCl standard solution of the variable frequency dosing pump according to the conductivity signal of the water sample after dosing fed back by a second online conductivity meter, so as to maintain the conductivity of the water sample after dosing within a preset conductivity range; and obtaining the pH measurement value of the water sample after dosing according to a pH meter.

[0012] The method for measuring the pH value of low conductivity water samples provided by this invention achieves closed-loop control of water sample temperature by automatically adjusting the opening of an electric regulating valve to change the cooling water flow rate based on feedback signals from a thermometer using a PLC controller. This effectively avoids interference from temperature fluctuations in pH measurement. Furthermore, the PLC controller automatically controls a variable frequency dosing pump to add KCl standard solution to the water sample based on feedback signals from a first online conductivity meter, and automatically adjusts the pumping rate of KCl standard solution based on feedback signals from a second online conductivity meter. This achieves closed-loop precise adjustment of the dosage, effectively reducing the influence of liquid junction potential and avoiding measurement drift. This method requires no manual intervention and can achieve stable and accurate measurement of the pH value of low conductivity water samples, offering advantages such as high automation and reliable measurement results.

[0013] In one optional implementation, before acquiring the temperature signal of the cooled water sample from the thermometer, the method further includes: controlling the electric regulating valve to open, so that cooling water enters the cooler through the cooling water inlet pipe and is discharged through the cooling water outlet pipe; controlling the three-way valve to switch to the drain position, so that the water sample is discharged through the drain pipe; and controlling the electric regulating valve to open, adjusting the water sample flow rate to a preset flow rate value.

[0014] In one optional embodiment, before controlling the variable frequency dosing pump to pump the KCl standard solution into the mixing tube, the method further includes: controlling the three-way valve to switch to the drain position; receiving and determining that the displayed values ​​of the first online conductivity meter and the second online conductivity meter are the same and both are lower than the preset low conductivity threshold, and controlling the variable frequency dosing pump to start; receiving the conductivity signal fed back from the second online conductivity meter, and adjusting the frequency of the variable frequency dosing pump until the feedback value of the second online conductivity meter reaches the preset conductivity setting value.

[0015] In one optional implementation, the process of obtaining the pH value of the water sample after dosing based on the pH meter includes: controlling the three-way valve to switch to the drain position, allowing the water sample to be discharged through the drain pipe for a preset time, and then controlling the three-way valve to switch to the measurement position, allowing the water sample to flow into the pH meter for measurement. Attached Figure Description

[0016] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a diagram illustrating the composition of a system for measuring the pH value of low-conductivity water samples according to an embodiment of the present invention. Figure 2 This is a schematic flowchart of a method for measuring the pH value of a low-conductivity water sample according to an embodiment of the present invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] It is understood that before using the technical solutions disclosed in the various embodiments of the present invention, users should be informed of the types, scope of use, and usage scenarios of the personal information involved in the present invention and their authorization should be obtained in accordance with relevant laws and regulations through appropriate means.

[0020] The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0021] This embodiment provides a system for measuring the pH value of low-conductivity water samples, such as... Figure 1 As shown, the system includes: a cooler 2, an electric regulating valve 22, a thermometer 20, a first online conductivity meter 13, a mixing tube 5, a variable frequency dosing pump 16, a second online conductivity meter 19, a pH meter 7, and a PLC controller 10. The cooling water inlet of the cooler 2 is connected to the electric regulating valve 22; the water sample outlet of the cooler 2 is connected to the inlet of the mixing tube 5 via a pipeline; the thermometer 20 and the first online conductivity meter 13 are located on the pipeline between the cooler 2 and the mixing tube 5; the inlet of the variable frequency dosing pump 16 is injected with KCl standard solution 15, and the outlet of the variable frequency dosing pump 16 is connected to the dosing port of the mixing tube 5; the outlet of the mixing tube 5 is connected to the second online conductivity meter 19 via a pipeline, and the outlet of the second online conductivity meter 19 is connected to the pH meter 7 via a pipeline; the PLC controller 10 is electrically connected to the thermometer 20, the electric regulating valve 22, the first online conductivity meter 13, the second online conductivity meter 19, and the variable frequency dosing pump 16.

[0022] Figure 1In the system, cooler 2 is connected to a cooling water inlet pipe 11 and a cooling water outlet pipe 23, and an electric regulating valve 22 is installed on the cooling water inlet pipe 11. A flow meter 14 is also installed on the pipeline between cooler 2 and mixing pipe 5, and the flow meter 14 is located between the first online conductivity meter 13 and mixing pipe 5. A check valve 18 and a flow meter 17 are installed on the pipeline between variable frequency dosing pump 16 and mixing pipe 5, and both check valve 18 and flow meter 17 are electrically connected to PLC controller 10. A three-way valve 21 is installed on the pipeline between mixing pipe 5 and pH meter 7, and the third port of the three-way valve 21 is connected to drain pipe 9. An electric regulating valve 12 is installed on the inlet pipe of cooler 2, and the electric regulating valve 12 is electrically connected to PLC controller 10.

[0023] Figure 1 In the cooling inlet tube 1, an electric regulating valve 12 is installed. After cooling, one end of the inlet tube 3 is connected to the outlet of the cooler 2, and the other end is connected to the mixing tube 5. A flow meter 14 and a first online conductivity meter 13 are installed sequentially on the inlet tube 3 near the mixing tube 5. One end of the dosing tube 4 is connected to the KCl standard solution 15, and the other end is connected to the mixing tube 5. A check valve 18, a flow meter 17, and a variable frequency dosing pump 16 are also installed on the dosing tube 4 near the mixing tube 5. One end of the inlet tube 6 is connected to the mixing tube 5, and the other end is connected to the pH meter 7. A drain pipe 8 is installed on the pH meter 7.

[0024] Specifically, Figure 1 In the process, a low-conductivity water sample enters the cooler 2 through the cooling inlet tube 1. Cooling water enters the cooler 2 through the cooling water inlet pipe 11 to cool the water sample. The cooled water after heat exchange is discharged through the cooling water outlet pipe 23. The PLC controller 10 automatically adjusts the opening of the electric regulating valve 22 to control the cooling water flow based on the temperature signal of the cooled water sample fed back by the thermometer 20, so that the temperature of the cooled water sample is kept constant within the preset temperature range, eliminating the interference of temperature changes on pH measurement. The cooled water sample flows into the mixing tube 5 through the cooling inlet tube 3. The first online conductivity meter 13 detects the conductivity of the water sample before dosing and feeds it back to the PLC controller 10. The PLC controller 10 controls the variable frequency dosing pump 16 to start according to the signal, pumping the KCl standard solution 15 into the mixing tube 5 through the dosing tube 4 to fully mix with the water sample, so as to increase the ionic strength and conductivity of the water sample.

[0025] After adding KCl standard solution, the water sample flows through the injection tube 6 and passes through the second online conductivity meter 19. The second online conductivity meter 19 detects the conductivity of the water sample after dosing and feeds it back to the PLC controller 10. The PLC controller 10 automatically adjusts the pumping volume of KCl standard solution in the variable frequency dosing pump 16 according to the difference between the conductivity signals fed back by the first online conductivity meter 13 and the second online conductivity meter 19, so that the conductivity of the water sample after dosing is kept constant within the preset conductivity range, effectively reducing the influence of liquid junction potential and avoiding measurement drift. The water sample after dosing finally flows into the pH meter 7 for pH measurement to obtain a stable pH measurement value.

[0026] The pH measurement system for low-conductivity water samples provided in this embodiment cools the water sample using a cooler and adjusts the cooling water flow rate with an electric regulating valve, effectively reducing the interference of temperature changes on pH measurement. A variable frequency dosing pump adds KCl standard solution to the water sample, increasing its ionic strength and conductivity, effectively reducing the influence of liquid junction potential. A first online conductivity meter and a second online conductivity meter detect the conductivity of the water sample before and after dosing, respectively. A PLC controller automatically regulates the electric regulating valve and the variable frequency dosing pump, ensuring the stability and accuracy of pH measurement without manual intervention. The system has a compact structure, a reasonable layout, and integrates automatic and intelligent measurement functions.

[0027] This embodiment provides a method for measuring the pH value of a low-conductivity water sample, applied to the PLC controller in the measurement system described above, such as... Figure 2 As shown, the method includes: Step S1: Based on the temperature signal of the cooled water sample fed back by the thermometer, adjust the opening of the electric regulating valve to keep the temperature of the water sample after flowing through the cooler within the preset temperature range. Before acquiring the temperature signal of the cooled water sample from the thermometer, the process also includes: (1) Control the electric regulating valve to open so that the cooling water enters the cooler through the cooling water inlet pipe and is discharged through the cooling water outlet pipe.

[0028] (2) Control the three-way valve to switch to the sewage discharge position so that the water sample is discharged through the sewage discharge pipe.

[0029] (3) Control the electric regulating valve to open and adjust the water sample flow rate to the preset flow rate value.

[0030] Specifically, refer to Figure 1Before acquiring the temperature signal of the cooled water sample from thermometer 20, system commissioning preparation is performed: PLC controller 10 controls the electric regulating valve 22 to open, allowing cooling water to enter cooler 2 through cooling water inlet pipe 11 and then exit through cooling water outlet pipe 23, enabling cooler 2 to have cooling capacity; simultaneously, it controls the three-way valve 21 to switch to the drain position, allowing the water sample to be discharged through drain pipe 9, preventing water samples that do not meet the measurement conditions from entering pH meter 7; it controls the electric regulating valve 12 to open, adjusting the water sample flow rate to the preset flow rate value, ensuring that the water sample passes through the system at a stable flow rate. Subsequently, PLC controller 10 acquires the temperature signal of the cooled water sample from thermometer 20. When the temperature deviates from the preset temperature range, PLC controller 10 automatically adjusts the opening of electric regulating valve 22 to change the cooling water flow rate, thereby precisely controlling the temperature of the cooled water sample to remain constant within the preset temperature range, eliminating the interference of temperature changes on subsequent pH measurements from the source.

[0031] Specifically, refer to Figure 1 Subsequently, the PLC controller 10 acquires the temperature signal of the cooled water sample fed back by the thermometer 20. When the temperature deviates from the preset temperature range, the PLC controller 10 automatically adjusts the opening of the electric regulating valve 22 to change the cooling water flow rate, thereby accurately controlling the temperature of the cooled water sample to be kept constant within the preset temperature range, eliminating the interference of temperature changes on subsequent pH measurements from the source.

[0032] Step S2: Based on the conductivity signal of the water sample before dosing fed back by the first online conductivity meter, control the frequency conversion dosing pump to pump the KCl standard solution into the mixing tube and mix it with the cooled water sample.

[0033] In some optional embodiments, before controlling the variable frequency dosing pump to pump the KCl standard solution into the mixing tube, the following steps are also included: (1) Control the three-way valve to switch to the sewage discharge position.

[0034] (2) When the displayed values ​​of the first online conductivity meter and the second online conductivity meter are the same and both are lower than the preset low conductivity threshold, control the variable frequency dosing pump to start.

[0035] (3) Receive the conductivity signal fed back by the second online conductivity meter, and adjust the frequency of the variable frequency dosing pump until the feedback value of the second online conductivity meter reaches the preset conductivity setting value.

[0036] Specifically, refer to Figure 1Before the variable frequency dosing pump 16 pumps the KCl standard solution 15 into the mixing tube 5, the dosing system is pre-commissioned: the PLC controller 10 controls the three-way valve 21 to switch to the drain position so that the water sample does not enter the pH meter 7; the first online conductivity meter 13 and the second online conductivity meter 19 are activated, and at this time the displayed values ​​of the two are the same and both are less than 1µS / cm; after the flow meter 17 is activated, the variable frequency dosing pump 16 is slightly opened, and the KCl standard solution 15 is pumped into the mixing tube 5 through the dosing tube 4 to mix with the water sample, and discharged through the sample inlet tube 6 to the drain tube 9. At this time, the displayed value of the second online conductivity meter 19 is greater than the displayed value of the first online conductivity meter 13. The frequency of the variable frequency dosing pump 16 is manually adjusted until the displayed value of the second online conductivity meter 19 reaches 5±0.5µS / cm, and the pre-commissioning calibration of the dosing system is completed.

[0037] Specifically, refer to Figure 1 After calibration, during the formal measurement process, the PLC controller 10 dynamically controls the variable frequency dosing pump 16 to pump the KCl standard solution 15 into the mixing tube 5 to mix with the cooled water sample based on the real-time feedback of the water sample conductivity signal before dosing from the first online conductivity meter 13, ensuring that the water sample has a stable ionic strength before entering the subsequent measurement stage.

[0038] Step S3: Based on the conductivity signal of the water sample after dosing fed back by the second online conductivity meter, adjust the KCl standard solution pumping rate of the variable frequency dosing pump to keep the conductivity of the water sample after dosing within the preset conductivity range.

[0039] Specifically, refer to Figure 1 After the water sample containing KCl standard solution is fully mixed through mixing tube 5, it flows into the second online conductivity meter 19. The second online conductivity meter 19 detects the conductivity of the water sample after dosing in real time and feeds it back to the PLC controller 10. The PLC controller 10 compares the conductivity feedback value of the water sample after dosing with the preset conductivity range (5±0.5µS / cm). When the conductivity of the water sample after dosing is lower than 5±0.5µS / cm, it indicates that the pumping volume of KCl standard solution is insufficient, and the PLC controller 10 controls the frequency conversion dosing pump 16 to increase the pumping volume of KCl standard solution 15. When the conductivity of the water sample after dosing is higher than 5±0.5µS / cm, it indicates that the pumping volume of KCl standard solution is excessive, and the PLC controller 10 controls the frequency conversion dosing pump 16 to decrease the pumping volume of KCl standard solution 15. Through this closed-loop regulation method, the PLC controller 10 tracks and maintains the conductivity of the water sample after dosing within the preset conductivity range in real time, thereby effectively reducing the concentration gradient difference between the water sample and the reference electrode, reducing the liquid junction potential and its drift, and ensuring the stability of pH measurement.

[0040] Step S4: Obtain the pH value of the water sample after dosing based on the pH meter.

[0041] The process of obtaining the pH value of the water sample after dosing based on the pH meter includes: controlling the three-way valve to switch to the sewage discharge position, allowing the water sample to be discharged through the sewage pipe for a preset time, and then controlling the three-way valve to switch to the measurement position, allowing the water sample to flow into the pH meter for measurement.

[0042] Specifically, refer to Figure 1 After the conductivity of the water sample stabilizes within the preset conductivity range following the addition of chemicals, and the PLC controller 10 has maintained the temperature at 25±1℃, the pH meter is put into operation. The PLC controller 10 controls the three-way valve 21 to switch to the measurement position, allowing the water sample to flow into the pH meter 7 for measurement. At this time, the value displayed on the pH meter 7 is the apparent pH value. Since the water sample has undergone cooling and temperature control treatment and chemical dosing and conductivity control treatment, the effects of temperature fluctuations and liquid junction potential have been effectively eliminated. However, the addition of KCl standard solution affects the H+ in the water sample. + The concentration caused a dilution effect, resulting in an apparent pH value lower than the true pH value of the water sample. Therefore, dilution compensation correction is required: read the flow rate T1 of the water sample in the cooled injection tube 3 through flow meter 14, and read the flow rate T2 of the KCl standard solution in the dosing tube 4 through flow meter 17. Substitute these values ​​into the correction formula pH true value = pH apparent value + Lg[T1 / (T1+T2)] to calculate the true pH measurement value of the water sample.

[0043] The method for measuring the pH value of low conductivity water samples provided in this embodiment achieves closed-loop control of water sample temperature by automatically adjusting the opening of the electric regulating valve to change the cooling water flow rate based on the feedback signal from the thermometer using a PLC controller. This effectively avoids the interference of temperature fluctuations on pH measurement. Furthermore, the PLC controller automatically controls the variable frequency dosing pump to add KCl standard solution to the water sample based on the feedback signal from the first online conductivity meter, and automatically adjusts the pumping volume of KCl standard solution based on the feedback signal from the second online conductivity meter. This achieves closed-loop precise adjustment of the dosage, effectively reducing the influence of liquid junction potential and avoiding measurement drift. This method requires no manual intervention and can achieve stable and accurate measurement of the pH value of low conductivity water samples, offering advantages such as high automation and reliable measurement results.

[0044] While embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and all such modifications and variations fall within the scope defined by the appended claims.

Claims

1. A system for measuring the pH value of low-conductivity water samples, characterized in that, include: Cooler (2), electric regulating valve (22), thermometer (20), first online conductivity meter (13), mixing pipe (5), variable frequency dosing pump (16), second online conductivity meter (19), pH meter (7) and PLC controller (10), among which, The cooling water inlet of the cooler (2) is connected to the electric regulating valve (22); The water sample outlet of the cooler (2) is connected to the inlet of the mixing tube (5) via a pipeline, and the thermometer (20) and the first online conductivity meter (13) are installed on the pipeline between the cooler (2) and the mixing tube (5); The inlet of the variable frequency dosing pump (16) is injected with KCl standard solution (15), and the outlet of the variable frequency dosing pump (16) is connected to the dosing port of the mixing tube (5). The outlet of the mixing tube (5) is connected to the second online conductivity meter (19) via a pipeline, and the outlet of the second online conductivity meter (19) is connected to the pH meter (7) via a pipeline. The PLC controller (10) is electrically connected to the thermometer (20), the electric regulating valve (22), the first online conductivity meter (13), the second online conductivity meter (19), and the variable frequency dosing pump (16).

2. The system for measuring the pH value of low-conductivity water samples according to claim 1, characterized in that, The cooler (2) is connected to a cooling water inlet pipe (11) and a cooling water outlet pipe (23), and the electric regulating valve (22) is installed on the cooling water inlet pipe (11).

3. The system for measuring the pH value of low conductivity water samples according to claim 1, characterized in that, A flow meter (14) is also provided on the pipeline between the cooler (2) and the mixing pipe (5). The flow meter (14) is located between the first online conductivity meter (13) and the mixing pipe (5).

4. The system for measuring the pH value of low-conductivity water samples according to claim 1, characterized in that, A check valve (18) and a flow meter (17) are installed on the pipeline between the variable frequency dosing pump (16) and the mixing pipe (5). The check valve (18) and the flow meter (17) are electrically connected to the PLC controller (10).

5. The system for measuring the pH value of low-conductivity water samples according to claim 1, characterized in that, A three-way valve (21) is installed on the pipeline between the mixing pipe (5) and the pH meter (7), and the third port of the three-way valve (21) is connected to the drain pipe (9).

6. The system for measuring the pH value of low-conductivity water samples according to claim 1, characterized in that, An electric regulating valve (12) is installed on the inlet pipe of the cooler (2), and the electric regulating valve (12) is electrically connected to the PLC controller (10).

7. A method for measuring the pH value of a water sample with low electrical conductivity, characterized in that, The method, applied to a PLC controller in the measurement system according to any one of claims 1 to 6, comprises: Based on the temperature signal of the cooled water sample fed back by the thermometer, the opening of the electric regulating valve is adjusted so that the temperature of the water sample after flowing through the cooler is maintained within the preset temperature range. Based on the conductivity signal of the water sample before dosing fed back by the first online conductivity meter, the variable frequency dosing pump is controlled to pump the KCl standard solution into the mixing tube to mix with the cooled water sample; Based on the conductivity signal of the water sample after dosing fed back by the second online conductivity meter, the pumping rate of KCl standard solution of the variable frequency dosing pump is adjusted so that the conductivity of the water sample after dosing is maintained within the preset conductivity range. The pH value of the water sample after dosing was obtained from the pH table.

8. The method for measuring the pH value of low conductivity water samples according to claim 7, characterized in that, Before acquiring the temperature signal of the cooled water sample fed back by the thermometer, the method further includes: The electric regulating valve is opened to allow cooling water to enter the cooler through the cooling water inlet pipe and then be discharged through the cooling water outlet pipe. Control the three-way valve to switch to the sewage discharge position so that the water sample is discharged through the sewage discharge pipe; The electric regulating valve is opened to adjust the water sample flow rate to the preset flow rate value.

9. The method for measuring the pH value of a low-conductivity water sample according to claim 7, characterized in that, Before controlling the variable frequency dosing pump to pump the KCl standard solution into the mixing tube, the method further includes: Control the three-way valve to switch to the sewage discharge position; When the displayed values ​​of the first online conductivity meter and the second online conductivity meter are the same and both are lower than the preset low conductivity threshold, the variable frequency dosing pump is controlled to start. The frequency of the variable frequency dosing pump is adjusted after receiving the conductivity signal from the second online conductivity meter until the feedback value of the second online conductivity meter reaches the preset conductivity setting value.

10. The method for measuring the pH value of a low-conductivity water sample according to claim 7, characterized in that, The process of obtaining the pH value of the water sample after dosing based on the pH table includes: Control the three-way valve to switch to the sewage discharge position, so that the water sample is discharged through the sewage pipe for a preset time. Then control the three-way valve to switch to the measurement position, so that the water sample flows into the pH meter for measurement.

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