Automatic calibration method for resistivity meter

Abstract

To shorten the time required for calibrating a specific resistance meter for measuring the quality of water incorporated in a pure water manufacturing device, and to reduce the error and variation in setting.SOLUTION: An automatic calibration method for calibrating a specific resistance meter 28 for measuring water quality includes: a connection step of temporarily connecting a specific resistance meter 60 for comparison, which has already been calibrated, to pipelines 27, 30 through which pure water flows in a pure water manufacturing device 10; a step of starting operation of a pure water production device 10 after the connection step; and a calibration step of automatically setting the cell constant of the specific resistance meter 28 for measurement of water quality so that a specific resistance value based on measurement by the specific resistance meter 28 for measuring water quality coincides with a specific resistance value measured by the specific resistance meter 60 for comparison while continuing operation of the pure water production device 10.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for automatically calibrating a resistivity meter built into a pure water production system or an ultrapure water production system. [Background technology]

[0002] A resistivity meter is provided in a pure water production system or an ultrapure water production system to measure the quality of the pure water or ultrapure water produced. In the following description, the term "pure water" includes ultrapure water, and the term "pure water production system" includes the ultrapure water production system. The resistivity meter provided in the pure water production system measures the resistivity of the pure water based on the electrical resistance between the electrodes when a cell in which a pair of electrodes are provided facing each other is filled with pure water. In general, in a resistivity meter, the resistivity is calculated by dividing the measured electrical resistance value between a pair of electrodes by the cell constant. The cell constant is a parameter equivalent to the value obtained by dividing the electrode gap by the electrode area, and is determined based on the physical shape and size of the cell, the shape and size of the electrodes, the electrode gap, etc., and differs for each cell of the resistivity meter. Therefore, in order to perform accurate measurements using a resistivity meter, it is necessary to calibrate the cell constant in advance.

[0003] The theoretical resistivity of water that does not contain impurities is 18.2 MΩ·cm at 25°C. Therefore, the cell constant of a resistivity meter installed in the pure water path of a pure water production system can be calibrated by preparing a comparison resistivity meter that has already been calibrated, connecting the resistivity meter to be calibrated and the comparison resistivity meter in series, and performing online measurements while running pure water with a resistivity of, for example, 18.0 MΩ·cm or more through these resistivity meters. In this case, the measured values ​​from each resistivity meter are compared, and the cell constant of the resistivity meter to be calibrated is manually adjusted and set so that the values ​​are the same.

[0004] The resistivity of water also changes with temperature; as the water temperature rises, the resistivity decreases. From the viewpoint of water quality management, it is more convenient to be able to manage the water quality based on the resistivity alone, regardless of the water temperature, so it is preferable to measure the water temperature as well as the resistivity, and to convert the measured resistivity value into a value at 25°C according to the water temperature and display it. For this reason, resistivity meters generally also have a temperature sensor such as a thermistor to measure the water temperature, and the temperature sensor is also subject to calibration. If the temperature sensor is a thermistor, the measured temperature value is the actual measured value of the electrical resistance of the thermistor multiplied by the temperature constant, so the temperature constant must be calibrated.

[0005] Patent Document 1 relates to the measurement of conductivity, which is the reciprocal of resistivity, and discloses that a resistor for identifying a cell constant is provided in an electrode probe including a cell, and when the electrode probe is connected to a converter, which is the main body of the conductivity meter, an identification signal is generated in the converter according to the value of the identification resistor, a cell constant is called up from a memory unit according to the identification signal, and the conductivity is calculated using the called-up cell constant. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Utility Model Application Publication No. 63-195269 Summary of the Invention [Problem to be solved by the invention]

[0007] When a resistivity meter to be calibrated (i.e., a resistivity meter used for measuring water quality in a pure water production system) and a resistivity meter for comparison are connected in series, pure water is run through them, and the cell constant of the resistivity meter to be calibrated is adjusted based on the measured values ​​of both resistivity meters, which requires repeated trial and error, and the calibration work takes time. The calibration of the resistivity meter for measuring water quality built into the pure water production system is generally performed as one of the items of shipping inspection when the pure water production system is shipped, and since it is desired to reduce the time required for the shipping inspection, it is also desired to reduce the time required for the calibration work of the resistivity meter. In addition, manual calibration work may cause variations in the setting of the cell constant depending on the worker in charge of the calibration work, or may cause mistakes in the setting itself. In addition, the technology described in Patent Document 1 is based on the premise that the cell constant of the cell installed in the electrode probe is already known, and therefore cannot be used to determine the cell constant itself.

[0008] An object of the present invention is to provide an automatic calibration method capable of shortening the time required to calibrate a resistivity meter for measuring water quality that is built into a pure water production system and reducing setting errors and variations. [Means for solving the problem]

[0009] The automatic calibration method of the present invention is an automatic calibration method for calibrating a resistivity meter for measuring water quality that is provided in a piping through which pure water flows in a pure water manufacturing apparatus in order to measure the quality of the pure water produced in the pure water manufacturing apparatus, and includes a connection step of temporarily connecting a comparison resistivity meter that has already been calibrated to the piping, a step of starting operation of the pure water manufacturing apparatus after the connection step, and a calibration step of automatically setting the cell constant of the resistivity meter for measuring water quality so that the resistivity value based on measurement with the resistivity meter for measuring water quality matches the resistivity value measured with the comparison resistivity meter while continuing to operate the pure water manufacturing apparatus. Effect of the Invention

[0010] According to the present invention, it is possible to shorten the time required to calibrate a resistivity meter for measuring water quality that is built into a pure water production system, and to reduce setting errors and variations. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of a pure water producing apparatus. [Diagram 2] 4 is a flowchart showing a procedure for calibrating a resistivity meter in the pure water producing system shown in FIG. [Diagram 3] 1 is a flowchart showing a procedure for determining a cell constant. [Figure 4] FIG. 13 is a diagram showing another example of the configuration of a pure water producing apparatus. [Diagram 5] FIG. 13 is a diagram showing another example of the configuration of a pure water producing apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Next, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a diagram showing the configuration of a pure water production system to which a method for automatically calibrating a resistivity meter according to an embodiment of the present invention is applied.

[0013] The illustrated pure water production system 10 produces pure water from the supply water such as tap water, and includes an activated carbon device (AC) 11 that first treats the supply water, and a reverse osmosis membrane device (RO) 12 that is equipped with a reverse osmosis membrane and treats the outlet water of the activated carbon device 11. Permeated water from the reverse osmosis membrane device 12 is stored in a tank 21, and concentrated water from the reverse osmosis membrane device 12 is discharged to the outside. A pump 22 that feeds the water stored in the tank 21 is provided at the outlet of the tank 21, and a flow sensor (FI) 23, an ultraviolet oxidation device (UV) 24, and a non-regenerative ion exchange device (cartridge polisher (CP)) 25 are provided in this order at the outlet of the pump 22, i.e., the secondary side. A pure water pipe 27 is provided at the outlet of the non-regenerative ion exchange device 25 to supply the pure water produced by the pure water production system 10 to a use point, and a solenoid valve 29 is provided in the pure water pipe 27 to control the supply of pure water to the use point. A resistivity meter 28 for measuring water quality is provided at a position upstream of the solenoid valve 29 in the pure water pipe 27 to measure the quality of the pure water flowing through the pure water pipe 27. The resistivity meter 28 for measuring water quality includes a resistivity sensor (RI) for measuring the resistivity of the pure water and a temperature sensor (TI) for measuring the temperature of the pure water. The resistivity sensor is configured by arranging a pair of electrodes in a cell through which the pure water flows, and measures the electrical resistance between the electrodes and outputs a resistance value. The resistivity of the pure water can be obtained by dividing the electrical resistance by a cell constant J specific to the cell. The automatic calibration method according to the present invention is intended to automatically set the cell constant J of the resistivity sensor of the resistivity meter 28 for measuring water quality. The temperature sensor is configured by a temperature sensor such as a thermistor, and an accurate water temperature can be obtained by applying a temperature constant specific to the temperature sensor to the measurement output value of the temperature sensor.

[0014] In the pure water production system 10, a circulation pipe 30 is provided which branches off from the pure water pipe 27 at a position between the connection position of the resistivity meter 28 for measuring water quality and the solenoid valve 29 and connects to the tank 21. A solenoid valve 31 is provided on the circulation pipe 30. The circulation pipe 30 is provided to return the pure water that has not been supplied to the point of use to the tank 21, and this allows the pure water production system 10 to constantly produce pure water and circulate it within the system regardless of whether there is a demand for pure water at the point of use. The pure water circulates within the system, and each time it circulates, the pure water is further purified and the water quality is further improved, so that the pure water production system 10 can supply pure water or ultrapure water of better quality to the point of use according to demand.

[0015] The pure water production system 10 is further provided with a control device 50 that controls the entire system, and an operation panel 51 that is connected to the control device 50 to display information to a user and accept input from the user. The control device 50 is configured with a processor such as a microprocessor (MPU) or a microcomputer, and in particular controls the opening and closing of solenoid valves 29, 31. The detection output of the flow rate sensor 23 and the detection output of the resistivity meter 28 for measuring water quality are input to the control device 50. The control device 50 has a function of storing the cell constant J and temperature constant of the resistivity meter 28 for measuring water quality, and receives the detected resistance value and detected temperature value of the resistivity meter 28 for measuring water quality. The control device 50 divides the detected resistance value by the cell constant J to calculate the resistivity value, and further has a function of converting the calculated resistivity value into a value when the water temperature is 25°C and outputting it to the operation panel 51. A measured value of the water temperature is necessary to convert the resistivity value to a value at 25° C., and the control device 50 calculates an accurate water temperature by applying a temperature constant to the temperature detection value of the resistivity meter 28 for measuring water quality. The water temperature calculated by the control device 50 can also be output to the operation panel 51.

[0016] The pure water production system 10 has several operation modes for supplying pure water to a point of use. For example, when a specified amount of pure water is to be supplied to a point of use, the control device 50 performs control to close the solenoid valve 31 and open the solenoid valve 29 until the integrated value of the flow rate measured by the flow rate sensor 23 reaches the specified amount. Alternatively, when pure water is to be supplied to the point of use only when there is an input, the control device 50 performs control to open the solenoid valve 31 only when there is an input.

[0017] Next, there will be described automatic calibration of the cell constant of the resistivity meter 28 for measuring water quality in the pure water production system 10 shown in Fig. 1. Fig. 2 is a flow chart showing the procedure of the automatic calibration.

[0018] First, in step 101, a comparative resistivity meter 60 is temporarily connected to the piping through which the pure water flows in the pure water production system 10. In the example shown in FIG. 1, the comparative resistivity meter 60 is attached between the solenoid valve 31 and the tank 21 in the circulation piping 30. In the pure water production system 10, flexible piping members such as flexible tubes are often used for the piping, and these piping members are connected to each other by one-touch joints or the like. Therefore, it is easy to temporarily connect the comparative resistivity meter 60 to the circulation piping 30 by changing the connection relationship of the piping members via the one-touch joints. In the example shown in the figure, the comparative resistivity meter 60 is connected to a position downstream of the resistivity meter 28 for measuring water quality in the path through which the pure water flows. It is also possible to connect the comparative resistivity meter 60 to a position upstream of the resistivity meter 28 for measuring water quality in the path through which the pure water flows, but it is preferable to place the comparative resistivity meter 60 downstream of the resistivity meter 28 for measuring water quality in order to prevent the occurrence of contamination in the pure water production system 10. The comparative resistivity meter 60 has already been calibrated, and is equipped with a resistivity sensor (RI) and a temperature sensor (TI) like the resistivity meter for measuring water quality. The comparative resistivity meter 60 is configured to be able to output or display the measured resistivity value and temperature value (water temperature value). The resistivity value output or displayed by the comparative resistivity meter 60 is generally converted to a value when the water temperature is 25°C.

[0019] Next, solenoid valve 29 is closed, solenoid valve 31 is opened, and feed water is supplied to the pure water production system 10, and in this state, operation of the pure water production system 10 is started in step 102. The pure water produced by the pure water production system 10 is returned to tank 21 via circulation piping 30, so that the pure water is circulated and purified, and the quality of the pure water in the pure water production system 10 improves over time, and eventually the resistivity indicated by the comparative resistivity meter 60 reaches 18.2 MΩ·cm (i.e., the specified value). As shown in step 103, the process is suspended until the resistivity value measured by the comparative resistivity meter 60 reaches the specified value (18.2 MΩ·cm converted at 25° C.).

[0020] When the resistivity value measured by the comparative resistivity meter 60 reaches the specified value, next, in step 104, the temperature constant of the temperature sensor of the resistivity meter 28 for measuring water quality is calibrated based on the temperature measurement value of the comparative resistivity meter 60. The temperature constant is calibrated by manually or automatically changing the temperature constant so that the temperature measurement value of the temperature sensor of the resistivity meter 28 for measuring water quality coincides with the temperature measurement value of the comparative resistivity meter 60. When manually calibrating the temperature constant, the temperature measurement value of the resistivity meter 28 for measuring water quality is displayed on the operation panel 51, and a command to increase or decrease the temperature constant is input to the control device 50 via the operation panel 51 until the temperature measurement value displayed on the operation panel 51 coincides with the temperature measurement value of the resistivity meter 28 for measuring water quality. When automatically calibrating the temperature constant, the control device 50 itself executes this process of increasing or decreasing the temperature constant. The calibrated temperature constant is stored in a memory (not shown) in the control device 50 and is used for subsequent water temperature measurements.

[0021] After the calibration of the temperature constant is completed, the automatic calibration process in the control device 50 is started. The automatic calibration process is executed by the processor in the control device 50 executing a program. When the automatic calibration process is started, in step 105, the control device 50 substitutes an initial value for the value of the cell constant J. For example, a value that is predetermined for each model of the resistivity meter 28 for measuring water quality can be used as the initial value. If the cell constant has been calibrated in the past, the cell constant obtained by the calibration may be used as the initial value. After that, in step 106, the control device 50 measures the resistivity using the resistivity meter 28 for measuring water quality. The resistivity value is calculated by dividing the resistance detection value input from the resistivity meter 28 for measuring water quality by the cell constant J and further compensating for the water temperature. At this point, the pure water production apparatus 10 continues to produce pure water by circulation purification, and the resistivity of the pure water should be the above-mentioned specified value (18.2 MΩ·cm) converted to a value at a water temperature of 25°C. Therefore, in step 107, the control device 50 judges whether the resistivity value calculated as described above based on the measurement by the resistivity meter 28 for measuring water quality matches the specified value, i.e., whether the resistivity has been measured correctly. If the resistivity has been measured correctly, the process proceeds to step 109, and if it has not been measured correctly, in step 108 the control device 50 changes the cell constant J so that the measurement can be performed correctly, and then repeats the process from the measurement of the resistivity in step 106. Ultimately, the adjustment of the cell constant is repeated until the resistivity value based on the measurement by the resistivity meter 28 for measuring water quality matches the specified value.

[0022] In step 109, the control device 50 stores the cell constant calibrated by repeating the processes from step 106 to step 108 in its memory (not shown), and displays it on the operation panel 51 to prompt the user to confirm. This ends the automatic calibration process. After this, when the quality of pure water is measured using the water quality measurement resistivity meter 28, the resistivity value can be calculated using the cell constant calibrated and stored as described above, as shown in step 110. Once the cell constant has been calibrated, the comparison resistivity meter 60 is no longer necessary, so in step 111, the production of pure water in the pure water production system 10 is stopped and the pump 22 is also stopped, and in step 112, the comparison resistivity meter 60 is removed.

[0023] FIG. 3 is a flow chart showing an example of the process in step 108, and for the sake of clarity, steps 106 to 109 in FIG. 2 are also shown. If the resistivity is not measured correctly in step 107, the resistivity value (calculated value) calculated from the resistance detection value of the resistivity meter 28 for measuring water quality and the water temperature may be larger or smaller than the above-mentioned specified value (described as "target value" in FIG. 3). Therefore, in step 108A, it is determined whether the calculated value is smaller than the target value. The goal of the calibration is that the calculated value and the target value match. Here, "match" means that the resistivity values ​​match as numbers up to the first decimal place when expressed in MΩ·cm units, taking into account measurement accuracy, etc. If the calculated value is smaller than the target value, the cell constant is decreased by Δ in step 108B and the process returns to step 106. If the calculated value is not smaller than the target value, the cell constant is increased by Δ in step 108B and the process returns to step 106. If the calculated value and the target value match, it has already been determined in step 107 that the resistivity has been measured correctly, and therefore no change in the cell constant occurs. The increment Δ of the change in the cell constant is a predetermined value, for example, 0.01 m. -1 It is.

[0024] According to the automatic calibration method of this embodiment described above, it is possible to automate the calibration of the cell constant of the resistivity meter 28 for measuring water quality built into the pure water production system 10, thereby shortening the time required for calibration and reducing setting errors and variations. For example, while manual calibration of the cell constant takes about 3 to 4 minutes, according to the method of this embodiment, calibration of the cell constant can be completed in about 10 seconds.

[0025] The comparative resistivity meter 60 may be one that outputs the detection result as a signal in a data format acceptable to the control device 50. FIG. 4 shows a pure water production system 10 to which such a comparative resistivity meter 60 can be attached. In the pure water production system 10 shown in FIG. 4, when the comparative resistivity meter 60 is temporarily connected to the circulation piping 30, the comparative resistivity meter 60 and the control device 50 can be connected by a signal line 61. The control device 50 has an input port to which the signal line 61 can be connected. By connecting by the signal line 61, the resistivity value and water temperature measured by the comparative resistivity meter 60 are input to the control device 50 via the signal line 61. The calibration of the water quality measuring resistivity meter 28 in the pure water production system 10 shown in FIG. 4 can be performed in a more automated form than the process shown in FIG. 2.

[0026] When calibrating the resistivity meter 28 for measuring water quality in the pure water production system 10 shown in Fig. 4, the comparison resistivity meter 60 is connected to the circulation pipe 30 as in the above case, and further, the signal line 61 is connected to the control device 50. Then, the operation of the pure water production system 10 is started, and the control device 50 is set to the automatic calibration mode. When the automatic calibration mode is set, the control device 50 reads the measured temperature value from the comparison resistivity meter 60 and automatically calibrates the temperature constant as described above, according to a program executed by the processor in the control device 50, and then reads the measured resistivity value from the comparison resistivity meter 60 and automatically calibrates the cell constant by carrying out the same process as shown in Fig. 3 with the read measured value as a target value, and then stores the calibrated temperature constant and cell constant in a memory (not shown), and outputs the calibrated cell constant to the operation panel 51. Since the resistivity measurements taken by the comparison resistivity meter 60 are constantly input to the control device 50, in the case of the pure water production system 10 shown in FIG. 4, calibration of the cell constant can be completed even before the resistivity of the pure water circulating within the pure water production system 10 reaches 18.2 MΩ cm at 25° C.

[0027] FIG. 5 shows an example of another pure water production apparatus to which the automatic calibration method of the present invention is applied. When pure water is collected in a laboratory or the like, for example, in a beaker, flask, or test tube, a water collection dispenser connected to the pure water production apparatus is used. In order to improve the quality of the pure water discharged from the water collection dispenser, JP 2020-006295 A discloses extending the piping through which the pure water circulates from the pure water production apparatus to the water collection dispenser for circulatory purification. FIG. 5 shows an example in which the piping for circulatory purification is extended to the water collection dispenser. The pure water production apparatus 10 shown in FIG. 5 is the pure water production apparatus 10 shown in FIG. 4 in which primary pure water is supplied to the tank 21 without providing an activated carbon device or a reverse osmosis membrane device, and further includes an ultrafiltration membrane device (UF) 26 connected to the outlet of the non-regenerative ion exchange device 25 and a circulation outlet 32 ​​and a circulation inlet 33 for circulating pure water between the water collection dispenser 70 and the pure water collection dispenser 70. The pure water pipe 27 is provided between the ultrafiltration membrane device 26 and the circulation outlet 32. The pure water pipe 27 is not provided with an electromagnetic valve. The circulation pipe 30 is provided between the circulation inlet 33 and the tank 21, rather than branching off from the water supply pipe 27. In order to enable the production of pure water by circulation purification in the pure water production apparatus 10 even when the water sampling dispenser 70 is not connected to the pure water production apparatus 10, a bypass pipe 34 is provided connecting the pure water pipe 27 and the circulation pipe 30, and a bypass valve 35 is provided in the bypass pipe 34. The position where the bypass pipe 34 connects to the circulation pipe 30 is between the electromagnetic valve 31 and the tank 21. The resistivity meter 28 for measuring water quality is provided in the pure water pipe 27 between the position where the bypass pipe 33 branches off and the circulation outlet 32.

[0028] The pure water production apparatus 10 and the water sampling dispenser 70 are connected by a pipe 71 connected to the circulation outlet 32 ​​through which the pure water flows from the pure water production apparatus 10 to the water sampling dispenser 70, and a pipe 72 connected to the circulation inlet 33 through which the pure water flows from the water sampling dispenser 70 to the pure water production apparatus 10. Flexible tubes, for example, are used as the pipes 71 and 72. The water sampling dispenser 70 is provided with pipes 73 and 74, an electromagnetic valve 75, a nozzle 76, and a switch 77. One end of the pipe 73 is connected to the pipe 71, and one end of the pipe 74 is connected to the pipe 72. The other end of the pipe 73 and the other end of the pipe 74 are connected to each other, and an electromagnetic valve 75 is provided on the pipe connecting this connection point to the nozzle 76. Therefore, if the electromagnetic valve 75 is opened when the pure water is circulating to the water sampling dispenser 70 through the pipes 71 and 72, at least a part of the circulating pure water will be discharged from the nozzle 76. The pure water ejected from the nozzle 76 can be poured into, for example, a test tube or a beaker.

[0029] The switch 77 is electrically connected to the control device 50, and a command input by a user operating the switch 77 is transmitted to the control device 50. The solenoid valve 75 is controlled by the control device 50. As a result, if the control device 50 executes control to open the solenoid valve 75 for which the user operates the switch 77, pure water is discharged from the nozzle 76 only while the switch 77 is being operated. Alternatively, when the user operates the switch once, the control device 50 can perform control so that a predetermined volume of pure water is discharged from the nozzle 76 based on the detection result of the flow rate sensor 23.

[0030] When calibrating the resistivity meter 28 for measuring water quality of the pure water production system 10 shown in Fig. 5, it is sufficient to connect the comparison resistivity meter 60 to the pipe 71 through which pure water flows from the pure water production system 10 toward the water sampling dispenser 70. At this connection position of the comparison resistivity meter 60, pure water circulates and flows due to circulatory purification, so the resistivity meter 28 for measuring water quality can be calibrated using the same procedure as in the above example.

[0031] The embodiment of the present invention has been described above. The calibration of the resistivity meter 28 for measuring water quality provided in the pure water production apparatus 10 is performed when the pure water production apparatus 10 is shipped from the factory, and is often performed after the shipment, for example, once a year. When there are many pure water production apparatuses 10 to be managed and therefore many resistivity meters 28 for measuring water quality, it is laborious to manage the calibration records of each resistivity meter 28 for measuring water quality. Therefore, by connecting the control unit 50 of each pure water production apparatus 10 to a server provided on the cloud side via a network, transmitting the cell constant of each resistivity meter 28 for measuring water quality automatically calibrated by the method based on the present invention to the server, and recording the cell constant on the cloud side, it becomes possible to collectively manage the periodic calibration records of many pure water production apparatuses 10 on the cloud. [Explanation of symbols]

[0032] 21 Tank 24. Ultraviolet Oxidation Device 25 Non-regenerative ion exchange device 26 Ultrafiltration Membrane Device 27 Pure water piping 30 Circulation piping 28 Resistivity meter for water quality measurement 50 Control device 51 Operation Panel 60 Comparison resistivity meter 70 Water Dispenser 76 Nozzle

Claims

1. 1. An automatic calibration method for calibrating a resistivity meter for measuring water quality, which is provided in a pipe through which pure water flows in a pure water production apparatus in order to measure the quality of the pure water produced in the pure water production apparatus, comprising: a connecting step of temporarily connecting a comparison resistivity meter that has already been calibrated to the piping; a step of starting operation of the pure water producing apparatus after the connecting step; a calibration step of automatically setting a cell constant of the resistivity meter for measuring water quality while continuing the operation of the pure water production apparatus so that the resistivity value based on the measurement by the resistivity meter for measuring water quality coincides with the resistivity value measured by the comparative resistivity meter; having an automatic calibration method in which, in the calibration step, after the resistivity value measured by the comparison resistivity meter becomes a specified value by continuing operation of the pure water production apparatus, the cell constant is automatically set so that the resistivity value based on the measurement by the water quality measurement resistivity meter becomes the specified value.

2. An automatic calibration method for calibrating a resistivity meter for measuring water quality, which is provided in a pipe through which pure water flows in a pure water production apparatus in order to measure the quality of the pure water produced in the pure water production apparatus, comprising: a connecting step of temporarily connecting a comparison resistivity meter that has already been calibrated to the piping; a step of starting operation of the pure water producing apparatus after the connecting step; a calibration step of automatically setting a cell constant of the resistivity meter for measuring water quality while continuing the operation of the pure water production apparatus so that the resistivity value based on the measurement by the resistivity meter for measuring water quality coincides with the resistivity value measured by the comparative resistivity meter; Has The automatic calibration method includes connecting the comparison resistivity meter downstream of the installation position of the water quality measurement resistivity meter in a pipe through which the pure water flows.

3. The resistivity meter for measuring water quality and the resistivity meter for comparison each have a temperature measurement function, 3. The automatic calibration method according to claim 1, wherein in the calibration step, a temperature constant in a temperature measurement function of the resistivity meter for measuring water quality is corrected so that a water temperature value based on measurement by the resistivity meter for measuring water quality coincides with a water temperature value measured by the comparative resistivity meter, and then the cell constant is automatically set.

4. 4. The automatic calibration method according to claim 3, further comprising automatically correcting the temperature constant.

5. the pure water production apparatus has a processor that receives a resistance detection value of the water quality measuring resistivity meter and calculates the resistivity of the pure water based on the resistance detection value and the cell constant; 5. The automatic calibration method according to claim 1, wherein in the calibration step, the processor sets the cell constant so that a resistivity value calculated from the resistance detection value and the cell constant matches a resistivity value measured by the comparison resistivity meter, and stores the set cell constant.

6. The resistivity meter for measuring water quality and the resistivity meter for comparison each have a temperature measurement function, the pure water production apparatus has a processor which receives the resistance detection value and the temperature detection value of the resistivity meter for measuring water quality and calculates the resistivity of the pure water based on the resistance detection value, the temperature detection value, the cell constant, and a temperature constant of a temperature measurement function of the resistivity meter for measuring water quality; In the calibration step, the processor corrects the temperature constant so that the water temperature calculated from the temperature detection value of the water quality measurement resistivity meter coincides with the water temperature measured by the comparison resistivity meter, then sets the cell constant so that the resistivity value calculated from the resistance detection value and the cell constant coincides with the resistivity value measured by the comparison resistivity meter, and stores the corrected temperature constant and the set cell constant. The automatic calibration method according to claim 2 .

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