pH MEASUREMENT SYSTEM AND METHOD

Abstract

The present disclosure provides for an on-line system and method to determine a pH of low conductivity water. The system includes a first flow cell and a second flow cell to receive the water at a first flow rate and a second flow rate, respectively; a first pH probe positioned in the first flow cell to measure a first pH at a predetermined temperature and the first flow rate; a second pH probe positioned in the second flow cell to measure a second pH at the predetermined temperature and the second flow rate; a pH analyzer coupled to the first pH probe and the second pH probe, where the pH analyzer allows for a comparison of the first pH value and the second pH value; and an outlet for the water from the on-line system.

Description

[0001] pH MEASUREMENT SYSTEM AND METHOD

[0002] Field of Disclosure

[0003]

[0001] The present disclosure relates generally to pH measurements and in particular to systems and methods of pH measurements of high purity water.

[0004] Background

[0005]

[0002] Accurate and reliable determination of the pH of high purity water is difficult due to its very low conductivity and limited buffering capacity. Common problems with conventional pH measuring of such water include large drift, unacceptable flow sensitivity and poor temperature compensation. Such problems impact the pH measurement for high purity water systems used in a variety of industrial processes, such as hydrocarbon production, power production units, plastics manufacturing and other industrial operations that rely heavily on having high purity water systems. For example, it is very difficult to measure the pH of high purity water having a conductivity of less than 100 microsiemens per centimeter (pS / cm). This is due to, among other things, the very low ion levels in the water. The temperature of the water and its flow rate to the pH analyzer can also have a significant impact on the accurate and precise monitoring of the pH value. It is also the case that a lot of the pH measurements for these industrial processes are done manually, which can lead to higher levels of error due to the measurement process. As a result, there is a need in the art for a pH measurement system that can provide a uniform and standard pH measurement of high purity water that can meet the stringent pH measuring needs of the wide variety of industrial processes.

[0006] Summary

[0007]

[0003] The present disclosure provides for a system and method that can allow for an accurate and reliable determination of the pH of low conductivity water (e.g., conductivity of less than 100 microsiemens per centimeter (pS / cm)). Such low conductivity water can typically be found in a wide variety of industrial processes, such as hydrocarbon production, power production units, plastics manufacturing and other industrial operations that rely heavily on having high purity water systems. Precise determination of the pH of low conductivity water samples in such industrial processes is difficult due to the very low conductivity and limited buffering capacity of the water sample. The result is difficulties in accurately and reliably measuring pH values of the water used in these industrial processes due to problems such as large pH drift, unacceptable flow sensitivity and poor temperature compensation.

[0008]

[0004] The subject matter of the present disclosure addresses the above identified problems and those as discussed herein, which can allow for the accurate and reliably pH measurement of high purity water used in industrial processes. In addition, the system and method of the present disclosure can perform the pH measurements automatically (e.g., not necessarily manually achieved) at a predetermined temperature, both of which can lead to lower levels of pH measurement errors. As such, the system and method of the present disclosure addresses the need in the art for a pH measurement system that can provide a uniform and standard pH measurement that can meet the stringent pH measuring needs of the wide variety of industrial processes.

[0009]

[0005] Embodiments of the present disclosure provide for an on-line system for measuring a pH value of water from an industrial process, where the water has a conductivity of 0.03 to 100 pS / cm as measured at a predetermined temperature of 15 to 40 °C. The on-line system of the present disclosure includes an inlet for the water from the industrial process; a first flow cell and a second flow cell fluidly coupled in parallel to receive the water at the predetermined temperature; a first flow control fluidly coupled to the first flow cell and a second flow control fluidly coupled to the second flow cell, where each of the first flow control and the second flow control independently controls a first flow rate of the water through the first flow cell and a second flow rate of the water through the second flow cell; a first pH probe positioned in the first flow cell to measure a first pH value of the water at the predetermined temperature and the first flow rate; a second pH probe positioned in the second flow cell to measure a second pH value of the water at the predetermined temperature and the second flow rate; a pH analyzer coupled to the first pH probe and the second pH probe, where the pH analyzer allows for a comparison of the first pH value and the second pH value; and an outlet for the water from the on-line system.

[0010]

[0006] Embodiments of the present disclosure further provide for a method of measuring a pH of water from an industrial process with the on-line system, where the water has a conductivity of 0.03 to 100 S / cm as measured at a predetermined temperature of 15 to 40 °C. The method includes receiving water into the on-line system from the industrial process; measuring a first pH value of the water at a first flow rate and at the predetermined temperature with a first pH probe in the on-line system; measuring a second pH value of the water at a second flow rate and at the predetermined temperature with a second pH probe in the on-line system, where measuring the first pH and the second pH is done on the water flowing in parallel to the first pH probe and the second pH probe; determining a difference between the first pH value and the second pH value; measuring a third pH value of the water at a third flow rate and at the predetermined temperature with a third pH probe, portable or permanently installed, in the online system when the difference between the first pH value and the second pH value exceeds a predetermined threshold; and recalibrating at least one of the first pH probe and the second pH probe when the difference between the first pH value and the second pH value does exceed the predetermined threshold.

[0011]

[0007] Other principal features and advantages of the disclosure will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.

[0012] Brief Description of the Figures

[0013]

[0008] FIG. 1 illustrates an embodiment of an on-line system for measuring a pH value of water from an industrial process according to an embodiment of the present disclosure.

[0014]

[0009] FIG. 2 pH sensor readings of redundant pH probes and deviation between the sensors.

[0015] Detailed Description

[0016]

[0010] The present disclosure provides for a system and method that can allow for an accurate and reliable determination of the pH of low conductivity water (e.g., conductivity of less than 100 microsiemens per centimeter (pS / cm)). Such low conductivity water can typically be found in a wide variety of industrial processes, such as hydrocarbon production, power production units, plastics manufacturing and other industrial operations that rely heavily on having high purity water systems. Precise determination of the pH of low conductivity water samples in such industrial processes is difficult due to the very low conductivity and limited buffering capacity of the water sample. The result is difficulties in accurately measuring pH values in these industrial processes due to problems such as large pH drift, unacceptable flow sensitivity and poor temperature compensation. [Oi l] The subject matter of the present disclosure addresses the above identified problems and those as discussed herein, which can allow for the accurate pH measurement of high purity water systems used in industrial processes. In addition, the system and method of the present disclosure can perform the pH measurements automatically (e.g., not necessarily manually achieved) at a predetermined temperature, both of which can lead to lower levels of pH measurement errors. As such, the system and method of the present disclosure addresses the need in the ail for a pH measurement system that can provide a uniform and standard pH measurement that can meet the stringent pH measuring needs of the wide variety of industrial processes.

[0017]

[0012] As used herein, "a," "an," "the," "at least one," and "one or more" are used interchangeably. The term "and / or" means one, one or more, or all of the listed items. The recitations of numerical ranges by endpoints include all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.). It is emphasized that the purpose of the figures is to illustrate and the figures are not intended to be limiting in any way. The figures herein may not be to scale and relationships of elements in the figures may be exaggerated. The figures are employed to illustrate conceptual structures and methods herein described.

[0018]

[0013] Referring now to FIG. 1, there is shown an embodiment of an on-line system 100 of the present disclosure, which includes pH probes arranged in redundant and parallel fashion to which water from an industrial process is supplied at a predetermined temperature and independently controlled flow rate for pH value measurement. For the various embodiments, the on-line system 100 of the present disclosure can be used for measuring a pH value of water from an industrial process. As discussed herein, industrial processes can include those in which high purity water is an important aspect of production. Such industrial processes can include hydrocarbon production; power production units; plastics manufacturing; semiconductor manufacturing; pharmaceutical and biotechnology manufacturing and production; food and beverage production; aerospace manufacturing processes; and photovoltaic (solar cell) production, among other industrial operations. As used herein, the water for such industrial processes can have, among other properties, a conductivity of 100 or less microsiemens per centimeter (pS / cm) as measured at a predetermined temperature of 15 to 40 °C. In one embodiment, the water for such industrial processes can have a conductivity of 0.03 to 100 pS / cm as measured at a predetermined temperature of 15 to 40 °C.

[0014] As is known in the art, conductivity is a measure of the ability of a solution (e.g., water) to conduct an electric current. In the context of water for the industrial processes discussed herein, conductivity is a parameter used to assess the level of impurities and ions present in the water. Common sources of conductivity in water include dissolved salts, minerals, and other impurities. To achieve high-purity water, various water purification technologies such as reverse osmosis, deionization, and distillation are employed to remove these impurities and reduce the conductivity to the required levels. The goal in applications requiring water having a high degree of purity, such as in the aforementioned industrial processes, is to minimize the conductivity, which indicates a very low concentration of ions and impurities.

[0019]

[0015] Regular monitoring of conductivity is part of the quality control measures in industries that rely on high-purity water, ensuring that the water meets the stringent standards necessary for the specific industrial processes. To that end, the various embodiments of the online system 100 of the present disclosure can include an inlet 102 for the water from an industrial process, as discussed herein. As used herein, the inlet 102 can be in the form of tubing that prevents the water from the industrial process from being exposed to atmospheric air and / or any other environment or structure that is outside the industrial process (e.g., the closed system of the industrial process). In other words, the water in the on-line system 100 does not contact atmospheric air. Examples of such tubing can include, but are not limited to, non-reactive, low or no leaching and corrosion resistant materials. Such materials can include, but are not limited to, high-density polyethylene (HDPE), ultra-high-molecular-weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perfluoroalkoxy (PEA), and stainless steel tubing (e.g., 316L stainless steel), as are known in the art. Preferably, the preferred material is stainless steel. Connectors and fittings are used for integration of the tubing into the overall on-line system 100, where such connectors and fittings can also be made of the same material as the tubing or from the materials provided here.

[0020]

[0016] The on-line system 100 further includes a first flow cell 106 and a second flow cell 108 fluidly coupled in parallel to receive the water from the industrial process at the predetermined temperature. For the various embodiments, the first flow cell 106 and the second flow cell 108 allow the water from the industrial process to contact the hydrogen ion sensing electrode of a pH probe, as discussed herein, in the closed system of the on-line system 100 so as to allow for real-time monitoring of the pH value of the water. Examples of such flow cells includes a flow cell housing that defines a chamber or enclosure through which the water flows and is designed to ensure a consistent and controlled flow of water over the pH sensor. In other words, the first flow cell 106 and the second flow cell 108 allow the water from the industrial process to contact the pH probe, as provided herein, in a consistent and controlled manner (e.g., consistent flow rate and consistent temperature). The first flow cell 106 and the second flow cell 108 also allow for the continuous or controlled flow of water for pH measurements, as discussed herein. An example of a suitable flow cell can include the flow cells sold under the ThermoFisher Scientific Orion™ trade designator and can include 2001FC Online Flow Cell. Other suitable flow cells are known in the art.

[0021]

[0017] The on-line system 100 further includes a first flow control 110 fluidly coupled to the first flow cell 106 and a second flow control 112 fluidly coupled to the second flow cell 108. For the various embodiments, each of the first flow control 110 and the second flow control 112 independently controls a first flow rate of the water through the first flow cell 106 and a second flow rate of the water through the second flow cell 108. Each of the first flow control 110 and the second flow control 112 control the flow rate of water flowing through each of the respective flow cells 106 and 108, which helps to maintain a consistent and controlled flow of water over the pH probe sensing element and thereby helping to optimize the accuracy and reliability of the measurements. Each of the first flow control 110 and the second flow control 112 can include a flow control mechanism (e.g., knob, dial or adjustment) for adjusting, setting and maintaining the flow rate of the water through the respective flow cell. Such mechanisms can include but are not limited to control valves and / or pumps that can be automatically and / or manually controlled for the desired water flow rate and pressure of the water. The first flow control 110 and the second flow control 112 can each also include a flow sensor to monitor and provide feedback on the actual flow rate, which can allow for real-time adjustments to maintain the desired flow conditions (e.g., flow rate and pressure) through the flow cells.

[0022]

[0018] The on-line system 100 further includes a first pH probe 114 and a second pH probe 116. For the various embodiments, the first pH probe 114 is positioned in the first flow cell 106 to measure a first pH value of the water at the predetermined temperature and the first flow rate. Similarly, the second pH probe 116 is positioned in the second flow cell 108 to measure a second pH value of the water at the predetermined temperature and the second flow rate. For the present disclosure, the pH probes for the present disclosure (e.g., the first pH probe and the second pH probed) can each preferably be a reference pH system that are preferably for use in low ionic strength solutions (e.g., the water as described herein). Preferably, reference electrodes for such pH systems (e.g., the first pH probe 114 and the section pH probe 116) are formed with platinum and the electrodes can have electrolyte solution of, for example, formed with KC1 or KI. Examples of the first pH probe 114 and the second pH probe 116 can include pH electrodes provided under the ThermoFisher Scientific Orion™ ROSS™ trade designator and can include 2001SC ROSS™ high purity pH electrode, 2001FS ROSS™ high purity pH electrode, 2001TM ROSS™ ATC pH electrode and 2001F4 ROSS™ high purity pH electrode. Other suitable pH probes are known in the art, including those provided in the Examples section herein. In addition to the pH probe, the on-line system 100 of the present disclosure can also include an automatic temperature compensation probe that can help to automatically compensate for temperature variations in the water being measured, ensuring more accurate and reliable readings.

[0023] [0191 The on-line system 100 further includes a pH analyzer 118 coupled to the first pH probe 114 and the second pH probe 116. For the various embodiments, the pH analyzer 118 allows for a comparison of the first pH value and the second pH value, as described herein. For the various embodiments, the pH analyzer 118 can be a multi-input intelligent process analyzer having a display, multiple sensor (e.g., pH probe) inputs, multiple current outputs, alarm / control relays and multiple digital communication protocols, among other things, that allow the pH analyzer 118 to receive a signal from each of the pH probes (e.g., the first pH probe 114 and the second pH probe 116) along with temperature measurements from the respective pH probe, display the respective pH values, temperature and calibration status, preforming math functions such as percent rejection and / or percent pass among others, data and event log for each channel, calibration functions, loop output for each channel, automatic temperature compensation (ATC), solution temperature compensation (STC) and alarm and control functions based on each channel, among other functions. An example of a suitable pH analyzer 118 include those sold under the trade designator AquaPro™ Multi- Input Intelligent Process Analyzer (ThermoFisher Scientific). Other examples of suitable pH analyzers are also known in the ait.

[0024]

[0020] For the various embodiments, the on-line system 100 further includes an outlet

[0025] 120 for the water from the on-line system 100. For the various embodiments, the outlet 120 can be connected to the industrial process so that the water returns to the industrial process. In other words, from the inlet 102 to the outlet 120 the water always remains within the on-line system 100. In an additional embodiment, some or all of the water having been tested in the on-line system 100 can be discarded. In other words, some or all of the water having been tested in the on-line system 100 can drain through the outlet 120 of the on-line system 100.

[0026]

[0021] FIG. 1 also shows that the on-line system 100 can further include a third pH measuring system 122, which provides the capability to take a representative sample of water from the industrial process “off-line” to verify one or more of the pH values provided by one or both of the first pH probe 114 and the second pH probe 116. For the various embodiments, the third pH measuring system can include a third pH probe 124 and a third fixed flow cell 126 fluidly coupled to the inlet 102 to receive the water at the predetermined temperature. For the various embodiments, the fixed flow cell 126 can be like or identical to the flow cells discussed above (first flow cell 106 and / or second flow cell 108). Similarly, the third pH probe 124 can be like or identical to the first pH probe 114 and / or second pH probe 116 discussed above. In one embodiment, the third pH probe 124 can be a permanent pH probe mounted in the third fixed flow cell 126. Alternatively, the third pH probe 124 can be a portable pH probe that is coupled to the third fixed flow cell 126 and the third pH measuring system 122 when one or more of a predetermined condition arises with the pH values measured using the first pH probe 114 and / or second pH probe 116. As used herein, a portable pH probe can include pH measuring systems that are designed for “on-the-go” measurements of pH values and can include a compact size with integrated electrodes for pH measurements, be optionally battery powered, with a built-in digital display for, among other things, pH readings, calibration settings and temperature measurements and compensation. Examples include of such portable pH probes include Thermo Orion Star meter with ThermoFisher Scientific Orion™ ROSS® Ultra Refillable 8157BNUMD probe; and ThermoFisher Scientific Orion™ Star meter with ThermoFisher Scientific Orion™ ROSS® Ultra Low Maintenance 8107UWMMD probe. Other examples of portable pH probes are also known in the art.

[0027]

[0022] For the various embodiments, the on-line system 100 of the present disclosure can further include a third flow control 128 fluidly coupled to the third fixed flow cell 126 to control a third flow rate of the water at the predetermined temperature through the third fixed flow cell 126. As illustrated, the third pH probe 124 can be positioned (releasably or not) in the third fixed flow cell 126 and coupled to an independent pH analyzer 119 (e.g., a pH analyzer other than pH analyzer 118) to measure a third pH value of the water at the predetermined temperature and the third flow rate. For the various embodiments, the third pH value of the water can be measured at the third flow rate and predetermined temperature with the third pH probe 124 in the on-line system 100. For the various embodiments, the third pH value of the water can be measured when a comparison of the first pH value and the second pH value, as discussed herein, show a difference between the first pH value and the second pH that exceeds a predetermined threshold. In other words, the third pH value of the water is measured when the difference between the first pH value and the second pH value exceeds the predetermined threshold. For the various embodiments, the predetermined threshold can be set at a value of at least 0.1 pH units. In one embodiment, the predetermined threshold is set at a value of 0.1 pH units. Other values for the predetermined threshold are also possible, for example, values of 0.2, 0.3, 0.4 and 0.5 pH units are also possible.

[0028]

[0023] As illustrated in Fig. 1, the third fixed flow cell 126 can be fluidity coupled in parallel with the first flow cell 106 and the second flow cell 108 to receive the water at the predetermined temperature. For the various embodiments, it is possible to further include a valve 130 along the inlet 102 to the third pH measuring system 122. For the various embodiments, the valve 130 can be used to either provide or stop the flow of the water to the third pH measuring system 122. In other words, the valve 130 can be used to either isolate or bring the third pH measuring system 122 on-line with the flow of water from the industrial process. For the various embodiments, the valve 130 can be operated manually or automated, where such values are known in the art.

[0029]

[0024] FIG. 1, also illustrates an embodiment where the inlet 102 can be fluidly coupled to a heat exchanger 132 to adjust a temperature of the water coming from the industrial process to the predetermined temperature. As appreciated, the temperature and the flow rate can have an impact on the accurate and precise pH values measurements of the water. Examples of such a heat exchanger 132 can include shell and tube heat exchangers, plate heat exchangers, air cooled heat exchangers and plate-fin heat exchangers, among others. Heat can either be added to or removed from the water from the industrial process so as to arrive at the predetermined temperature for the water.

[0030]

[0025] The present disclosure also provides for a method of measuring a pH of water from an industrial process with the on-line system as provided herein. As discussed herein, the water from the industrial process has a conductivity of 0.03 to 100 pS / cm as measured at a predetermined temperature of 15 to 40 °C. For the various embodiments, the method can include receiving water into the on-line system from the industrial process through the inlet. Optionally, the temperature of the water can be adjusted, as needed, via the heat exchanger to arrive at the predetermined temperature. The first pH value of the water can be measured at a first flow rate and at the predetermined temperature with the first pH probe in the on-line system along with measuring the second pH value of the water at a second flow rate and at the predetermined temperature with the second pH probe in the on-line system. As illustrated in FIG. 1, measuring the first pH and the second pH is done on the water flowing in parallel to the first pH probe and the second pH probe. As discussed herein, a difference between the first pH value and the second pH value can be determined, where a third pH value of the water can be measured at a third flow rate and at the predetermined temperature with the third pH probe in the on-line system when the difference between the first pH value and the second pH value exceeds a predetermined threshold. For the various embodiments, at least one of the first pH probe and the second pH probe can be recalibrated when the difference between the first pH value and the second pH value exceeds the predetermined threshold. Values for the predetermined threshold are as provided herein.

[0031]

[0026] For the various embodiments, the method of the present disclosure can further include adjusting the temperature of the water in the on-line system to the predetermined temperature. An example of the on-line system with this ability is seen and discussed in relation to FIG. 1. In addition, for the various embodiments, it is possible through control of the first flow control, second flow control and / or the third flow control to independently control the first flow rate, the second flow rate and the third flow rate in the on-line system.

[0032] Examples

[0033]

[0027] The examples of the present disclosure provide information on the limitations and errors of using a single pH probe. Six total pH probes where tested, where each pH probe had solution temperature compensation functionality. The six pH probes analyzed were: pH Probe #1 - Thermo Fisher Scientific 200 ISC Ross® high purity pH electrode - refillable pH Probe #2 - Mettler Toledo 2003i-UPW (Mettler-Toledo Thornton, Inc.) - refillable pH Probe #3- Mettler Toledo 3201 UPW / 120 (Mettler-Toledo Thornton, Inc.) - pressurized pH Probe #4 - HACH® 08362 - pressurized pH Probe #5 - Yokogawa FU24 (Yokogawa Electric Corporation) - pressurized pH Probe #6- Yokogawa Bellomatic (Yokogawa Electric Corporation) - refillable

[0028] For the tests, a custom rig with the ability to simultaneously test the pH probes, listed above in redundancy at 25 °C and temperature varying from 15 °C to 40 °C, was constructed. Each pH probe was provided with an individual flow cell and flow control as described above for FIG. 1. The custom rig also included multiple pH analyzers to receive both pH and temperature values from the respective probes. Each of the pH probes was calibrated using the same standard at 25 °C. The water used to test the pH probes had a temperature varying between 15 °C to 40 °C and the solution temperature compensation functionality of the probes auto compensated for the temperature using the correction factor built in with the transmitter. Such testing also helped to illustrate the importance of having redundant pH probes as provided in the on-line system of the present disclosure so as to improve the reliability and accuracy of pH measurements as compared to currently practiced single pH probe use.

[0029] FIG. 2 provides the results from a 1 -month long study to compare and test the reliability of the pH probes at varying water temperature between 25 °C to 40 °C using the process water. The flow rate for each pH probe were: #1 - Flow rate: 20-100 ml / minute; #2 - Flow rate: 50-150 ml / minute; #3 - Flow rate: 50-150 ml / minute; #4 - Flow rate: 100-300 ml / minute; #5 - Flow rate: 50-125 ml / minute; and #6 - Flow rate: 50-125 ml / minute.

[0030] A desired goal is to provide the on-line system as described in the present disclosure having redundant pH values measurements having pH value measurement deviations of less than 0.1 pH units all while operating in a very narrow pH operational window (e.g., 9.2- 9.8). As it can be seen from the FIG. 2, the pH probes showed excellent pH control and solution temperature compensation along with consistency and reliability.

[0034]

[0031] The above results provided that the pH probes would be suited for use with low conductivity water and that any such pH probe should be installed in an on-line system as provided in the present disclosure: redundant and in parallel with independent flow control along with a flow cell for the manual pH probes to improve the reliability of measurements.

Claims

What is Claimed is:

1. An on-line system for measuring a pH value of water from an industrial process, wherein the water has a conductivity of 0.03 to 100 microsiemens per centimeter (pS / cm) as measured at a predetermined temperature of 15 to 40 °C, the on-line system comprising: an inlet for the water from the industrial process; a first flow cell and a second flow cell fluidly coupled in parallel to receive the water at the predetermined temperature; a first flow control fluidly coupled to the first flow cell and a second flow control fluidly coupled to the second flow cell, wherein each of the first flow control and the second flow control independently controls a first flow rate of the water through the first flow cell and a second flow rate of the water through the second flow cell; a first pH probe positioned in the first flow cell to measure a first pH value of the water at the predetermined temperature and the first flow rate; a second pH probe positioned in the second flow cell to measure a second pH value of the water at the predetermined temperature and the second flow rate; a pH analyzer coupled to the first pH probe and the second pH probe, wherein the pH analyzer allows for a comparison of the first pH value and the second pH value; and an outlet for the water from the on-line system.

2. The on-line system of claim 1, further including a third pH measuring system that includes a third pH probe and a third fixed flow cell fluidly coupled to the inlet to receive the water at the predetermined temperature; a third flow control fluidly coupled to the third fixed flow cell to control a third flow rate of the water at the predetermined temperature through the third fixed flow cell; the third pH probe positioned in the third fixed flow cell and coupled to the pH analyzer to measure a third pH value of the water at the predetermined temperature and the third flow rate, wherein the third pH value of the water is measured at the third flow rate when the comparison of the first pH value and the second pH value shows a difference between the first pH value and the second pH of at least 0.1 pH units.

3. The on-line system of claim 2, wherein the third fixed flow cell is fluidity coupled in parallel with the first flow cell and the second flow cell to receive the water at the predetermined temperature.

4. The on-line system of any one of claims 2-3, wherein the third pH probe is a portable pH probe.

5. The on-line system of any one of claims 1-4, wherein the inlet is fluidly coupled to a heat exchanger to adjust a temperature of the water from the industrial process to the predetermined temperature.

6. The on-line system of any one of claims 1-5, the water from the outlet of the on-line system returns to the industrial process.

7. The on-line system of any one of claims 1-6, wherein from the inlet to the outlet the water always remains within the on-line system.

8. The on-line system of any one of claims 1-7, wherein the water in the on-line system does not contact atmospheric air.

9. A method of measuring a pH of water from an industrial process with an on-line system, wherein the water has a conductivity of 0.03 to 100 microsiemens per centimeter (p.S / cm) as measured at a predetermined temperature of 15 to 40 °C, the method comprising: receiving water into the on-line system from the industrial process; measuring a first pH value of the water at a first flow rate and at the predetermined temperature with a first pH probe in the on-line system; measuring a second pH value of the water at a second flow rate and at the predetermined temperature with a second pH probe in the on-line system, wherein measuring the first pH and the second pH is done on the water flowing in parallel to the first pH probe and the second pH probe; determining a difference between the first pH value and the second pH value;measuring a third pH value of the water at a third flow rate and at the predetermined temperature with a third pH probe in the on-line system when the difference between the first pH value and the second pH value exceeds a predetermined threshold; and recalibrating at least one of the first pH probe and the second pH probe when the difference between the first pH value and the second pH value exceeds the predetermined threshold.

10. The method of claim 9, further including adjusting a temperature of the water in the online system to the predetermined temperature.

11. The method of any one of claims 9-10, wherein the third pH probe in the on-line system is a portable pH probe.

12. The method of any one of claims 9-11, further including independently controlling the first flow rate, the second flow rate and the third flow rate.

13. The method of any one of claims 9-12, wherein the predetermined threshold is 0.1 pH units or greater.

14. The method of any one of claims 9-13, wherein the water flowing through the on-line system does not come into contact with atmospheric air.

15. The method of any one of claims 9-14, including either returning the water flowing through the on-line system to the industrial process or draining the water away from the industrial process.