pH Sensor with Secondary Reference Electrode

Abstract

A pH sensing probe (103) configured to be exposed to a process fluid is provided. The pH sensing probe (103) includes a sensor body (206) and a pH electrode (110) attached to the sensor body (206). A primary reference electrode (202) is attached to the sensor body (206) and has a primary reference junction configured to be exposed to the process fluid. A secondary reference electrode (204) is attached to the sensor body (206) and has a secondary reference junction configured to be exposed to the process fluid. A seal (208) isolates the secondary reference junction from the process fluid until degradation of the primary reference junction. A pH sensing system (300) and a method (320) of operating a pH sensing system are also provided.

Description

[Technical field]

[0001] background pH is defined as the negative logarithm of the hydrogen ion concentration. pH is expressed on a scale of 0 to 14, with pH values ​​below 7 identifying acidic conditions in the process and pH values ​​above 7 identifying basic conditions. Typical strong acids such as hydrochloric acid (stomach acid) and battery acid (sulfuric acid) have pH values ​​below 1 and are very corrosive. Similarly, strong bases such as caustic soda or bleach and drain cleaners have pH values ​​above 13 and are also very corrosive. Pure water has a pH value of 7.

[0002] Commercially available pH sensors are used in a wide range of applications. One application is neutralizing drinking water up to urban pH limits, where the pH is essential for the safety and health of the community. pH control is used in caustic scrubbers to determine the amount of caustic that has reacted with harmful gases and therefore the amount of caustic that should be replenished. pH sensors are also used to control and monitor industrial processes. For example, it has been shown that the optimum pH for penicillin production in bioreactors is between 6.8 and 7.8. Therefore, for safety and efficiency reasons, the accuracy of the pH measurement is crucial in many processes. Summary of the Invention

[0003] overview A pH sensing probe configured to be exposed to a process fluid is provided. The pH sensing probe includes a sensor body and a pH electrode attached to the sensor body. A primary reference electrode is attached to the sensor body and has a primary reference junction configured to be exposed to the process fluid. A secondary reference electrode is attached to the sensor body and has a secondary reference junction configured to be exposed to the process fluid. A seal separates the secondary reference junction from the process fluid until degradation of the primary reference junction. A pH sensing system and a method of operating a pH sensing system are also provided. [Brief description of the drawings]

[0004] [Figure 1]FIG. 1 is a schematic diagram of a known pH measurement system. [Diagram 2] FIG. 1 is an electrical circuit diagram of a pH electrode. [Diagram 3] 1 is a pH versus voltage chart showing the difference between a new pH sensor and a contaminated pH sensor. [Figure 4] 1 is a chart showing the change in reference offset of a pH sensor over time. [Figure 5a] 1A-1C are schematic diagrams of a number of different types of reference junctions that may be used in accordance with various embodiments of the present invention to improve the stability of the reference voltage over time for different applications. [Figure 5b] 1A-1C are schematic diagrams of a number of different types of reference junctions that may be used in accordance with various embodiments of the present invention to improve the stability of the reference voltage over time for different applications. [Figure 5c] 1A-1C are schematic diagrams of a number of different types of reference junctions that may be used in accordance with various embodiments of the present invention to improve the stability of the reference voltage over time for different applications. [Figure 6a] 1A and 1B are perspective and cross-sectional views, respectively, of a pH sensor having a secondary reference electrode according to an embodiment of the present invention. [Figure 6b] 1A and 1B are perspective and cross-sectional views, respectively, of a pH sensor having a secondary reference electrode according to an embodiment of the present invention. [Figure 7a] 1A-1C are schematic diagrams illustrating a number of different ways in which a secondary reference electrode can be connected in accordance with embodiments of the present invention. [Figure 7b] 1A-1C are schematic diagrams illustrating a number of different ways in which a secondary reference electrode can be connected in accordance with embodiments of the present invention. [Figure 7c] 1A-1C are schematic diagrams illustrating a number of different ways in which a secondary reference electrode can be connected in accordance with embodiments of the present invention. [Figure 8a] FIG. 2 is a schematic diagram of a portion of a pH sensor according to an embodiment of the present invention. [Figure 8b] FIG. 2 is a schematic diagram of a portion of a pH sensor according to an embodiment of the present invention. [Figure 9] FIG. 1 is a schematic diagram of a pH transmitter according to an embodiment of the present invention. [Figure 10] FIG. 2 is a schematic diagram of a pH sensing system according to another embodiment of the present invention. [Figure 11] FIG. 2 is a flow diagram of a method of operating a pH sensor having a secondary reference electrode in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0005] Many current pH sensors generally include a single reference electrode that is used to complete the circuit for pH measurement. This "reference" must remain stable for accurate pH measurements. The process may attack the reference so that the sensor goes out of calibration. The sensor may be recalibrated to correct this drift or offset until the offset becomes too large (e.g., typically + / - 60 mV). Outside this range, the reference is already contaminated or fouled and should be replaced. In accordance with various embodiments described below, a secondary or backup reference electrode is provided to the pH sensor or pH sensing system that is sealed from the process until needed. When the primary reference fails or degrades, the secondary reference is exposed to the process. As used herein, "degradation" of a reference electrode includes breakage, blockage, contamination, or other conditions that reduce the ability of the reference electrode to provide a proper reference. The secondary reference electrode serves as a backup while a new sensor is ordered or while the reference electrode is used to extend the sensor life in applications where the reference electrode is known to be the cause of sensor failure.

[0006] FIG. 1 is a schematic diagram of a known pH measurement system. The loop 100 includes a transmitter 102 operably coupled to a pH sensor 103 having a glass electrode 104, a temperature element 106, and a reference electrode 108. The glass electrode 104, the temperature element 106, and the reference electrode 108 are typically provided within a single pH sensor probe housing arranged to interact with a process fluid. Thus, the pH glass bulb 110 is configured to be immersed in or in contact with the process fluid, while the temperature element 106 is configured to provide an electrical indication of the temperature of the process fluid. Typically, the temperature element 106 is a resistance temperature device (RTD). Although a single wire is shown coupling the temperature element 106 to the transmitter 102, one skilled in the art will recognize that the single connection represents any suitable number of physical conductors that operably couple the temperature sensor to the transmitter. For example, if the temperature element 106 is a four-wire RTD, the single wire represents four different conductors. The reference electrode 108 typically includes a reference junction 112 that electrically couples the pH electrode 104 to the reference electrode 108.

[0007] The transmitter 102 detects or measures the electrical signals from the pH glass electrode 104, temperature element 106, and reference electrode 108 to provide an indication of the pH of the process fluid. As shown, the loop 100 is comprised of the transmitter 102, glass electrode 104, reference electrode 108, and RTD 106 for temperature compensation. The sensing technology is at the end of the glass electrode 104, called the pH glass bulb 110. Here, the hydrogen ions in the process are absorbed into a leached layer of the pH glass. The interior of the glass electrode 104 is filled with a pH 7 buffer, so the difference in concentration of hydrogen ions across the pH glass (between the buffer solution in the glass electrode 104 and the hydrogen ions in the process) creates a millivolt (mV) potential. This millivolt potential drives the flow of electrons in the pH loop and can be modeled by the Nernst equation shown below:

[0008]

number

[0009] where E is the reduction potential, Eo is the standard potential, R is the universal gas constant, T is the process temperature in Kelvin, z is the ionic charge (moles of electrons), F is the Faraday constant, and Q is the reaction quotient.

[0010] An example of a known pH sensor using a pH glass electrode is sold under the trade name Model 3300HT PERph-X High Performance pH / ORP Sensor available from Emerson Corporation's Rosemount Inc. Additionally, a commercially available example of a transmitter 102 is sold under the trade name Model 56 Dual Input Analyzer available from Rosemount Inc.

[0011] FIG. 2 is an electrical circuit diagram of a pH electrode. In the circuit shown, the pH voltage changes with changes in process fluid pH, while the reference voltage remains constant for different process fluids. The reference electrode (such as electrode 108) in the pH loop is open to the process and provides a path for electrons to return to the transmitter. FIG. 2 shows the flow of electrons in the path. In general, the flow is from the transmitter or meter 102, down the Ag / AgCl wire of the glass electrode, through the buffer solution in the glass electrode, out the pH glass, into the process, up the reference junction, through the reference electrode, up the Ag / AgCl wire of the reference electrode, and back to the transmitter. FIG. 2 shows an electrical schematic of a pH loop. The voltage read by the transmitter 102 is the sum of the voltages across the pH glass, the reference junction, and the reference electrode. The reference electrolyte is typically a mixture of potassium chloride. The reference electrode potential is theoretically calculated as the potential between the Ag / AgCl wire and the chloride ions in the reference electrolyte.

[0012] pH sensors can fail in several ways. First, the pH glass can be degraded by temperature or attacked by process chemicals such as sodium hydroxide. Second, the pH glass can be cracked by operator manipulation or by impact with undissolved solids. Third, the reference electrode can be contaminated by ions such as cyanide or sulfide that form a precipitate that blocks the reference junction. Fourth, the reference potential can be contaminated by diffusion of ions from the solution through the reference junction. The reference potential is determined by the potential difference between the Ag / AgCl wire and the chloride ions surrounding the wire in the reference electrolyte. Process ions can diffuse through the reference junction and displace chloride ions, which diffuse out of the reference junction. A pH sensor with a contaminated reference electrode will have a change in the reference voltage. This will cause an error in the reported pH value. Fifth, the reference electrode can wear out over time by diffusion out of the reference junction. This is typically the case in high purity water applications. The concentration of the ion is higher at the reference electrode than at the process and the ion diffuses away from the reference junction.

[0013] The output of a pH sensor can shift over time and with process conditions. Thus, pH sensors often require frequent calibration to compensate for these shifts. Changes in the calibration results (slope and offset) indicate the health of the sensor. A change in offset is most often due to degradation of the reference system, while a change in slope in the calibration is an indication of degradation of the pH sensing element. Additional diagnostics include glass impedance measurements to determine if the glass pH electrode is cracked or broken. The reference impedance measurement determines if the reference system is blocked as a result of noise from the pH measurement.

[0014] FIG. 3 is a pH vs. voltage chart showing the difference between a new pH sensor and a contaminated pH sensor. Calibrating a pH sensor is generally a two-step process in which the user places the probe in two different buffers, such as pH 4 and pH 7 buffers. The transmitter then measures the mV signal from the pH sensor while it is placed in one of the two buffers and calculates the slope and offset. At 25°C, the millivolt value in pH 4 buffer is approximately 180 mV and the millivolt value in pH 7 buffer is 0.00 mV. These two points are used by the transmitter to calculate the calibration line. As shown in FIG. 3, contamination of the reference electrode results in a shift of the calibration line around the y-axis from the ideal calibration line shown by a new sensor. Some transmitters have a reference offset fail criterion of + / - 60 mV as a factory default. If the reference offset changes more than this default offset fail criterion, an offset error will appear on the transmitter display. The transmitter allows the user to increase the offset range of the transmitter, thereby providing ample time to obtain a replacement. However, in many cases, replacement sensors are not available on-site and the lead time to obtain a new pH sensor can be long. Furthermore, some users may be reluctant to extend the offset range of the reference offset simply to pass calibration, as this will introduce errors in the pH measurement. In addition to post-calibration offset diagnostics, many transmitters also offer reference impedance diagnostics to detect blocked references. Diagnostics can be performed periodically, on demand, or continuously.

[0015] FIG. 4 is a chart showing the change in reference offset of a pH sensor over time. The chart shown in FIG. 4 shows the reference offset obtained by time calibration of three different pH sensors in 0.5 M sodium sulfide aqueous solution at 50° C. Sulfides are contaminating ions, and they react with the silver surrounding the wire to form silver sulfide precipitates that can plug the reference junction. The test in FIG. 4 was carried out in the laboratory over a period of about four months. It can be seen that the reference offset changes dramatically in all three sensors at about 60 days. By 100 days, the reference electrodes of all sensors are contaminated such that the reference offset exceeds + / - 60 mV from the original value.

[0016] 5a-5c are schematic diagrams of many different types of reference junctions that may be used in accordance with various embodiments of the present invention. As shown and described above, the reference electrode is typically open to the process through the reference junction. Thus, the reference electrode is subject to contamination over time. Chemical processes may contaminate the reference electrode in several ways. Ion diffusion from the process into the reference electrolyte is one way that the reference electrode may become contaminated. When ions are on the Ag / AgCl wire from chloride to other types of ions, the reference potential may be altered and distort the pH reading. In more extreme cases, ions such as cyanide and sulfide may react with the reference electrode and form precipitates. The precipitates may actually block the reference junction and cause unstable and erroneous pH readings. Some analytical transmitters, such as transmitter 102, may provide a reference impedance diagnostic that may account for either a blockage of the reference junction by precipitate or a dried reference electrolyte, but it is generally preferred to try to suppress all such blockages.

[0017] As mentioned above, because the reference electrolyte is open to the process, contamination of the reference electrode occurs over time. However, various reference junction configurations can be used to slow contamination. The most basic reference junction configuration is a double junction reference shown in FIG. 5a. In such a double junction reference, each junction 120, 122 is made of polytetrafluoroethylene. pH sensors using such a double junction reference can be used in drinking water plants and other relatively clean applications. In less clean applications, further adaptations can be useful. FIG. 5b and FIG. 5c show a triple junction reference and a spiral reference junction, respectively. Such junctions are useful for dirtier and more demanding applications, such as scrubbers and wastewater treatment plants. Process ions must jump through three reference junctions in such cases before attacking the reference electrode. There are many types of reference systems that can be used in accordance with the embodiments described herein. Examples of materials that can form the reference junctions shown in FIG. 5a-5c include, but are not limited to, polytetrafluoroethylene, wood, ceramic, and capillary tubes. Additionally, refillable reference systems may also be used in accordance with the embodiments described herein.

[0018] 6a and 6b are schematic diagrams of a pH sensor with a secondary reference electrode according to an embodiment of the present invention. Some commercially available pH sensors provide a complete concentric cylinder around the pH glass electrode for the reference electrolyte of the reference electrode. The reference wire and the reference electrolyte are placed within this concentric circle inside the sensor. According to at least one embodiment described herein, each of the primary and secondary reference electrodes includes its own reference wire and reference electrolyte in its own half of the concentric cylinder around the pH glass electrode. Although one particular embodiment is shown in FIGS. 6a and 6b, those skilled in the art will recognize that variations in shape and size may be made according to various embodiments described herein. Regardless of the configuration of the second reference electrode, it is preferred that the primary and secondary reference electrodes are separated from each other. Thus, the complete concentric cylinder around the pH glass electrode is divided into two separate half-cylinders, each supporting a different reference electrode. Each reference electrode is provided with the same number and type of reference junctions and reference electrolyte.

[0019] As can be seen, each reference electrode 202, 204 is positioned in close proximity to the pH glass bulb 110. However, one of the reference electrodes is, at least initially, isolated from the process. As shown in Figures 6a and 6b, this is accomplished by providing a molded plastic cover 208 over the reference electrode 204 to isolate it from the process. As can be seen, the reference electrode 202 is opened to the process by an opening 210 in the sensor body 206. When it is determined that the primary reference electrode 202 is contaminated or unacceptably damaged, the plastic cover 208 is unscrewed or otherwise removed and / or moved. Immediately, an electrical switch between the primary and secondary electrodes is made. This can be accomplished by manually changing the wiring from the conductor 212 that was coupled to the transmitter or other suitable sensing circuit before the reference electrode 202 was contaminated, and coupling the new conductor 214 to the transmitter, thus electrically coupling the reference electrode 204 to the transmitter. Alternatively, this switching may occur automatically by providing both conductors 212, 214 to a switching circuit that may be controlled by a suitable processor. Thus, when the transmitter's processor determines that the reference electrode 202 is already contaminated or has degraded or aged beyond an acceptable level, the transmitter may automatically connect the reference electrode 204 and provide notification to a responsible party that the pH sensor 200 should be replaced. After the electrical switching has been performed, either manually or automatically, a recalibration of the pH sensor should be performed.

[0020] 7a-7c are schematic diagrams showing many different ways a secondary reference electrode can be connected according to embodiments of the present invention. Modern pH sensors typically have an analog connection system (analog connector and cable) that is wired directly to the transmitter or integrated electronics (preamp or digital electronics). Adding a secondary reference system may require new electronics to manage the new signal. In embodiments, this is accomplished using an analog or digital system along with the creation of new hardware such as a switch that receives both signals and provides one of the selected signals to the digital or analog sensing system based on a control signal received from a processor such as a microprocessor. When the secondary reference is sealed from the process, the secondary reference has a high resistance from the process. This signal can be used to determine if the reference should be used. The transmitter electronics, or even the probe system, may report a pH based on the primary reference and / or a pH based on the secondary reference. If the primary reference fails, removing the seal provides additional sensor useful life. The electrical circuit requires another measurement to evaluate the secondary reference voltage.

[0021] Various options are possible, as shown in Figures 7a-7c. One option, shown in Figure 7a, is for the analog signal to be provided directly to the transmitter via a suitable cable, such as a VP8 / analog cable, without a preamplifier, where a conductor for the secondary reference is simply added to the cable. When a secondary reference electrode is used, the wire in the cable that connects to the secondary reference is simply coupled to the reference connection on the transmitter. In another embodiment, shown in Figure 7b, the pH sensing system is included in a smart preamplifier circuit board. This smart preamplifier circuit board is provided with a reference switch that receives both conductors 212, 214, and then provides one of the selected signals to the transmitter based on a reference control transmitted from the transmitter via smart 1-wire communication. One advantage of this system is that the transmitter electronics do not need to be modified from a conventional system, instead a software update can be provided to the transmitter so that when the transmitter determines that the primary reference is no longer available, it can generate the necessary reference control signal. In yet another embodiment, shown in Figure 7c, an analog-to-digital Modbus circuit board is added to the pH probe system. The analog-to-digital Modbus circuit board includes a reference switch to the circuit board. The circuit board itself determines when the primary reference has degraded to an unacceptable degree and automatically switches the reference switch to the secondary reference. The pH sensing probe is typically coupled to the transmitter via an appropriate DeviceNET Modbus cable. One advantage of this particular embodiment is that there is no effect on the transmitter electronics. In yet another embodiment, the transmitter itself may be provided with a software update such that the transmitter provides a signal to the analog-to-digital Modbus circuit board to cause the reference switch to switch to the secondary reference electrode. As can be appreciated, many options are available for manually and / or automatically switching to the secondary reference when the primary reference is no longer available.

[0022] 8A and 8B are schematic diagrams of a portion of a pH sensor according to an embodiment of the present invention. Although the previous embodiments described switching from the first reference electrode to the second reference electrode as occurring by removing the sensor cover 208 (shown in FIG. 6) and electrically coupling the secondary reference electrode to a transmitter (either manually or automatically), both the physical coupling of the secondary reference electrode to the process and the electrical coupling of the secondary reference electrode to the transmitter can occur in other ways.

[0023] FIG. 8A shows a pH sensor 250 having a sensor cover 252 with a glass bulb electrode 110 located proximate the center of the sensor cover 252. The sensor cover 252 is rotatable (as shown in FIG. 8B) to select between reference 202 and reference 204. In an embodiment, the rotation shown in FIG. 8B automatically isolates reference 202 from the process and exposes reference 204 to the process. In yet another embodiment, this rotation also shifts internal electrical components, such as switch contacts, to electrically couple reference 204 to the sensor's reference connection when sensor cover 252 is rotated to the reference 204 position. Although rotation is shown in FIGS. 8A and 8B, this is merely to illustrate how the reference electrodes can be physically and electrically switched. One of ordinary skill in the art will recognize other types of motions and actuations that may be used in accordance with the embodiments described herein. Additionally, while the embodiments described herein have generally been directed to a pair of reference electrodes (reference 202 and reference 204), it is expressly contemplated that additional reference electrodes (such as a tertiary or quaternary reference electrode) may be added, so long as appropriate physical coupling and electrical connections can be accommodated.

[0024] FIG. 9 is a schematic diagram of a pH glass sensor system according to another embodiment of the present invention. As shown, the system 300 includes a transmitter 250 coupled to a pH sensor 200 including a secondary reference electrode. In the illustrated embodiment, the reference electrode, the pH sensing electrode, and the temperature sensing device are coupled to a measurement circuit 260 via respective conductors 254, 212, 214, and 264. The measurement circuit 260 may include any suitable amplification, linearization, analog-to-digital conversion, and / or multiplexing circuitry to individually interact with the conductors 254, 212, 214, and 264 to measure signals associated with the sensor. In the example of a pH measurement, the measurement circuit 260 may detect a millivolt potential across the conductor 254 and one of the conductors 212, 214. Similarly, the measurement circuit 260 may measure the process fluid temperature based on a signal (e.g., resistance) of the temperature device 106 (shown in FIG. 10) via the conductor 264.

[0025] The measurement circuit 260 is operably coupled to a controller 270 and provides one or more signals indicative of various electrical parameters of the pH sensor and / or temperature element to the controller 270. The controller 270 may be any suitable combination of hardware or software capable of executing one or more program steps for obtaining an indication of the millivolt potential of the pH sensor, obtaining an indication of the process fluid temperature, and providing a temperature compensated pH output based on the millivolt potential. In an embodiment, the controller 270 is a microprocessor. The controller 270 is operably coupled to a display / output module 272 and one or more inputs 274. The display / output module 272 may include a liquid crystal display or other suitable type of display and one or more indicator lights. Additionally, the display / output module 272 may include an audible output, such as a local alarm. Additionally, the display / output module 272 may include a signaling circuit that may interact with one or more remote devices, such as via a wireless process communication protocol, such as WirelessHART (IEC 62591). The one or more inputs 274 may include suitable user-operable buttons, a keypad, a joystick, a microphone, or other suitable user input devices capable of receiving user input.

[0026] In an embodiment, controller 270, through hardware, software, or a combination thereof, is configured to perform a reference electrode test to identify broken, degraded, or aged reference electrodes and provide a signal indicative of such a condition. Additionally, controller 270 is configured to identify the time or occurrence when the reference electrode coupled via conductor 212 can no longer be used and automatically transition to providing a temperature compensated pH output based on a backup reference electrode via that signal over line 214.

[0027] FIG. 10 is a schematic diagram of a pH sensing system according to another embodiment of the present invention. System 310 has some similarities to system 300 (described with respect to FIG. 6), and like components are similarly numbered. System 300 may be the same as the embodiment shown in FIG. 7c. Unlike system 300, system 310 does not require a transmitter, such as transmitter 250. Instead, some components of transmitter 250 are instead provided within pH sensor probe housing 216. Thus, measurement circuit 260 and controller 270 are disposed within housing 216. Additionally, input / output circuit 312 is disposed within housing 216 and coupled to controller 270 to enable controller 270 to communicate with one or more external devices, preferably using digital communications. In one example, input / output circuit 312 is configured to communicate according to one or more process industry standard communication protocols, such as a wired Highway Addressable Remote Transducer (HART®) protocol, FOUNDATION™ Fieldbus, or a suitable wireless process communication protocol, such as WirelessHART, as listed above. Similar to system 300, controller 270, through hardware, software, or a combination thereof, is configured to perform a reference electrode diagnostic test to identify broken, degraded, or aged reference electrode and provide a signal indicative of such condition. Additionally, controller 270 is configured to identify the time or occurrence when the reference electrode signal can no longer be used and automatically transition to providing a temperature compensated pH output based on a backup reference electrode via that signal over line 214.

[0028] FIG. 11 is a flow diagram of a method of operating a pH sensor system according to an embodiment of the present invention. Method 320 begins at block 322 where a dual element pH sensor with a backup reference electrode is provided. In one example, this sensor is sensor 200 described above with respect to FIG. 6. Next, at block 324, a pH sensing loop is used to sense pH with a primary reference electrode, such as electrode 202 (shown in FIG. 6). At block 326, a controller of a transmitter, such as transmitter 250, detects degradation or failure of the primary reference electrode. This detection may be the result of a reference electrode diagnostic process performed by the transmitter or may be indicated by a user. Additionally, failure or degradation of the primary reference electrode may be detected during normal operation, such as a completely out-of-range value.

[0029] If degradation / failure of the primary reference electrode is detected, the method 320 proceeds to block 328 where the pH loop is switched to the secondary reference electrode. This may be a manual process as shown at reference numeral 330, where a technician physically disconnects the conductor of the primary reference electrode from the transmitter and connects the conductor of a capped or otherwise unused backup reference electrode to the transmitter. Alternatively, the switch may be automatic as shown at reference numeral 332, where a controller, such as controller 270 (shown in FIG. 9 ), automatically switches to determine pH based on the conductor already coupled to the backup reference electrode. In another example, the switching may occur in response to the transmitter receiving a command (e.g., via digital communication using a process industry standard communication protocol) from an external device (such as a process controller or other suitable device) that causes the transmitter to automatically switch to the secondary or backup reference electrode. Finally, in block 334, the pH is sensed by the secondary reference electrode and an output is provided to the transmitter display, etc. Additionally, the output may include an indication that the primary pH glass electrode has failed and a replacement needs to be obtained and installed.

Claims

1. 1. A pH sensing probe configured to be exposed to a process fluid, comprising: Sensor body; a pH electrode attached to the sensor body; a primary reference electrode attached to the sensor body and having a primary reference junction configured to be exposed to the process fluid; a secondary reference electrode attached to the sensor body and having a secondary reference junction configured to be exposed to the process fluid; and a seal isolating the secondary reference junction from the process fluid until degradation of the primary reference junction.

2. The pH sensing probe of claim 1 , further comprising a temperature sensing element configured to provide an indication of a process fluid temperature.

3. The pH sensing probe of claim 1 , wherein the primary reference electrode comprises a primary reference electrolyte disposed within a portion of a circle disposed around the pH electrode.

4. 4. The pH sensing probe of claim 3, wherein the secondary reference electrode comprises a secondary reference electrolyte disposed within another portion of a circle disposed around the pH electrode, the primary reference electrode and the secondary reference electrode being separated from one another.

5. The pH sensing probe of claim 1 , wherein the seal comprises a removable cover sealingly disposed over the secondary reference junction.

6. 2. The pH sensing probe of claim 1, wherein the seal includes a rotatable sensor cover having a first position in which the primary reference junction is exposed to the process fluid and the secondary reference junction is isolated from the process fluid, and a second position in which the secondary reference junction is exposed to the process fluid.

7. 7. The pH sensing probe of claim 6, wherein moving the rotatable sensor cover from the first position to the second position automatically electrically isolates the primary reference electrode from a sensor reference electrode output conductor and automatically couples the secondary reference electrode to the sensor reference electrode output conductor.

8. The pH sensing probe of claim 1 , wherein the primary reference junction is selected from the group consisting of a double junction, a triple junction, and a helix junction.

9. The pH sensing probe of claim 1 , wherein the secondary reference junction is selected from the group consisting of a double junction, a triple junction, and a helix junction.

10. display, at least one user input mechanism; a measurement circuit configured to measure at least one electrical characteristic of the attached device; a controller coupled to the display, the at least one user input mechanism, and the measurement circuitry, the controller configured to obtain pH information and process fluid temperature information and provide a pH process output; A pH transmitter comprising: Sensor body, a pH electrode electrically coupled to the measurement circuit and attached to the sensor body, the pH electrode configured to be exposed to a process fluid; a primary reference electrode electrically coupled to the measurement circuit and having a primary reference junction attached to the sensor body, the primary reference electrode configured such that the primary reference junction is exposed to the process fluid; and a secondary reference electrode electrically coupled to the measurement circuit and having a secondary reference junction attached to the sensor body, the secondary reference electrode being configured such that the secondary reference junction is exposed to the process fluid; a pH sensing probe comprising: A pH sensing system comprising:

11. The pH sensing system of claim 10 , wherein the controller is configured to detect degradation of the primary reference electrode and to perform an action if the primary reference electrode degrades.

12. 12. The pH sensing system of claim 11, wherein the action generates instructions to a user to manually switch wiring of the pH sensing probe to disconnect the primary reference electrode and connect the secondary reference electrode to the pH transmitter.

13. 12. The pH sensing system of claim 11, wherein the action automatically disconnects the measurement circuit from the primary reference electrode and automatically couples the secondary reference electrode to the measurement circuit.

14. 12. The pH sensing system of claim 11, wherein the action includes switching calculation of the pH process output from a combination of the pH electrode and the primary reference electrode to a combination of the pH electrode and the secondary reference electrode.

15. The pH sensing system of claim 10 , wherein the controller comprises a microprocessor.

16. 1. A method of operating a pH sensing system, comprising: providing a pH sensing probe having a pH sensing electrode, a primary reference electrode and a secondary reference electrode; sensing pH with said pH sensing electrode and said primary reference electrode; detecting degradation of the primary reference electrode; switching from the primary reference electrode to the secondary reference electrode; and Sensing pH with said pH sensing electrode and said secondary reference electrode; A method comprising:

17. 17. The method of claim 16, wherein the primary and secondary reference electrodes comprise primary and secondary electrolytes disposed within separate portions of a circle disposed around the pH sensing electrode.

18. 17. The method of claim 16, wherein switching from the primary reference electrode to the secondary reference electrode occurs automatically upon detecting degradation of the primary reference electrode.

19. 17. The method of claim 16, wherein switching from the primary reference electrode to the secondary reference electrode occurs in response to receiving a command from an external device.

20. Sensor body; a pH electrode electrically coupled to a measurement circuit and attached to the sensor body, the pH electrode configured to be exposed to the process fluid; a primary reference electrode electrically coupled to the measurement circuit and having a primary reference junction attached to the sensor body, the primary reference electrode configured such that the primary reference junction is exposed to the process fluid; a secondary reference electrode electrically coupled to the measurement circuit and having a secondary reference junction attached to the sensor body, the secondary reference electrode configured to be isolated from the process fluid until a seal is opened to expose the secondary reference junction to the process fluid; a measurement circuit operably coupled to said pH electrode, said primary reference electrode, and said secondary reference electrode; input / output circuitry configured to provide digital communications; and a controller coupled to the measurement circuit and the input / output circuit, the controller configured to obtain pH information from the measurement circuit and provide a pH process output via the input / output circuit, the controller configured to automatically detect degradation of a primary reference electrode and switch from the primary reference electrode to a secondary reference electrode; A pH sensing probe comprising:

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