pH sensor with backup pH sensing electrode

Abstract

A pH sensing probe (200) configured to be exposed to a process fluid is provided. The pH sensing probe (200) includes a sensor body (202) and a pH glass electrode (104) attached to the sensor body (202). A reference electrode has a junction (112) attached to the sensor body (202) configured to be exposed to the process fluid. A backup pH electrode (214) is attached to the sensor body (202) and configured to be exposed to the process fluid. A pH sensing system (300) and a method (320) of operating the pH sensing system (300) are also provided. In one example, the backup pH electrode (214) is an ISFET electrode that can be automatically switched when the pH glass electrode (104) is compromised.

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 glass electrode attached to the sensor body. A reference electrode is attached to the sensor body and has a junction configured to be exposed to the process fluid. A back-up pH electrode is attached to the sensor body and configured to be exposed to the process fluid. A pH sensing system and a method of operating the pH sensing system are also provided. In one example, the back-up pH electrode is an ISFET electrode. [Brief description of the drawings]

[0004] [Figure 1] FIG. 1 is a schematic diagram of a known pH measurement system. [Diagram 2] 1 is a chart of sensor life in months versus process temperature. [Diagram 3] 1 is a graph of the slope of a new versus an aged pH sensor. [Figure 4] FIG. 2 is a perspective view of a pH sensor having a backup ISFET electrode according to an embodiment of the present invention. [Diagram 5] FIG. 2 is a schematic diagram of a pH sensor having a back-up ISFET electrode in accordance with an embodiment of the present invention. [Figure 6] FIG. 1 is a schematic diagram of a pH transmitter according to an embodiment of the present invention. [Figure 7] FIG. 2 is a schematic diagram of a pH sensing system according to another embodiment of the present invention. [Figure 8] FIG. 2 is a flow diagram of a method of operating a dual electrode pH sensor in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0005] Many current pH sensors include a single pH glass electrode. The electrode's pH glass can break for many reasons, including operator handling or from undissolved solids striking the glass. Additionally, in applications using caustic agents such as sodium hydroxide, which are known to attack glass, the pH glass can wear over time. In either case, the user must replace the pH sensor. In many cases, replacement pH sensors are not available in the field, and the lead time to obtain a new pH sensor can be long. In accordance with various embodiments described below, the sensor and / or pH sensing system has a glass electrode and one ion-sensing field effect transistor (ISFET) electrode. The ISFET is a non-glass pH technology that calibrates similarly to pH glass technology and can be used as a backup electrode until a new sensor is procured. Compared to glass electrode technology, the ISFET electrode technology can withstand environments that affect glass wear.

[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 may detect or measure 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] As mentioned above, the glass bulb 110 of the glass electrode 104 is prone to failure and when such failure occurs, the entire pH sensor must be replaced. There are several failure modes for the pH glass bulb 110. The first failure mode is glass breakage. This can be caused by either (a) handling, installation, calibration, or storage of the sensor, (b) extreme cold temperatures where the buffer solution in the pH glass becomes less dense as it increases in volume, or (c) undissolved solids in the process impinging on the pH glass. Since ISFET is a non-glass sensing technology, it is not susceptible to cracking or breakage and is suitable as a backup for glass pH electrodes.

[0012] The second failure mode of pH glass is aging, which is usually accelerated by high process temperatures. These high process temperatures slowly degrade the pH glass. Figure 2 shows a graph of sensor life in months versus process temperature to illustrate the effect of temperature on pH glass life. As shown in Figure 2, it is generally understood that the life of standard pH glass is halved with a 25°C increase in temperature. In the example described, the sensor has a typical life of 12 months when operated at 25°C. Some pH sensors are offered in glass specially formulated for high temperature applications, called AccuGlass, but even these specially adapted pH sensors are aged by increased process temperatures, although to a much lesser extent. In comparison, ISFET electrodes do not age as quickly as glass electrodes and are not degraded as quickly by temperature.

[0013] FIG. 3 is a chart of the slope of a new pH sensor versus an aged pH sensor illustrating the change in slope as the sensor ages. A new pH sensor is shown at line 130 and an aged sensor at line 132. The change in slope can be corrected by periodic calibration. pH calibration is generally a two-step process on both the pH glass and the ISFET electrode where the user places the probe in two separate buffers, such as pH 4 and pH 7. The transmitter measures the millivolt signals from the two buffers and calculates the slope and offset based on the measurements. At 25° C., the millivolt value in a pH 4 buffer is typically 177.48 mV and the millivolt value in a pH 7 buffer is 0.00 mV. As shown in FIG. 3, the ideal slope is therefore −59.16 mV / pH. As the pH glass ages over time, the slope decreases. Some transmitters can be programmed with a pH slope failure criterion of −35 mV / pH, which is considered too low for many applications. Additionally, the transmitter may provide the ability to change the slope failure criterion. It is generally accepted that the decrease in gradient over time is due to degradation of the pH glass.

[0014] A third failure mode for pH glass is from attack by process chemicals such as sodium hydroxide.

[0015] Conventional industrial transmitters have diagnostics to identify and notify the user of glass failure. Known transmitters, such as the transmitter 102 described above, typically provide glass impedance diagnostics for pH glass sensors. Healthy pH glass for pH glass sensors typically has a glass impedance between 50 and 500 megaohms, and broken glass will exhibit a glass impedance much lower than 50 megaohms. A broken glass failure is indicated on the transmitter display and the pH sensor will no longer respond to the buffer. In other words, the broken glass prevents millivolt potential differences from being generated across the pH glass. It should be noted that sometimes pH glass cracks are too small to be discerned by the naked eye and therefore can only be seen under a microscope.

[0016] FIG. 4 is a perspective view of a pH sensor with a backup ISFET electrode according to an embodiment of the present invention. The sensor 200 generally includes a relatively cylindrical sensor body 202 having a proximal portion 204 with an electrical connector 206 configured to couple to an instrument cable or wire for connecting the pH sensor 200 to a pH transmitter device such as the transmitter 102. The body 202 also includes an externally threaded distal region 208 configured to be received by a process coupling or other suitable device for physically mounting the pH sensor. To aid in such mounting, the pH sensor body 202 typically includes a number of wrench flats 210 to facilitate rotation of the body 202 during mounting. The pH sensor 200 includes a pH glass electrode 104, which may take any suitable form, including the glass electrode shown and described with respect to FIG. 1. This glass electrode is assumed to fail in the various failure modes described above. The primary pH electrode function is provided by the pH glass electrode 104 and the reference junction 112.

[0017] According to one embodiment, the pH sensor 200 includes an ISFET electrode 214 located in close proximity to the temperature element 106. Thus, when the sensor 200 is installed in a process, the pH glass electrode 104, the temperature element 106, the reference junction 112, and the ISFET electrode 114 are all positioned to contact the process fluid. In some embodiments, the pH glass electrode 104 and the ISFET electrode 214 may be calibrated at the same time. However, it is expressly contemplated that these electrodes may be calibrated at different times. For example, the ISFET electrode 214 may be calibrated when required to provide its back-up function. It is preferred that the ISFET electrode 214 always be in line with the glass electrode 104. Additionally, it is also preferred that the ISFET electrode 214 and the glass electrode 104 share the same reference electrode, temperature sensing element (such as an RTD or thermocouple), and solution ground because the glass electrode 104 usually fails before the other components fail. As soon as it becomes apparent that the pH glass electrode 104 has cracked or has degraded to an unacceptable degree, as indicated by broken glass failure, low glass impedance, or unresponsiveness to the buffer, the pH wire coupled to the pH glass electrode 104 is disconnected and the capped ISFET wire coupled to the ISFET electrode 214 is connected to the pH signal board. This can be done manually via a technician responding to a fault indication on the pH transmitter display, or automatically via electrical switching within the pH transmitter.

[0018] 5 is a schematic diagram of a pH sensor 200 operably coupled to a pH transmitter 250 in accordance with an embodiment of the present invention. As shown, the pH sensor 200 includes a pH glass electrode 104, an ISFET electrode 214, a reference junction 112, and a temperature element 106, all disposed within a common pH sensor body 202. A transmitter 250 is shown coupled to the pH glass electrode 104, the reference junction 112, and the temperature element 106. In an embodiment, when the transmitter 250 determines that a broken glass failure, low glass impedance, or no response to the buffer has occurred, the transmitter 250 generates an indication of such on a display 252. The operator may then physically remove the lead 254 of the glass electrode 104 and instead couple the lead 256 to the transmitter 250. Alternatively, in some embodiments, the transmitter 250 may be electrically coupled to the lead 256 and may automatically begin using the signal from the ISFET electrode 214 upon detection of a broken glass fault, low glass impedance, or no response to the buffer of the glass electrode 104.

[0019] FIG. 6 is a schematic diagram of a pH glass sensor loop according to another embodiment of the present invention. As shown, the loop 300 includes a transmitter 250 coupled to a pH sensor 200 including a backup ISFET electrode 214, as described in more detail above (with respect to FIG. 5). In the embodiment shown in FIG. 6, the backup ISFET electrode 214 (shown in FIG. 4) is coupled to a measurement circuit 260 of the transmitter 250 via a conductor 256. Additionally, the glass bulb pH electrode 104 is coupled to the measurement circuit 260 via a conductor 254. The reference electrode and the temperature sensing device are coupled to the measurement circuit 260 via respective conductors 262 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, 256, 262, and 264 to measure a signal associated with the sensor. In the example of a pH measurement, the measurement circuit 260 may detect a millivolt potential across the conductor 262 and one of the conductors 254, 256. Similarly, the measurement circuit 260 may measure the process fluid temperature based on a signal (eg, resistance) of the temperature device 106 via a conductor 264 .

[0020] 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.

[0021] In an embodiment, the controller 270, through hardware, software, or a combination thereof, is configured to perform glass bulb diagnostic tests to identify breakage, degradation, or aging of the glass bulb pH electrode and provide a signal indicative of such a condition. Additionally, the controller 270 is configured to identify the point or occurrence when the pH glass electrode can no longer be used and automatically transition to providing a temperature compensated pH output based on a back-up ISFET pH sensor providing that signal via line 256.

[0022] 7 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. 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 202. Thus, measurement circuit 260 and controller 270 are disposed within housing 202. Additionally, input / output circuit 312 is disposed within housing 202 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 communications protocols, such as a wired Highway Addressable Remote Transducer (HART®) protocol, FOUNDATION™ Fieldbus, or a suitable wireless process communications protocol, such as WirelessHART, as listed above. Similar to system 300, controller 270 is configured, through hardware, software, or a combination thereof, to perform glass bulb diagnostic tests to identify breakage, degradation, or aging of the glass bulb pH electrode and provide a signal indicative of such condition. Additionally, controller 270 is configured to identify points or occurrences when the pH glass electrode signal can no longer be used and automatically transition to providing a temperature compensated pH output based on a backup ISFET pH sensor providing its signal via line 256.

[0023] FIG. 8 is a flow diagram of a method of operating a pH glass sensor loop according to an embodiment of the present invention. The method 320 begins at block 322 where a dual element pH sensor with a backup element is provided. In one example, this sensor is the sensor 200 described above with respect to FIG. 5. However, it is expressly contemplated that the backup element of the dual element sensor may be a second pH sensing element of a different type, or even the same type as the glass bulb electrode 104. Next, at block 324, the pH sensing loop is used to sense pH with a glass electrode, such as electrode 104 (shown in FIG. 5). At block 326, a controller of a transmitter, such as transmitter 250, detects deterioration or failure of the pH glass electrode. This detection may be the result of a glass bulb diagnostic process performed by the transmitter, or may be caused by detection of low glass impedance or general non-response to the buffer. Additionally, pH glass deterioration may be performed by comparing the detected pH glass deterioration to a selected threshold value.

[0024] If a deterioration / failure of the glass electrode is detected, the method 320 proceeds to block 328 where the pH loop is switched to the secondary electrode. This may be a manual process as shown at reference numeral 330, where a technician physically disconnects the conductor of the glass bulb electrode from the transmitter and connects the conductor of a backup pH electrode, such as a capped or otherwise unused ISFET 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. 6 ), automatically switches to determine the pH based on the conductor already coupled to the backup pH 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 ISFET electrode. Finally, in block 334, the pH is sensed by the secondary 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. Sensor body; a pH glass electrode attached to the sensor body and configured to be exposed to a process fluid; a reference electrode having a junction, the junction configured to be attached to the sensor body and exposed to the process fluid; and A pH sensing probe comprising: a back-up pH electrode attached to the sensor body and configured to be exposed to the process fluid.

2. The pH sensing probe of claim 1 , wherein the backup pH electrode is an ISFET electrode.

3. The pH sensing probe of claim 2 , wherein the ISFET electrode shares a reference electrode with the pH glass electrode.

4. The pH sensing probe of claim 2 , wherein the ISFET electrode shares a solution ground with the pH glass electrode.

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

6. The pH sensing probe of claim 5 , wherein the temperature sensing element is selected from the group consisting of an RTD and a thermocouple.

7. the sensor body having a cylindrical shape with a distal portion and a proximal portion; 2. The pH sensing probe of claim 1, wherein the pH sensing probe includes an electrical connector mounted near the proximal portion and operably coupled to the pH glass electrode, the temperature sensing element, the reference electrode, and the back-up pH electrode.

8. The pH sensing probe of claim 1 , wherein the sensor body includes at least one externally threaded portion configured to attach the pH sensing probe to a process.

9. The pH sensing probe of claim 1 , wherein the reference junction is electrically coupled to the process fluid.

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, 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 temperature compensated pH process output; A pH transmitter comprising: Sensor body, a pH glass electrode electrically coupled to the measurement circuit and attached to the sensor body, the pH glass electrode configured to be exposed to a process fluid; a temperature sensing element electrically coupled to the measurement circuit and configured to provide an indication of a process fluid temperature; a reference electrode electrically coupled to the measurement circuit and having a reference junction attached to the sensor body, the reference junction configured to be exposed to the process fluid; and a backup pH electrode attached to the sensor body and configured to be exposed to the process fluid; a pH sensing probe comprising: A pH sensing system comprising:

11. 11. The pH sensing system of claim 10, wherein the controller is configured to detect degradation of the pH glass electrode and to perform an action if detected pH glass degradation exceeds a selected threshold.

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 pH glass electrode and connect the backup pH electrode to the pH transmitter.

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

15. The pH sensing system of claim 11 , wherein the backup pH electrode is an ISFET electrode.

16. The pH sensing system of claim 11 , wherein the controller comprises a microprocessor.

17. 1. A method of operating a pH sensing system, comprising: providing a pH sensing probe having a glass pH sensing electrode and a back-up pH sensing electrode; sensing pH with said glass pH sensing electrode; Detecting degradation of the pH sensitive glass; switching from the glass pH sensing electrode to the backup pH sensing electrode; and sensing pH with said backup pH sensing electrode; A method comprising:

18. 20. The method of claim 17, wherein the backup pH sensing electrode is an ISFET electrode.

19. 20. The method of claim 17, wherein switching from the glass pH sensing electrode to the backup pH sensing electrode occurs automatically upon detecting degradation of the pH sensing glass.

20. 20. The method of claim 17, wherein switching from the glass pH sensing electrode to the backup pH sensing electrode occurs in response to receiving a command from an external device.

21. Sensor body; a pH glass electrode electrically coupled to a measurement circuit and attached to the sensor body, the pH glass electrode configured to be exposed to a process fluid; a reference electrode electrically coupled to the measurement circuit and having a reference junction attached to the sensor body, the reference electrode being configured such that the reference junction is exposed to the process fluid; a backup pH electrode attached to the sensor body and configured to be exposed to the process fluid; a measurement circuit operably coupled to said pH glass electrode, said reference electrode, and said back-up pH 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 process fluid temperature information from the measurement circuit and provide a pH process output via the input / output circuit, the controller configured to automatically detect degradation of the pH glass electrode and switch from the pH glass electrode to the backup pH sensing electrode; A pH sensing probe comprising:

22. The pH sensing probe of claim 21 , wherein the backup pH sensing electrode is an ISFET electrode.

23. a temperature sensing element coupled to the measurement circuit and configured to provide an indication of a process fluid temperature; 22. The pH sensing probe of claim 21, wherein the controller is configured to provide a temperature compensated pH process output.

Images