Patent Publication Number: US-2011048971-A1

Title: Robust potentiometric sensor

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/239,274, entitled Robust pH Sensor, filed on Sep. 2, 2009, the contents of which are incorporated herein by reference in their entirety for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention generally relates to electrochemical sensors and more particularly to sensor assemblies including both sensing and reference half-cells in a single robust configuration. 
     (2) Background Information 
     Electrochemical potential measurements are commonly used to determine solution pH, other selective ion activities, ratios of oxidation and reduction activities, as well as other solution characteristics. A pH/ion selective electrode/oxidation reduction potential meter (hereafter referred to as a pH/ISE/ORP meter) is typically a modified voltmeter that measures the electrochemical potential between a reference half-cell (of known potential) and a measuring half-cell. These half-cells, in combination, form a cell, the electromotive force (emf) of which is equal to the algebraic sum of the potentials of the two half-cells. The meter is used to measure the total voltage across the two half-cells. The potential of the measuring half-cell is then determined by subtracting the known potential of the reference half-cell from the total voltage value. 
     The measuring half-cell typically includes an ion selective material such as glass. The potential across the ion selective material is well known by those of ordinary skill in the art to vary in a manner that may generally be described by the Nernst Equation, which expresses the electrochemical potential as a logarithmic function of ion activity (thermodynamically corrected concentration). A pH meter is one example of a pH/ISE/ORP meter wherein the activity of hydrogen ions is measured. pH is defined as the negative logarithm of the hydrogen ion activity and is typically proportional to the measured electrochemical potential. 
       FIG. 1  is a schematic of a typical, prior art arrangement  21  for measuring electrochemical potential. Arrangement  21  typically includes a measuring half-cell  30  and a reference half-cell  40  immersed in a process solution  6  and connected to an electrometer  50  by connectors  38  and  48 , respectively. Measuring half-cell  30  and reference half-cell  40  are often referred to commercially (as well as in the vernacular) as measuring electrodes and reference electrodes, respectively. Electrometer  50  functions similarly to a standard voltage meter in that it measures a D.C. voltage (electrochemical potential) between measuring half-cell  30  and reference half-cell  40 . Measuring half-cell  30  typically includes a half-cell electrode  36  immersed in a half-cell electrolyte  32 , which is typically a standard solution (e.g., in pH measurements). For some applications, such as pH measurement, measuring half-cell  30  also includes an ion selective material  34 . Alternatively, when measuring ORP (oxidation-reduction potential) the half-cell electrode  36  is immersed directly into the process solution  6 . 
     The purpose of the reference half-cell  40  is generally to provide a stable, constant (known) potential against which the measuring half-cell may be compared. Reference half-cell  40  typically includes a half-cell electrode  46  immersed in a half-cell electrolyte  42  ( FIG. 1 ). As used herein, the term “half-cell electrode” refers to the solid-phase, electron-conducting material in contact with the half-cell electrolyte, at which contact the oxidation-reduction reaction occurs that establishes an electrochemical potential. Half-cell electrolyte  42  ( FIG. 1 ) is hereafter referred to as a reference electrolyte. Electrochemical contact between the reference electrolyte  42  ( FIG. 1 ) and the process solution is typically established through a reference junction  44 , which often includes a porous ceramic plug or the like, for achieving restricted fluid contact. Ideally, the reference junction  44  is sufficiently porous to allow a low resistance contact (which is important for accurate potential measurement) but not so porous that the solutions become mutually contaminated. 
     12-mm diameter glass membrane pH sensors are a standard configuration in process and laboratory analytical environments. Over years, users have looked for more and more features in this relatively small envelope. In addition to housing both the sensing and reference half-cells of the electrochemical measuring system as an integrated “combination” probe, incorporation of additional features may be desired. Furthermore—and especially for process analytical applications—the sensor is often required to operate in harsh chemical environments over a wide range of temperatures and pressures and in the presence of shock, vibration, electrical currents in the test fluid, and electromagnetic radiation. Compromises in functionality and performance have been made in order to meet these requirements and/or to conform to specific form factors such as the 12-mm diameter form factor. 
     As one example of such a compromise, the Ceragel CPS71 series of 12-mm pH probes commercially available from Endress+Hauser of Switzerland, does not provide a fluid/solution ground contact. The user is required to run a separate ground wire from the process fluid near the deployment location of the sensor back to the measuring instrument. 
     An example of a conventional combination glass pH electrode may be found in U.S. Pat. No. 7,176,692 to Adami, et al. As shown in  FIG. 2 , Adami, et al. discloses an outer glass tube  10 , inner glass stem  8 , liquid junction  24 , pH glass membrane  16 , electrolytes  18  and  28 , and seal plugs  22  and  32 . 
     Adami, et al. directly fuse the outer glass tube  10  to the inner stem  8 . This approach is typical of many conventional combination probes, and tends to be relatively expensive while restricting the ability to modify the configuration for alternate sizes. Adami, et al., address a drawback of the aforementioned Ceragel CPS71 device by incorporating a fluid ground contact into their assembly in the form of a metal coating  14  applied to the outside surface of the glass. This approach, however, is generally incompatible with applications requiring the use of non-metallic components. 
     U.S. Pat. No. 3,666,651 to Makabe discloses a thermosensitive resistance element, i.e., a temperature sensor, inside the glass envelope of a pH half-cell. A drawback of this approach, however, is that the time response of the temperature sensor to changes in process fluid temperature tends to be compromised by the thermal mass of the combination pH probe. 
     US Patent Publication No. 2008/0283399 to Feng and Benson discloses a configuration in which a temperature sensor and solution ground contact are disposed within the reference electrolyte compartment. This approach tends to suffer the same drawback as Makabe with regard to the response time of the temperature sensor. In addition, placement of the solution ground contact in the electrolyte tends to be limiting. 
     Therefore, there exists a need for an improved potentiometric sensor for use in pH, selective ion activity, oxidation-reduction potential (ORP), and other electrochemical potential measurements that addresses the aforementioned drawbacks. 
     SUMMARY 
     In accordance with one aspect of the invention, a modular electrochemical potential measurement sensor includes a housing having a transverse cross-sectional geometry sized and shaped for compatibility with industry standard mounting and insertion hardware. A measuring half-cell having a sensing element, and a reference half-cell, are both disposed within the housing. The reference half-cell includes a reference electrode, a reference electrolyte in electrolytic contact with the reference electrode, and a reference junction including an ion barrier configured to provide controlled flow of the reference electrolyte therein to form an electrical pathway extending through the reference junction. A temperature sensor and solution ground combination assembly is also disposed within the housing. The combination assembly includes an electrical conductor extending through the housing, while remaining electrically isolated from each of the housing, reference half-cell, and measuring half-cell, and terminating at an electrically and thermally conductive end cap. Resilient seals are disposed at proximal and distal ends of the housing, through which portions of the reference half-cell, the measuring half-cell, and the combination assembly extend. The seals in combination with the housing, the measuring half-cell and the combination assembly define an electrolyte compartment for the reference half-cell. The sensing element, porous member, and end cap extending through the seal, enable direct contact with a test fluid, wherein the end cap provides close thermal coupling to the test fluid while also serving as a test fluid ground that is electrically isolated from the electrolyte compartment. One or more of he housing, measuring half-cell, reference half-cell, and combination assembly are modular components, so that the measurement sensor may be fabricated in a plurality of lengths by altering the length of the housing independently of the measuring half-cell, reference half-cell, and combination assembly. 
     In another aspect of the invention, a method for measuring electrochemical potential includes providing the modular electrochemical potential measurement sensor of the foregoing aspect, inserting the sensor into a liquid, and electrically connecting the sensor to a meter. The method also includes using the meter to capture a total voltage value across the measuring half-cell and the reference half-cell, and subtracting the potential of the reference half-cell from the total voltage value. 
     In still another aspect of the invention, a method of fabricating a modular electrochemical potential measurement sensor includes providing a housing sized and shaped for compatibility with industry standard mounting and insertion hardware, placing a measuring half-cell having a sensing element, and a reference half-cell, within the housing. The reference half-cell includes a reference electrode, a reference electrolyte disposed in electrolytic contact with the reference electrode, and a reference junction including an ion bather configured to provide controlled flow of the reference electrolyte therein to form an electrical pathway extending through the reference junction. A temperature sensor and solution ground combination assembly are placed within the housing. The combination assembly is configured to have an electrical conductor extending through the housing, while remaining electrically isolated from each of the housing, reference half-cell, and measuring half-cell, and terminating at an electrically and thermally conductive end cap. Resilient seals are placed at proximal and distal ends of the housing, and portions of the reference half-cell, the measuring half-cell, and the combination assembly are extended therethrough, so that the seals in combination with the housing, the measuring half-cell and the combination assembly, define an electrolyte compartment for the reference half-cell. The sensing element, porous member, and end cap, are extended through the seal disposed at the distal end of the housing, to enable direct contact with a test fluid, so that the end cap provides close thermal coupling to the test fluid while also serving as a test fluid ground that is electrically isolated from the electrolyte compartment. One or more of the housing, measuring half-cell, reference half-cell, and combination assembly, are configured as modular components, so that the measurement sensor may be fabricated in a plurality of lengths by altering the length of the housing independently of the measuring half-cell, reference half-cell, and combination assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a typical electrochemical potential measurement system of the prior art; 
         FIG. 2  is a schematic representation of a sensor assembly of the prior art; 
         FIG. 3  is a schematic representation of sensor assembly embodying aspects of the present invention; 
         FIGS. 4-8  are schematic, not-to-scale representations of various optional aspects usable with the embodiment of  FIG. 3 ; and 
         FIGS. 9-14  include graphical representations of test results of embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention include a modular potentiometric sensor which combines various features in a single unified form factor, to address various drawbacks associated with the prior art. Briefly described, these features include a 12-mm diameter for compatibility with industry standard mounting and insertion hardware, glass or rugged plastic outer body, sensing half-cell(s) of various types (e.g., ion sensing half-cell for pH with spherical, domed, or flat glass membrane, ion-sensing half-cell for other ions, ORP half-cell of platinum or other inert metal), reference half-cell with ion barrier, and plastic or elastomeric seals which help define an electrolyte compartment while serving as primary structural elements. 
     Embodiments of the invention also facilitate the use of gelled electrolytes, and include a combination temperature sensor/solution ground assembly that provides close thermal coupling to a test fluid, while also providing a metallic or non-metallic solution ground contact that is electrically isolated from the internal electrolyte compartment. This ground contact may be used as a diagnostic test point or as an additional sensing half-cell. A porous liquid junction assembly serves as a fill-hole plug with high column strength. The assembly is steam-sterilizable, may include an internal pressure compensator, and its modularity enables convenient reconfiguration for different probe lengths. 
     Various embodiments of the present invention will be described in greater detail with specific reference to the accompanying Figures. For convenience of explication, these embodiments are described with respect to sensors having a measuring half-cell configured for the measurement of pH, since pH is a commonly measured analyte. However, it should be understood that other types of sensing half-cells, such as those used for ORP, fluoride ion detection, or other ion-selective measurements, may be substituted for, or added to, the described pH half-cell in order to create a sensor suitable for these other measurements. Moreover, although embodiments of the present invention are shown and described with respect to sensors of the 12-mm diameter form factor, it should be recognized that these embodiments may be configured with substantially any other form factor without departing from the scope of the present invention. 
     As mentioned above with respect to  FIG. 2 , because of their widespread use and acceptance, pH sensors/probes have often been fabricated from glass using various glassblowing techniques. As shown in  FIG. 2 , the pH-sensitive glass membrane  16  is typically fused to an inert glass tube or “stem”  8 . A wire  20  and electrolyte  18  is then sealed into the stem to form the pH sensing half-cell. In order to combine with a reference half-cell, the pH half-cell is inserted and sealed into a second larger inert glass tube  10 . The annulus thus formed serves as a compartment to house a reference electrolyte  28  and reference wire K 2 . The circuit between the pH and reference half-cells is completed by means of a liquid junction  24  between the reference electrolyte and test fluid  6 . A common means of creating the liquid junction in such a glass electrode is to seal a piece of porous ceramic into the wall of the outer glass tube  10 . The assembly described is referred to herein as a combination pH electrode or probe since it combines the sensing and reference half-cells in an integral unit. 
     With reference to  FIG. 3 , various components of a representative embodiment of the present invention will be described in detail. As shown, a potentiometric (e.g., electrochemical) probe  60  is configured for pH measurement. For convenience, the means of connecting probe  60  (including leads  38  and  48  from the measuring and reference half-cells, and leads  77  and  98  from the temperature detection and solution ground) to a measuring apparatus (e.g., electrometer  50 ,  FIG. 1 ) is not shown. Such connection may be accomplished by means of a conventional multiconductor cable, such as integrally built into the proximal end (e.g., the “top” of the assembly, in the orientation shown in  FIG. 3 ) or with a multi-contact connector  92  ( FIG. 5 ) which then interfaces with a cable. In particular embodiments, electrometer  50  takes the form of a conventional process variable transmitter (PVT) coupled to a factory automation network of the type sold by Invensys Systems, Inc., (Foxboro, Mass.) and which is configured to measure the electrochemical potential between the half-cells. 
     As shown, sensor  60  is a modular device including a housing  62  having a predetermined length and a predetermined diameter configured for compatibility with industry standard mounting and insertion hardware. As mentioned above, in particular embodiments the housing is provided with an industry standard diameter of 12 mm, and is fabricated from glass and/or plastic materials. 
     A measuring (e.g., pH) half-cell  64  extends longitudinally within housing  62 , and includes a stem glass housing  65  which terminates at a sensing element  66  (e.g., a pH glass membrane) at the distal end thereof. Also, in the embodiment shown, measuring half-cell  64  includes a measuring electrode  67  disposed therein, in electrolytic contact (e.g., via a half-cell electrolyte  32 ) with membrane  66 . Membrane  66  may take substantially any form factor, such as a spherical, domed, or flat configuration. 
     A reference half-cell  68  is also disposed within housing  62 , and includes a reference electrode  70 , a reference electrolyte  72  disposed in electrolytic contact with reference electrode  70 , and a reference junction  74  including an ion barrier, e.g., in the form of a porous member configured to provide controlled flow of the reference electrolyte  72  therein to form a primary electrical pathway extending through the reference junction  74 . 
     Reference junction  74  may take the form of a porous ceramic plug or the like (e.g., porous Teflon® (polytetrafluoroethylene, DuPont), porous KYNAR® (polyvinyldifluoride, Elf Atochem, N.A.), or wood) or nominally any other porous material) for achieving restricted fluid contact. Reference junction  74  is sufficiently porous to allow a low resistance contact (for accurate potential measurement) but not so porous that the solutions become excessively mutually contaminated. The skilled artisan will recognize that pore size, percent porosity, and effective cross-sectional area of the reference junction  74  must all be balanced, in conjunction with the particular electrolyte used, to achieve the desired restricted fluid contact. In particular exemplary embodiments, junction  74  may include a porous ceramic plug of the type conventionally used in the DolpHin™ sensor available from Invensys Systems, Inc., e.g., having an effective diameter of approximately 0.05 to 0.14 inches, pore sizes between about 0.2 to 0.3 μm, and total percent porosity of 20 to 30 volume percent. 
     As also shown, in particular embodiments, the reference electrode  70  is encased in a NAFION® (DuPont) tube  71 . Those skilled in the art will recognize that NAFION® is a permselective polymer that prevents complex silver anions in the reference half-cell from entering the bulk electrolyte  72  where they may diffuse to the liquid junction  74  and cause clogging. 
     It should be recognized that any number of materials may be used for electrolytes  32  and  72 . Some examples of reference electrolytes  72  include a solution including potassium chloride, silver chloride, and combinations thereof. One particular example includes a mixture of about 4 molar potassium chloride and saturated silver chloride. The reference electrode  70  may also be fabricated from any number of suitable materials, including, for example, silver, silver-silver chloride, mercury-mercurous sulfate, mercury-mercurous chloride, and other redox couples. 
     A temperature sensor/solution ground assembly  76  is disposed within housing  62 , and includes an electrical conductor  98  extending through the housing, while remaining electrically isolated from the housing  62 , from the reference half-cell  68 , and from the measuring half-cell  64 , and terminating at an electrically and thermally conductive metallic or non-metallic end cap  78  disposed at the distal end of the sensor  60 . In the particular embodiment shown, this electrical isolation is provided by use of a tubular electrically non-conductive sleeve  80 . Assembly  76  also includes electrical conductors  77  extending to a temperature detector (e.g., RTD  106 ,  FIG. 6 ) disposed at the distal end of the housing, e.g., within end cap  78 . 
     Seals  82  and  84 , e.g., fabricated from plastic, elastomeric, or other suitable electrically non-conductive and chemically inert resilient materials, are disposed at proximal and distal ends of the housing, respectively. Examples of suitable materials include various elastomers such as silicone rubber, EPDM, fluoroelastomers such as VITON® (DuPont), and perfluoroelastomers such as Kalzrez™ or Chemraz™ may be chosen for their mechanical and chemical properties. Polymers such as PTFE, PFA, or PEEK may also be used, with or without elastomeric O-rings. Similar seals may be used in a conventional manner within the half-cells  64  and  68 , such as shown at  85 . 
     Proximal and distal portions of the reference half-cell  68 , the measuring half-cell  64 , and the temperature sensor assembly  76  extend through the seals  82 ,  84 , as shown. The seals  82 ,  84 , in combination with housing  62 , measuring half-cell  64 , and the temperature sensor assembly  76 , effectively define an electrolyte compartment for the reference electrolyte  72 . Optionally, the reference electrolyte  72  may take the form of a conventional gelled electrolyte. It should be recognized that gelled electrolytes tend to provide for relatively slow diffusion, which advantageously tends to slow electrolyte contamination during use. 
     It is noted that in the configuration shown, the conductive (optionally gelled) electrolyte  72  in the annular electrolyte compartment surrounds the high-impedance pH half-cell  64  to effectively shield it from electromagnetic radiation. Moreover, an optional internal pressure compensator  86  may be disposed within the reference electrolyte compartment. Compensator  86  is configured to expand or contract in response to relatively low or high external pressures on the housing  62 , to help compensate for pressure variations in the test (process) fluid  6 . In particular embodiments, pressure compensator  86  may take the form of a sealed, gas (e.g., air)-filled polymeric tube. The gas may thus compress when subjected to higher pressure from the process  6 , or due to thermal expansion of the reference electrolyte  72 . This compression should help guard against components rupturing or the seals  82 ,  84  or liquid junction  74  being blown out of the body  62 . In this regard, it is noted that tube compression due to external process pressure generally has not been problematic with conventional glass electrodes due to the inherent rigidity of their glass housings. The pressure compensator  86  may, however, be desired in embodiment hereof, which employ plastic housings  62 . 
     It should be recognized that in addition to use within the reference cell  68 , a pressure compensator  86  may also be disposed within the measuring half cell  64 . However, such use may be unnecessary in the event the measuring half-cell is a pH half cell or other half-cells fabricated from hermetically sealed glass, since such glass is relatively unaffected by the pressures and temperatures experienced in typical applications. 
     Thus as shown, membrane  66 , porous member  74 , and the end cap  78  extend through seal  84  disposed at the distal end of the housing  62 , to enable direct contact with test fluid  6 . In this configuration, end cap  78  provides close thermal coupling to test fluid  6 , to facilitate temperature measurement. In addition, because the end cap  78  is electrically conductive, while also being electrically isolated (e.g., by sleeve  80 ) from the reference electrolyte  72 , the cap  78  may serve as a test fluid ground which may be used as a diagnostic test point or as an ORP sensor, etc., as discussed in greater detail hereinbelow. 
     It should be recognized that the above-described construction of sensor  60 , the housing  62 , measuring half-cell  64 , reference half-cell  68 , and temperature sensor assembly  76  are each configured as modular components which are substantially independent of one another. This modular construction enables the measurement sensor  60  to be fabricated in a plurality of lengths simply by altering the length of the housing  62  independently of the measuring half-cell  64 , reference half-cell  68 , or temperature sensor/ground assembly  76 . This length modification option will be described in greater detail hereinbelow with respect to  FIG. 5 . 
     The various components may be fabricated from steam-sterilizable materials, i.e., materials that maintain their structural and chemical integrity through repeated steam sterilizations and operation at elevated temperatures and pressures. The body  62 , for example, may be fabricated from glass. However, while glass has advantages such as transparency, inertness, and low cost, it suffers from fragility, particular when fabricating sensors of relatively long length. Thus, particular embodiments may use a body fabricated from a relatively rugged plastic tube. Examples of suitable plastics may include any number of structurally rugged, chemically inert materials, such as PEEK (polyetheretherketone), Ryton® PPS (polyphenylene sulfide, Chevron Phillips Chemical Company), or Kynar® (PVDF). In various embodiments, these polymeric materials may provide the desired resistance to breakage, while also providing sufficient structural rigidity to protect relatively fragile interior components such as the stem glass  65 , etc., from damage both during use and during installation and removal from the process  6 . 
     It will be noted that the above-described modular construction provides for enhanced flexibility of construction relative to conventional approaches. For example, a membrane  66  (e.g., a pH glass membrane) of substantially any desired configuration may be used, including spherical, domed, or flat membranes. An embodiment including a substantially flat membrane  66 , positioned flush to the distal process seal  84 , combined with a body  62  fabricated from PEEK, provides for an especially robust sensor. Also, unlike glass, a plastic such as PEEK is readily machined or molded. This allows, for example, incorporation of protective fluting  90  to further protect the glass membrane  66  against damage or breakage, such as shown in  FIG. 4 . 
     As mentioned above, the modular configuration described above provides for conveniently adapting the various embodiments of sensor  60  to different overall lengths. Not only do different applications require different process insertion depths, but mounting and insertion hardware for electrochemical (e.g., pH) sensors is becoming more and more standardized as well. Hardware for 12-mm diameter pH probes is commonly available that accommodates lengths of 120, 220, 360, and 425 mm However, in prior art approaches, such as described hereinabove with respect to  FIG. 2 , changing the overall length is relatively complex, generally requiring changing the lengths of many other components in addition to the outer housing, including the measuring half-cell, wiring and insulation for temperature sensors and other components, etc. Moreover, as also mentioned above, difficulties associated with glass housings tend to be greatly exacerbated when length is doubled or trebled. 
     In contrast, as mentioned above, the modularity of embodiments of the present invention enables probes to be conveniently provided in various sizes (e.g., lengths), without requiring all of the components to be resized. For example, turning now to  FIG. 5 , a plastic outer body  62  of a relatively short length (e.g., 120 mm), may be lengthened by adding a plastic body extender  62 ′ and running longer wire leads  94  to the proximal end (e.g., to connector  92 ). Such an embodiment may be sold as a kit including the shortest (e.g., 120 mm) housing  62 , one or more extenders  62 ′, and internal wiring  94  (including leads  77 ,  98  and tubing  80 ) which is long enough for use with the extender(s)  62 ′. (The wiring  94  may be shortened by the user in the event the extender is not to be used.) The extender(s)  62 ′ may be connected to the shortest version by any suitable means, such as a bayonet or snap-fit connector, threaded connections, and/or glue, etc. Similarly, embodiments having a housing with a user-adjustable length may be provided. For example, a relatively long housing may be provided with transverse score lines spaced along its length to enable a user to conveniently cut or break the housing to a desired length. This flexibility to conveniently provide for variable lengths may provide significant advantages to a manufacturer in terms or product cost, inventory, and cycle time. 
     Turning now to  FIG. 6 , a particular embodiment of temperature sensor/solution ground assembly  76  is shown and described as assembly  76 ′. This particular embodiment includes an electrically conductive tube  96 , e.g., of stainless steel or other metal, which provides electrical contact between the solution ground contact (e.g., end cap)  78  and ground wire lead  98  at the proximal end of the probe. The tube  96  is closed, e.g., by welding, at the distal end to preventingress of the process fluid. It is noted that in some applications, the tube  96  itself may serve as a satisfactory solution ground contact. However, metals—even stainless steels—are subject to corrosion in some process fluids. For this reason, particular embodiments employ an end cap  78  which is electrically conductive, but non-metallic, to serve as a solution ground contact. End cap  78  may be fabricated from any number of electrically conductive, non-metallic materials known to those skilled in the art. In particular embodiments, end cap  78  is fabricated from PVDF, due to its wide applicability to various applications and its general acceptance by users in the field of electrochemical sensing. 
     The solution (process fluid) ground contact, such as provided by end cap  78 , may be used to provide a reference potential that may be subtracted from the potentials provided by sensing and reference half-cells  64  and  68 , respectively ( FIG. 3 ). Such use may effectively prevent variable, spurious currents and potentials in the process fluid  6  ( FIG. 3 ) from interfering with the measured pH signal. In addition, as mentioned above, a solution ground contact  78  may enable useful diagnostics when the readout instrumentation (e.g., electrometer  50 ,  FIG. 1 ) has such capabilities. For example, monitoring the electrical resistance between the ground contact  78  and the internal pH half-cell wire  38  ( FIG. 3 ) may indicate a break or crack in the glass membrane  66  ( FIG. 3 ). Likewise, monitoring the resistance of the liquid junction  74  may have diagnostic value. However, in order to make this measurement it is necessary to electrically insulate the ground contact  78  from the reference electrolyte  72  ( FIG. 3 ). Therefore, in order to provide this functionality, the (stainless steel) tube  96  may be provided with insulating barrier  80  where tube  96  passes through the reference electrolyte  72  as shown in  FIG. 3 . As shown in  FIG. 6 , barrier  80  may take the form of conventional heat-shrinkable tubing. Other insulation schemes would occur to those versed in the art in light of the instant disclosure. 
     As mentioned briefly above, the ground contact  78  may also serve another purpose. If the solution-contacting end cap  78  is fabricated from an inert metal, such as platinum, it may serve as an ORP sensing half-cell. In such an embodiment, the 12-mm probe becomes a multi-measurement device capable of measuring pH and ORP simultaneously when connected to an appropriately configured electrometer  50  ( FIG. 1 ). 
     Further, as discussed hereinabove, the solution ground assembly  76 ′ may serve as a housing for a temperature sensor  106  in the form of an RTD or other element, e.g., disposed within end cap  78 , to thus serve as a combination solution ground and RTD assembly. As also discussed above, this configuration brings the RTD  106  relatively close to the process fluid  6  ( FIG. 3 ), with separation provided by (e.g., end cap) materials with relatively good heat conducting properties. Moreover, the temperature sensor may be thermally isolated from the thermal mass of the probe by embedding it in the weakly heat-conducting process seal  84 , while it is thermally coupled to the process fluid  6  by means of the thin-walled and relatively strongly heat-conducting end cap  78 . This embodiment has been shown to achieve relatively rapid response to changes in process temperature, as discussed hereinbelow with respect to exemplary test results. 
     Turning now to  FIGS. 7 and 8 , particular embodiments of the present invention may benefit from liquid junction assemblies  108  or  110 . As mentioned above, in many conventional pH products, a porous ceramic element is sealed directly into the glass body. This approach may be satisfactory for many applications. However, this approach generally requires glass working skill to manufacture, involves the possibility that fused glass may penetrate some of the pores causing blockage, and the inability of a user to replace the junction if clogged. 
     Embodiments of the present invention address these concerns by use of seal  84 , into which the reference junction  74  may be press-fit. Moreover, the optional assemblies  108  and  110  facilitate this insertion while helping to avoid damage to the porous junction  74 . 
     Referring in particular to  FIG. 7 , a relatively hard polymeric (plastic) sleeve  112  may be formed, e.g., by slicing a portion of a plastic tube lengthwise so that its inner diameter could be opened slightly. A porous rod  74  may then be slipped into the sleeve  112  with the inner diameter of the sleeve allowed to clamp over it in a press-fit manner as shown. The resulting assembly  108 , although not a perfect cylinder, may be press-fit into a hole in elastomeric seal  84 , which then conforms to the “half moon” shape of the assembly  108 . 
     Alternatively, as shown in  FIG. 8 , a porous rod  74  may be encased in heat-shrinkable polymeric (e.g., PVDF) tubing/sleeve  114 , or other tubing of suitable material, to form assembly  110 . This assembly forms a substantially cylindrical cross-section, which enables it to be press fit into a seal  84 . In this embodiment, seal  84  may be fabricated from a less resilient material than that used with assembly  108 , since there are essentially no gaps around the assembly  110  which need to be filled by a resilient seal. Thus, while a seal  84  fabricated from an elastomeric material may be used, other materials, such as relatively hard plastic, may also be used in combination with this assembly  110 . 
     These liquid junction sub-assemblies  108 ,  110 , tend to enable simplified installation, which does not require any specialized skill. In addition, the porosity of the junction  74  is not compromised by fused glass. Moreover, prior to installation, the hole in the process seal  84  may be conveniently used for filling the electrolyte chamber with electrolyte  72 . If the junction becomes clogged or coated during use, it may be conveniently replaced (e.g., with an appropriate tool, the old junction may be pulled out or pushed into the electrolyte compartment, or a replacement junction may be used to displace the old one). (The electrolyte  72  may also be replenished once the junction is removed, prior to replacement.) This approach may also accommodate the use of materials for junction  74 , which may not be well suited for sealing to glass and/or for press fit installation, such as various porous ceramics or other porous media such as porous PTFE, since sealing into glass is not required and the plastic sleeve or shrink tubing, etc., provides the column strength necessary for a press fit. 
     The following illustrative examples are intended to demonstrate certain aspects of the present invention. It is to be understood that these examples should not be construed as limiting. 
     EXAMPLES 
     Example 1 
     Robustness against Sterilization 
     An electrode  60  ( FIG. 3 ) was fabricated with a domed pH glass membrane  66 , a 12 mm diameter PEEK housing  62 , Viton® seals  82 ,  84 , a liquid junction assembly  114  including a ceramic rod within a PEEK sleeve, a Kynar® RTD/solution ground end cap  78 , a PFA pressure equalization bladder  86 , and a NAFION® ion-barrier inner reference assembly  70 . 
       FIG. 9  shows the test results of the pH sensor of Example 1 after multiple 30-minute autoclave cycles (steam-sterilizations) at 125° C. Slope between pH 4 and 7 buffers remained above 90% of theoretical (Nernst) after 60 cycles. 80% slope was chosen as benchmark for acceptable performance. 
     Example 2 
     Operation at Process Pressure of 150 psi 
     An electrode  60  was fabricated substantially as in Example 1, but with the 12 mm diameter housing  62  fabricated from Pyrex® glass instead of PEEK. 
       FIG. 10  shows the response of the pH sensor of Example 2 in pH 4, 7, and 10 buffers under a process pressure of 150 psi. No physical damage nor abnormal response behavior was observed. 
     Example 3 
     Operation at elevated Temperature and Pressure 
       FIG. 11  shows the output of a probe configured as in Example 2 in pH 4 buffer at a temperature of 121° C. pressure of 150 psi. The output shown indicates successful operation. 
     Example 4 
     Operation in Partial Vacuum 
     An electrode  60  was fabricated substantially as in Example 2, but with a flat, instead of domed, pH glass membrane  66 . 
       FIG. 12  shows the response of the sensor of this Example 4 in pH 4 buffer at a pressure alternating between 1.0 and 0.5 atm (101 and 50.5 kpa). Results showed an output change of less than 1 mV (representing less than 0.02 pH). These results indicate successful performance. 
     Example 5 
     Performance at Lowered Temperature 
     An electrode was fabricated substantially as in Example 1, but with a flat pH glass membrane  66 , and with a Kynar® sleeve  114  in the liquid junction assembly. 
       FIG. 13  shows the response of the sensor of this Example 5 in pH 7, 4, 7, 10, and 4 buffers, respectively, at a temperature of −15° C. These results indicate successful performance. 
     Example 6 
     Low-Temperature Performance Compared to Prior Art 
       FIG. 14  shows the response of the probes of Examples 1 and 2 compared to prior art probes at −15° C. Probes of present invention showed fast and stable response to pH 4, 7 and 10 buffers, reaching 90% response less than 1 minute, while prior art domed-membrane probes from Suppliers 1 and 2 showed 90% response in about 2 and 1.3 minutes, respectively, and with flat-membrane probe from Supplier  2 , no stable response could be reached after 30 minutes. 
     Example 7 
     Thermal Shock 
     Probes with the features indicated in Table 1 were exposed to 100° C. boiling water for 5 minutes and then put into iced water for 5 minutes. The experiment was repeated 3 times and pH response performance was then tested at room temperature. No slope deterioration nor process seal nor liquid junction movement was observed. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Membrane 
                 Process 
                 Junction 
                 % Slope before 
                 % Slope after 
                   
               
               
                 Sensor 
                 Type/Body 
                 Seal 
                 Sleeve 
                 Thermal 
                 Thermal 
                 Seal/Junction 
               
               
                 ID 
                 Material 
                 Material 
                 Material 
                 Shock 
                 Shock 
                 Movement 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 1 
                 Domed/Glass 
                 EPDM 
                 PEEK 
                 97.97 
                 99.66 
                 None 
               
               
                 2 
                 Domed/Glass 
                 EPDM 
                 PEEK 
                 97.97 
                 99.10 
                 None 
               
               
                 3 
                 Domed/Glass 
                 EPDM 
                 PEEK 
                 98.28 
                 99.10 
                 None 
               
               
                 4 
                 Domed/PEEK 
                 EPDM 
                 PEEK 
                 98.54 
                 99.10 
                 None 
               
               
                 5 
                 Domed/PEEK 
                 EPDM 
                 PEEK 
                 98.54 
                 100.23 
                 None 
               
               
                 6 
                 Domed/PEEK 
                 EPDM 
                 PEEK 
                 97.97 
                 98.54 
                 None 
               
               
                 7 
                 Flat/PEEK 
                 Viton ® 
                 Kynar ® 
                 98.54 
                 99.66 
                 None 
               
               
                 8 
                 Flat/PEEK 
                 Viton ® 
                 Kynar ® 
                 98.54 
                 99.66 
                 None 
               
               
                 9 
                 Flat/PEEK 
                 Viton ® 
                 Kynar ® 
                 99.10 
                 99.66 
                 None 
               
               
                   
               
            
           
         
       
     
     In the preceding specification, the invention has been described with reference to specific exemplary embodiments for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. 
     While several embodiments of the present invention have been shown and described with various characteristics, it should be understood that one or more of these characteristics of one embodiment may be substituted or added to characteristics of other embodiments without departing from the spirit and scope of the present invention. 
     The modifications to the various aspects of the present invention described hereinabove are merely exemplary. It is understood that other modifications to the illustrative embodiments will readily occur to persons with ordinary skill in the art. All such modifications and variations are deemed to be within the scope and spirit of the present invention as defined by the accompanying claims.