IMPROVEMENTS IN ELECTROCHEMICAL SENSORS

Abstract: An electrochemical sensor unit (10) employs a resilient, porous plug (14) to form an ion conduction path between an external test fluid and an internal conductive liquid by which an electrical circuit to lead wire (44) of a measuring instrument is completed. Flow of fluid past the plug between the plug and an outer body (28) and between the plug and an inner sensor tube (38) is prevented by using the body to compress the plug in the longitudinal direction of the plug and the body. In one design the plug is compressed by a pair of flanges (32, 34) which press against the margin of the ends of the plug.

Description: IMPROVEMENTS IN ELECTROCHEMICAL SENSORS Technical Field This invention relates to improvemen s in elec rochemical sensors of the kind which utilize ion exchange across a barrier between a conductive liquid and a test fluid, either gas or liquid. Background Art The basic circuit for ' utilizing electrochemistry to detect and measure the concen ra ion of a particular chemical constituent, or pH , in a test fluid includes a voltaic cell formed by ions of the material to be detected. Charges are transported by those ions to or from a substance which transfers charges selectively to ions of that material. The concentration of the material to be detected is calculated on the basis of the magnitude of the exchange potential, and that requires a complete electrical circuit and electron flow. Creation of the electrical circuit necessarily involves junction between connector wires and liquids and gasses and the ion selective transfer substances. Elec¬ trical poten ials are developed at these junctions of electrochemically dissimilar materials. The magnitude of those potentials may be of the same order of magnitude as the potentials across the test cell. Because the addi¬ tional junction potentials cannot be eliminated, the measuring circuit is arranged so that the additional junctions are created in pairs arranged to cancel one another. In the present state of the art the most common arrangement is to have the test solution and the ion selective transfer material contact an electrically con¬ ductive solution and to connect the conductive solution to the voltmeter wires through a chemical which contains both the metal of the wires and the conducting ion of the con¬ ductive solution. In the typical case the copper wire from the voltmeter is joined to one end of a silver wire. A silver-silver chloride mixture is fixed to the other end of the silver wire. The. silver chloride is immersed in a saturated solution of sodium or potassium chloride which is contained in a vessel where it makes physical contact with the ion selective transfer substance. That substance is disposed to make physical contact with the test fluid. The circuit from the other side of the voltmeter is similar. It extends from copper wire to silver wire to silver-silver chloride to saturated salt solution to test fluid. There is a difficult mechanical problem in that arrangement. The saturated salt solution must make electrical contact with the test fluid without an inter¬ vening junction at one side of the sensing junction. That connection is called the reference cell or reference junction. The problem is how to complete the electrical contact at the reference junction without a transfer of test fluid to contaminate the saturated salt solution or loss of the saturated salt solution from its container. A like problem occurs at the other side of the test cell if the ion transfer material is not impervious to flow of test fluid or the flow of the salt solution. A special pH sensitive glass is used as the ion transfer material in most pH test cells. The glass is impervious to both test fluid and salt solution. In that case the separation probably occurs only at the reference junction, and it has become both possible and common to package the pH test cell and the reference cell in a single assembly called a combination electrode. In the case of units for measuring oxygen, potassium, C02, and other substances, the ion transfer material must be incorporated in a carrier material whose physical nature permits it to serve as a barrier to separate the salt solution from the test fluid. The carrier materials fall into two classes. In one, the carrier material is impervious to the flow of salt solution and of test fluid. In the other, the salt solution or test fluid, or both, are permitted to migrate into the body of carrier material to contact the ion transfer material jointly whereby to shorten the flow path and minimize motion artifacts and random flow variations. Carrier materials of the latter class are porous and, in the current state of the art, they are preferred. They are preferred for use at both the reference junction where they are called salt bridges , and at the measuring junction where they are called carrier or membrane mater¬ ials. It has become standard to dispose of them in one end of a tubular housing which serves as the container for the salt solution and the metal-metal salt connection from the solution to the electrode wire. Porous salt bridge and carrier materials have included wooden plugs, cotton threads, ceramic plugs, glass frit-filled rubbers , porous elastomers and other materials . One of the most successful is shown and des¬ cribed in United States Patent No. 4,128,468 to Bukamier. The plugs disclosed in that patent are resilient and porous. The plug shown in United States Patent No. 4,105,509 is porous but rigid. In each case, the .salt bridge or carrier material is formed as a plug, and in each case an 0-ri.ng is employed to aid in sealing against the flow of liquid past the plug. In practice, these units may be subjected to temperature and pressure cycling over a wide range. The forces urging contamination of salt solution by test fluid and leakag.e and weeping of salt solution contribute to shortened useful life. The construction shown in the Bukamier patent has done much to overcome the problem, but it is an object of this invention to provide even greater reliability. OMPI . Λ Disclosure of Invention It is an object of the invention to provide an electrochemical electrode structure which offers improved protection against contamination and internal liquid loss. A related object is to provide a structure which will permit use o.f more porous plug materials without loss of protection against such contamination or liquid loss. A further object is to provide those improve¬ ments and advantages at reduced cost. The requirement of the sealing structure is to prevent movement of liquid, or gas, between the outer wall of the plug and the inner wall of the electrode body and, in the case of combination electrodes, between the outer wall of the inner element and the wall of the plug opening. The theory of the prior art was to accomplish sealing with laterally directed forces - with compression of O-rings and interference fits. The invention utilizes forces applied in the longitudinal direction. Greater force can be applied without the kind of compression of the plug that reduces porosity. Sealing, at least primary sealing, is accom¬ plished by forming the housing with elements by which he plug can be stressed longi udinally either in compression or in tension. In the case of combination electrodes arranged concentrically, sealing of the opening through the plug is enhanced with longitudinally applied, again without adverse effect on the porosity of the plug. The preferred form uses a plug that is both resilient and porous. Resilience lends an ability to to conform and to engage all of the area available for sealing and to close off flow passages at the outer surface of the plug. Also, the renitence of a resilient plug urges pore openings to maximum size and volume. But, combined with porosity, resilience permits compression to smaller volume and decreased porosity. Sealing with forces in compression at the sides of the plug has minimum effect on porosity along the primary paths of test fluid and salt solution migration. The result is that increased porosity is feasible. A major advantage in the longitudinal compres¬ sion form of the invention is that the shape of the plug can be simplified and manufacturing cost reduced. The need for closely held plug dimensions is eliminated and assembly can be simplified. These and other objects and advantages of the invention will be apparent upon examination of the following description and the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 is an isometric view of electro¬ chemical sensor electrode in which the invention is embodied; Figure 2 is a cross-sectional view taken on the vertical midplane containing the axis of the unit of Figure 1 as indicated by line 2-2 of Figure 1; Figure 3 is an isometric view of the porous plug of the unit of Figures 1 and 2; Figure 4 is an enlarged cross-sectional view of the screw front of the unit of Figures 1 and 2; Figure 5 is an enlarged view, the lower half in section, of a fragment of the sensing end of said uni ; Figure 6 is an isometric view of a sensing electrode of another form in which the invention is embodied; Figure 7 is an enlarged cross-sec iona 1 view taken on line 7-7 of Figure 6 of the sensor end of the unit; and Figure 8 is an isometric view of the porous plug of the unit of Figure 6. Description of the Preferred Embodiment The sensing electrode 10 of Figure 1 is a combi¬ nation electrode arranged to sense pH of a fluid in which the sensor element 12 and the end of the porous plug 14 of the reference element are immersed. The exterior shape and appearance of the unit are common, even standard. The body 16 is an elongated cylindrical tube terminating at its sensor end in scallops 18 which are designed to permit fresh fluid to flow past and contact the sensor element and porous plug while keeping them free of any solids which may be contained in the test .fluid. The body contains saturated salt solution which is sealed in place by the porous plug 14 at the sensor end and by several layers of sealing material at the connector end 20. That end is closed, and a connector of coaxial cable end is held in place by an end cap 22. In this case the cap holds a BNC-type electrical connector 24 which extends axially from the body 16. There are two differences between this unit and the standard configura ion. One is the parting line 26. The body in this case is divided into a main body 28 and a clamping section 30. The other difference is not visible in Figure 1. The plug 14 is held in place and compressed in the direction of the longitudinal axis of the unit by a shoulder formed on the inner periphery of the clamping section . In this case the shoulder is formed by an annular flange 32 which extends in ardly into the central opening of the clamping section 30. In the assembled unit the plug 14 is trapped and compressed between that flange 32 and a second shoulder, also an inwardly directed flange in this case. The second flange 34 is formed on the in¬ terior of the main body section 28. It is preferred that the flanges 32 and 34 be molded integrally with the OMPI clamping section 30 and main body 28, respectively. However, other constructions are possible and acceptable. The shoulders may be the consequence of a change in diameter of the clamping section or main body, or both, or they might be formed by separate elements which are fixed to the interior of the body. The preferred construction is shown in Figure 2, 3 and 4. The sensor end of the main body 28 has reduced diameter. Its inner bore is smooth and its outer wall is formed with threads. The threads are threaded into threadways formed on the inner surface of the clamping section 30. When the clamping member is screwed onto the threads of the main body and turned up finger tight, the separation between flanges 32 and 34 is less than the length of the plug when the plug is in relaxed condition as shown in Figure 3. At the right of flange 34, in Figure 2, the unit is conventional. This particular one has its reference cell divided into two cells in series by a porous plug 36 held in place by a pair of 0-rings at its outer surface, and by tight embracement of the inner pH cell body 38. That arrangement is taught in Bukamier United States Patent No. 4,128,468. Division of the reference elec¬ trode, and of the sensing electrode for that matter, into a series of cells is used when there are likely to be large differentials between interior and exterior pres¬ sures or where there is a possibility that constituents of the test fluid might migrate into the reference fluid (the salt solution) of the reference electrode. Both of the cavities 40 at the right of plug 36, and 42 at the left , are filled with a conductive liquid. In this case both are filled with saturated potassium chloride solution. The body 38 of the pH sensing elec¬ trode is also filled with a saturated potassium chloride OMPI solution. The bulb 16 is made of pH sensitive glass. It is bonded to the glass tube which serves as the body 38. Contact with the reference electrode fluid is made through a silver wire 44 and a body of silver and silver chloride mixture 46 which is bonded to the wire and is disposed, in cavity 40. Contact with the sensing electrode -is made by silver wire 48 and a glob of silver-silver chloride 50. At the right, both electrode bodies are sealed by several layers 52 of sealing material. The two-conductor connec¬ tor 24 is held in place by the sealing material and the end cap 22. One of the silver wires is connected to one terminal of the connector and the other wire is connected to the other terminal. The plug itself is not new. It is made of a porous material which is inert to the materials with which it does or might come into contact. In the past selection of plug material required a compromise between porosity and rigidity. Increased porosity reduces the need for pre-soaking and increases response to change in test fluid chemistry, but it complicates the problem of sealing the unit to prevent flow or migration of test fluid or the electrode solution. Rigid materials simplify the sealing problem, but make the problem of achieving uniform poro¬ sity at a reasonable cost more complex. Resilience in the plug material reduces the need for close dimensional control, but it, too, complicates the sealing problem. While the renitence of the material aids sealing, too much compression collapses the internal voids and reduces porosity, and the resilient plug is deformed by pressurized fluid to permit leakage. The invention makes it possible to utilize softer, more porous plugs than was possible in prior OMPI technology. The plug material is stressed and the reaction force is utilized to accomplish sealing, but the stress is in the longitudinal direction. In the preferred arrangement, the sealing is accomplished at the ends of the plug instead of or in addition to sealing at the sides of the plug. In most cases it is desirable to employ both end sealing and side sealing, and both are employed in the unit of Figure 1 and 2. The diameter of the plug in relaxed condition are such as to embrace the inner tube 38 and to bear against the inner wall of the outer body with a significant force even without endwise compression of the plug. Assembling the compression section 30 with the main section 28 of the body has the effect of forcing the plug against the inner wall of the body and against the inner tube more tightly. Compression of the plug is limited to the region around its periphery in this case. Whatever the configur¬ ation of the surfaces hich press against the ends of the plug, some surface area at both ends must be left free to contact the test fluid and the conductive salt solution. In the preferred form the flange 32 and 34 are continuous whereby a continuous end sealing surface is provided around the margins of both ends of the plug. If neither end sealing surface is continuous, end compression must be sufficiently great to force the plug material laterally to accomplish sealing at the outer wall of the plug and at the inner wall around the opening which is numbered 54 in Figure 3. The compression section 30 and the sensor end of the remainder of the assembly are shown enlarged in OMPI Figures 4 and 5, respectively. The thread grooves 56 of the compression section are visible in Figure 4. They are engaged by the thread 58 of the reduced end portion 60 of the main body section 28. Except for the threads and thread grooves, and the scallops 18 of the- compression section,' both sections are symmetrical about their central longitudinal axis. The flanges 32 and 34 are continuous and extend inwardly only enough to engage the outer peripheral margin of the plug ends. As best seen in Figure 5, the plug is seated against the flange 34 at the right and it extends beyond the end of the end section 60 at the left. The right face of flange 32 being flat will bear against and compress the plug longitudinally when the compression section is screwed on to the main body. Other configurations can have the same result. The screw connection affords adequate mechanical advantage to permit tightening the compression section by hand. This unit is designed to permit easy removal and reassembly of the compression section 30 in the event that it is desired to clean the pH glass and the plug face. The lower half of Figure 2 is sectioned to make visible another feature of the invention. The surface area at the interface between the plug and the inner sensor body or tube 38 is much less than the area at the interface between the outer body an the outer surface of the plug. If a resilient plug which fits snugly on the inner body is compressed radially enough to accomplish sealing at the outer surface, more than enough force is applied to the inner body to seal the inner opening . However, in the invention the radial forces are usually less, especially if the plug is quite porous and resil¬ ient. In that case, while the renitence of the plug is still enough to affect the inner seal, it is preferred to augment that sealing force with a longitudinal force. That is done by enlarging the inner body or tube at the outer end of the plug and forcing the enlargement against the plug and into the outer end of the hole of the plug. In this case the pH glass forms a bulb 12 at the end of tub 38 whose diameter exceeds that of the tube 38. Inspection of Figure 5 shows that the bulb is pressed into the plug short distance. Returning to Figure 2, the bulb 12 is held in place part way into the plug by the tube 38 and the tube is held relative to the main body section 28 by the closure material 52. The longitudinal force of the bulb on the plug is transmitted to the flange 34 at the other end of the plu . The end result of that construction is an effec¬ tive seal against the flow or migration of fluid past the plug 14 notwithstanding that the plug is more resilient and more porous than the standard plugs of the past. The invention is not limited to the use of softer, more porous plugs, but it makes their use practical and a feature of the invention. The range of resilience and porosity when those qualities constitute a feature of the invention are: Porosity - - 25% to 60 %; and Resilience - 15 to 90 measured on the Shore A Dura eter scale. One of the consequences of plug sealing by longitudinal stressing is that non-cylindrical design is feasible. Other cross-sectional shapes, rectangular, tri¬ angular, and others, are entirely practical. An example of a rectangular electrode design with a mounting bracket formed integrally with the electrode body is shown in Figures 6, 7 and 8. This is a mono-electrode design for measuring potassium concentration. The plug 100 is resil¬ ient. It is porous, and it contains an ion transfer agent, phe ly nomycene, which is specific to potassium. It is produced in pads or sheets which are sliced into rec¬ tangular plugs, as shown in Figure 8, requiring no further shaping or processing. One surface 102 of the plug is exposed at the sensor end of the unit 104. The sensor body is divided into a compression section 106 and a main body section 108. The mounting bracket 110 is shown to be integrally formed with the main body section in Figure 6. The right end is closed by a cap 112 from which a coaxial cable 114 extends. The internal construction at the sensor end is shown in Figure 7. Not unlike the unit of Figures 1 through 5, the sensor end of the main body section has reduced outer dimensions at 116. The plug 100 is pressed into that reduced sized end section against an inwardly directed flange 118 which is continuous around the inner perimeter of the main body section. Longer than the distance from flange 118 to the sensor end of the main body section, the plug is squeezed in compression when the compression section 108 is telescoped on the reduced size section of the main body and forced into place. The com¬ pression section 106 and the main body section 108 are held in assembled condition with the plug compressed by co-acting locking conformatio s. In this case a latch projection, triangular in cross-section, fits into a complemen tally shaped keeper depression. As shown at 120, the keeper is formed in the compression section 106 and the latch element projects from the reduced end section of the main body. The body parts and the plug are distorted in shape to permit assembly, but return to the shape shown when the latch is seated in the keeper. The flange 122 at the sensor end of the compression section 106 bears on the periphery of the plug 100 at face 102 and forces it into compression against the flange 118 at its rear face 124. While certain preferred embodiments of the in¬ vention have be-en shown and described, it is to be understood that other embodiments and modifications of the invention are possible. Accordingly, the invention is to be limited only by the scope of the claims hereof. OMPI

Claims: The Claims We claim: 1. In an electrochemical measuring electrode of the kind in which a porous material is exposed at the tubular end of an electrolyte-filled electrode body, the improvement in wjαich the porous material occurs in an elongated elastomeric plug which plug is disposed in said tubular end with a surface exposed at the exterior of the body ; and plug retaining elements carried by the body and effective to compress the plug in the direction of its length and force sealing engagement between an en¬ compassing area of said plug with an inner circumferential surface area of said tubular end of the body. 2. The invention defined in Claim 1 in which the plug is formed with a central opening and which further comprises an inner tubular element disposed to extend through said opening substantially parallel with the axis of said tubular end of the body, said plug being compressed in the direction of its length such that an inner circumfere tial area of said plug is forced to sealing engagement with an outer peripheral area of said inner tubular element. 3. The invention defined in Claim 2 in which said inner tubular element has increased circumference at that portion of its length adjacent to and within that end of said plug which is exposed to the exterior of said body . 4. The invention defined in Claim 3 in which the inner tubular element is tapered to increased circum¬ ference at said portion of its length in the direction such that compression of said plug tends to urge said inner tubular member and said plug to relative axial movement . 5. The invention defined in Claim 4 which further comprises means for fixing said inner tubular member against such relative axial movement. 6. The invention defined in Claim 1 in which said plug retaining elements are effective to apply greater compressive force to outer peripheral portions of said plug than to inner portions of said plug. 7. An electrochemical chemical measuring element comprising: an electrode body; a resilient, porous plug disposed in one end of said body ; and means for sealing against passage of fluid between the exterior of the plug and the interior of the body by compressing the plug in the longitudinal direction of the body. 8. The invention defined in Claim 7 in which said body is divided into two portions each portion having an inwardly directed peripheral flange, said plug being compressed between said flanges. OMPI S WIPO ~~ 9. The invention defined in Claim 8 in which said body is tubular and which includes stop means in the form of a stop at the interior of said body for limiting the degree of insertion of the plug in said one end of said body, and a plug compressing means fixe to said body for compressing said plug between said plug compressing means and said stop. 10. The invention defined in Claim 9 in which said plug is both resilient in substantial degree and is porous. 11. The invention defined in Claim 10 in which said plug is formed with a through opening whose axis is parallel too the axis of said body, and which electrode further comprises a tubular member having an outer diameter no less than the inner diameter of said through opening of the plug when the plug is relaxed and said tubular member being disposed in said through opening. 12. The invention defined in Claim 11 in which said tubular member is formed with an enlarged diameter at one end, and in which a portion of said tubular member having enlarged diameter is disposed in the end of said through opening of the plug which is opposite said stop. 13. The invention defined in Claim 12 in which said compressing means is fixed to said body by being threadedly engaged therewith. OMPI

Inventors: BUKAMIER GARY L, RUPERT STEVEN L, BUKAMIER, GARY, L., RUPERT, STEVEN, L.
Applicants: BUKAMIER GARY L, RUPERT STEVEN L, BUKAMIER, GARY, L., RUPERT, STEVEN, L.
Application year: 1984
Main classification: G01N27
IPC codes: G01N27