IL34959A - Oxygen detector for analysis of oxygen in gaseous streams - Google Patents

Description

Oxygen detector HYDRCOTATJTICS, JEG9 09 33213 SPECIFICATION This invention relates to oxygen detectors, and more particularly to new and improved galvanic cells for detecting trace amounts of oxygen in gases .

In the past, various devices have been proposed for detecting and measuring the quantity of oxygen in a fluid by its heat conductivity, heat of combustion, electrolytic conductivity, paramagnetism, and many others .

It is known, for example, that gaseous oxygen can be produced in an electrolytic cell by electrolysis of a suitable electrolyte, and that the amount of oxygen evolved is proportional to the amount of current applied to the cell. Similarly, it is known that oxygen can be reduced in a galvanic cell at one of the electrodes and generate a current proportional to the amount of oxygen reduced .

Utilizing this latter principle, many and varied cell configurations have been provided in which oxygen from a gas stream is distributed over an inert cathode of the cell while a base metal anode undergoes anodic oxidation to produce a current proportional to the amount of oxygen reduced at the cathode. In a galvanic cell containing an aqueous electrolyte, the following overall anodic oxidation, electrical -current generating reaction occurs, where M represents a base metal anode: 1/202 + M .+ H20 M(OH)2 + electrical energy The overall cell reaction is actually the result of subreactions that occur at both of the cell's electrodes . In these subreactions, an oxygen containing gas supplied to the cell reacts with the water of the electrolyte at the cathode accepting electrons generated at the anode and forming negatively charged hydroxyl ions . At the anode, positively charged metal ions are formed in solution by giving up electrons. The base metal hydroxide produced in the cell generally precipitates out of solution due to its low solubilit in the electrolyte.

The electrons generated at the anode travel to the cathode through an external circuit between the electrodes, and the amount of current passing through the circuit is proportional to the amount of oxygen reduced at the cathode. Measurement of the current generated, therefore, is indicative of the amount of oxygen in the gas .

While the anode is consumable and must eventually be replaced, an advantage of a galvanic detector is that the cell immediately begins to operat as soon as oxygen is introduced into the electrolyte without the need for applying an external electromotive force . Galvanic cells, however, generall have an unfavorable cathode area to volume of carrier gas ratio, requiring long gas residence times to insure complete reduction of the oxygen in the gas . If complete reduction does not occur, of course, the current produced in the cell will only be a fraction of the current that should have been produced and will not be a true measurement of the quantity of oxygen in the carrier gas .

Attempts to provide more intimate contact of the carrier gas with the cathode have included an electrode assembly comprising a single anode sandwiched between a porous electrolyte-cathode structure on either side, thus doubling the exposed cathode area in contact with the gas for more complete oxygen cathode diffusion and its subsequent reduction. The electrode assembly must be suspended within a suitable gas -confining housin so that both surfaces of the cathode will be exposed to the gas . Such an assembly, however, not only complicates construction of the cell but does fluctuations in the amount of current produced and leading to erratic results, particularly at high gas flow rates . Further, the suspended electrode assembly requires indirect electrical connections between the electrodes and an external measuring circuit, which increases the electrical resistance encountered and also leads to inaccurate results .

It is therefore a primary object of this invention to provide a new and improved galvanic oxygen detector that can measure trace amounts of oxygen in gases and provide stable and meaningful results at acceptable gas flow rates .

Yet another object of this invention is to provide a new and improved electrode assembly for a galvanic oxygen detector that provides a large ratio of cathode surface area to gas volume, that insures complete reduction of the oxygen content of the gas, and that provides more direct electrical connections between the external measuring circuit and the electrodes to minimize current losses through electrical resistance .

A further object of this invention is to provide a galvanic oxygen detector having internal humidifying means for maintaining proper humidity in the detector .

Yet a further object of this invention is to provide a stronger, more durable, and simplified oxygen detector that can be readily assembled, disassembled, and repaired and that is capable of operating under high pressures to improve oxygen cathode diffusion and its subsequent reduction.

Additional objects and advantages will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention, the objects and advantages being realized and attained by means of the apparatus, methods, and To achieve the foregoing objects and in accordance with its purpose, this invention as embodied and broadly described provides apparatus for detecting oxygen in a gas comprising: (a) a tubular housing having gas inlet and outlet means; (b) a tubular consumable anode incapable of evolving hydrogen upon being circuited with a cathode, said anode being concentrically mounted adjacent the inner surface of the housing; (c) a tubular, porous, and nonconductive electrolyte-retentive matrix concentrically mounted in intimate contact with the surface of the anode opposite to the surface adjacent the housing; (d) a tubular and porous cathode concentrically mounted in intimate contact with the surface of the electrolyte matrix opposite to the surface in contact with the anode, said tubular cathode defining a central passage for confining the flow of gas that passes through the housing over the inner surface of the cathode; and (e) means for electrically connecting the anode and cathode to an external electrical circuit for measuring the current across said ahode and cathode.

In accordance with preferred embodiments of this invention, the housing, anode, electrolyte matrix, and the cathode are concentrically mounted cylinders; the housing is of metal and serves as the anode terminal of the cell; and the gas and electrical connections to the cell are all provided in a single, readily removable and reusable cap for the housing . In some embodiments, internal humidifying means are provided for maintaining the proper humidity within the housing.

The accompanying drawings which are incorporated in and constitute invention, and together with the description serve to explain the principles of the invention.

Of the drawings : FIG. 1 is a longitudinal cross-section of a galvanic oxygen detector embodying the present invention; FIG . 2 is a broken sectional view taken along the line 2-2 of FIG . 1; FIG . 3 is a sectional view taken along the line 3-3 of FIG, 1; FIG. 4 is a longitudinal cross -section of an alternative embodiment of this invention; ' ^ FIG. 5 is a broken sectional view taken along the line 5-5 of FIG . 4; FIG . 6 is a longitudinal cross -section of yet another and simplified embodiment of this invention; the figure also illustrates schematically a valving mechanism suitable for use with the detectors of this invention; and FIG . 7 is a sectional view taken along the line 7-7 of FIG . 6.

Reference will now be made in detail to the present preferred embodiments of this invention, examples of which are illustrated in the accompanying drawings .

As shown in FIG . 1, the galvanic oxygen detector of this invention includes a tubular housing, generally indicated as 10, having gas inlet means 12 , gas outlet means 14, and an axial chamber 15. Preferably, tubular housing 10 is a cylindrical metal tube constructed of stainless steel, such as 316 stainless, nickel, Inconel, and a variety of nickel- chromium steels .

Housing 10 is capped at both ends with suitable pressure-sealed fittings to provide a pressure type housing while permitting ready access to its interior. As best shown in FIG. 1, the pressure-sealed fitting on the - over the end of the housing and threads into a retaining ring 17. A similar cap 18 and retaining ring 19 are provided on the top of housing 10.

Gas inlet means 12 includes an inlet tube 2 that extends through an axial aperture 22 in cap 16 and into the chamber 15 of housing 10. Inlet tube 24 is of stainless steel and is fillet welded to the outer surface of cap 16 at 26.

Cap 16 is provided with a dished-out inner recess that is filled with a resinous material 28, such as an epoxy resin, to eliminate all voids around inlet tube 24. Preferably, a nonconductive sleeve 30 of Teflon or other suitable plastic material is fitted over the interior stub of inlet tube 24 to insulate the metal tube from possible electrical contact with any of the other elements located within housing 10.

In accordance with the invention, means are provided for diffusing the inlet gas and distributing the gas molecules over the entire cathode surface. As embodied, the means comprises a microporous, sintered plug 32, that is force-fitted into the inner open end of sleeve 30 to cause the inlet gas stream to break up into many minute streams as it passes through the plug and diffuse rapidly through chamber 15 in housing 10. Suitable materials for plug 32 include a sintered 316 stainless steel sheet having an average pore size of lO^t,, a sintered polyethylene sheet, or any other commercial porous metallic or non-metallic material.

In accordance with the invention, a tubular electrode assembly, generally indicated as 40, and preferably cylindrical in shape, is concentrically mounted adjacent the inner surface of the lower portions of housing 10.

As embodied, electrode assembly 40 includes a cylindrical anode incapable of evolving hydrogen upon being circuited with a cathode . A choice of metal for anode 42, therefore, is restricted to those having a position sufficiently below hydrogen in the galvanic series (more electronegative) to provide an acceptable electromotive force and yet not electrolyze water into its gaseous components of hydrogen and oxygen when connected to a suitable cathode, which would interfere with the function of the detector. Rather, the anode should dissolve in the electrolyte of the cell to form cations upon being circuited with a cathode . Suitable metals for use in the construction of the anode include cadmium, lead, antimony, and bismuth, and the like.

Exemplary of an unsuitable metal for the anode is zinc because of its higher electrochemical activity which would cause it to reduce water to hydrogen when connected to a noble cathode such as graphite or carbon.

. Preferably, anode 42 is a reticulated porous substrate impregnated with one of the above metals . Exemplary of a preferred anode is a commercially available cadmium impregnated nickel screen. The screen is rolled into a cylinder with its outer diameter slightly larger than the inner diameter of housing 10 so that it can be force-fitted into the housing and be in electrical contact with the inner surface of the metal housing. To insure a good electrical connection between housing 10 and anode 42, the ends 44 of the anode are dipped in a 1 normal hydrochloric acid solution to dissolve the cadmium from the nickel screen. The exposed nickel screen ends are then tack welded to the inner surface of the metal housing .

Electrode assembly 40 also includes a tubular, porous, and non-conductive electrolyte retentive matrix 46 that is concentrically mounted in intimate contact with the inner surface of anode 42. As embodied, electrolyte matrix 46 is cylindrical and is tightly fitted against the inner on nonconductive porous materials for use as an electrolyte matrix include finely woven cloths of polypropylene, Teflon, polyethylene, pre -formed sintered cylinders of similar plastic materials, and the like . A preferred material for use as electrolyte matrix 46 is a nylon felt, available from the Pellon Corporation. The nylon felt is rolled into a spirally- wrapped, four-layered cylinder and force -fitted into the anode cylinder .

As further described below in connection with the operation of the present device, matrix 46 is impregnated with an aqueous electrolyte solution. Satisfactory electrolytes include alkali metal hydroxides, such as potassium or sodium hydroxide . A preferred electrolyte for use in the present device is a 20% aqueous solution of potassium hydroxide.

Finally, electrode assembly 40 includes a tubular, porous cathode 48 concentrically mounted in intimate contact with the inner surface of electrolyte matrix 46. Preferably, cathode 48 is also cylindrical and defines a central passage 50 for confining the entire gas flow over the exposed inner surface of the cathode, thereby permitting the gas to diffuse through the cathode and establish a gas interface with the electrolyte in matrix 46. As embodied, cathode 48 is a reticulated porous cylinder whose length is slightly less than electrolyte matrix 46.

Cathode 48 can be formed of a porous layer of a variety of inert metals, such as silver, gold, carbon, or the precious metals of Group 8 of the Periodic Table (i. e . , having an atomic number of at least 44), such as platinum. Preferably, cathode 48 is a graphite impregnated woven satin cloth. The cloth is spirally-wrapped into a four-layered cylinder that fits tightly against the inner surface of electrolyte matrix 46.

As shown in FIG . 1, a split U-ring 51, preferably of silver, is - a relatively rigid, porous material is used for electrolyte retentive matrix 46, a recess 54, as shown in FIG . 1, can be provided around the inner top of the matrix to provide clearance for crimping ring 51.

A plurality of retaining rings 56, suitably of nonconductive plastic materials, such as polyvinylchloride, polyethylene, Teflon, and the like, are spaced axially along the inner surface of cathode 48 to hold the cathode in intimate contact with electrolyte matrix 46.

An electrical lead 52 , preferably of silver, is swagged at one end to ring 51 and at the other end to a gas-tight, ceramic or glass insulated electrical feed-through connector 60 that passes through an epoxy resin seal 62 in the side of housing 10. Connector 60 thus serves as the cathode terminal for the cell. Surrounding connector 60 and welded to the outer surface of housing 10 is a threaded fitting 64 that serves as the anode terminal for the cell since the metal housing is in electrical contact with anode 42 .

A shorting resistor 66, generally from about 10 and 100 ohms, is connected between cathode connector 60 and housing 10 to short out excessive current loads on an external measuring circuit (not shown) .

Gas outlet means 14 for exhausting carrier gas after it has passed through housing 10 includes an outlet tube 67 that extends through top cap 18 and communicates with the housing's chamber 15. Preferably, gas outlet tube 67 is horizontal, as shown in FIG . 1, so that it does not interfere with the servicing of the humidifying means, as more fully described below.

In accordance with the invention, internal humidifying means are provided for maintaining the proper humidity in the cell, and more particularly to prevent a drying out of the electrolyte . As embodied, and as shown in FIG . 1 the humidif in means enerall indicated as 68 includes an external transparent tube 69 that extends axially through an opening 70 in the top of cap 18 and into housing chamber 15. Tube 69 serves as a sight glass and is sealed into position by an epoxy seal 72 and an O-ring 74, retained in corresponding recesses in cap 18. A very fine porous glass tube 76 is fitted to the internal end of sight glass 69 and extends axially through housing chamber 15 into central passage 50, terminating a slight distance above plug 32 in gas inlet tube 24. The external end of sight glass tube 69 is fitted with a retaining ring 80 and a corresponding screw-type cap 82 containing a piercable elastometric seal 84. Seal 84 can be of butyl rubber or any other soft inert material that permits periodic injection of a humidifying solution into porous tube 76. , A thin vent tube 86 of polyethylene or other inert plastic is located axially within porous glass tube 76 and is held in place by an annular plug 88 in the bottom of tube 76. Vent tube 86 is open at both ends, the top being located adjacent plug 84 and the bottom adjacent the diffuser plug 32 in gas inlet tube 24. - As shown in the drawings, the axial construction of humidifier 68, in accordance with this invention, not only provides an internal means for maintaining proper humidity control in the cell, but it further confines the flow of gas through central passage 50, thereby increasing the ratio of cathod surface area to gas volume and insuring more complete reduction of the oxygen content of the gas .

In assembly, electrode assembly 40 is placed within housing 10 in the manner described above and the housing, without end caps 16 or 18, is placed in a suitable closed container and the container is evacuated . The container is then slowly filled with an aqueous electrolyte solution, such as a 20% potassium hydroxide, to impregnate electrolyte matrix 46 with the 0 electrolyte. The vacuum in the container is then released by admitting air and the entire procedure is repeated to insure complete impregnation of the electrolyte matrix. The cell should then be tested in air by a suitable meter shorted through a resistor to produce a low impedance of about 10 to 100 ohms . A current of about 100 ^ta is indicative of a good cell. If a lower output is obtained, impregnation steps should be repeated .

An insulating washer 90 is placed in the lower end of housing 10 to prevent cap 16 from shorting the ends of the electrodes, and the cap is then assembled into retaining ring 17.

Porous glass tube 76 of humidifier 68 is filled with a 20% potassium hydroxide solution (oxygen free) being careful to fill around and up to just below the level of vent tube 86. Upper cap 18 containing humidifier 68 is then assembled into retaining ring 19 to provide a gas-tight seal at the upper end of housing 10.

In operation, and as best shown by reference to the drawings, an oxygen containing gas is fed to housing 10 through inlet tube 24. The gas is dispersed through porous plug 32 and passes into central passage 50 and into contact with the inner surface of porous cathode 48. The flow of gas through chamber 50 and over cathode 48 is confined to a thin, wide sheet by the outer surface of axial tube 76 of humidifying means 68, thus increasing oxygen cathode contact and its subsequent reduction.

As the oxygen containing gas flows upwardly through passage 50 in the direction of arrows 96 under the pressure of the inlet gas, it diffuses through porous cathode 48 and comes into contact with the electrolyte, which is dispersed throughout matrix 46 , forming a gas -electrolyte interface within cathode 48. The oxygen content of the gas is reduced at the cathode by h droxyl ions are transported by the electrolyte to anode 42 where they oxidize the base metal of the anode and produce the electrical output of the detector.

External measuring means, not shown, are provided for measuring the electrical output. Thus, the amount of current flowing in the external circuit is proportional to the amount of oxygen consumed at cathode 48, and, therefore, can be used to measure the oxygen content of the gas .

Since accurate results are dependent upon complete reduction of all the gases 1 oxygen content, a relatively large cathode surface area to gas volume is provided in the device of this invention. Further, the inlet gas can be pressurized to insure more complete oxygen cathode diffusion and its subsequent reduction.

The oxygen depleted gas passes out of central passage 50 into the upper end of housing chamber 15 and out of the housing through outlet tube 67.

If a dry gas is passed through the cell and absorbs water from the electrolyte, the difference in vapor pressures thereby established between the electrolyte and the saturated condition of humidifier 68 causes porous tube 76 to sweat water vapor and restore the proper humidity in the cell.

Thus, the fine pores of tube 76, which permit only the penetration of water vapor and not water molecules, provides an internal means for maintaining humidity control without flooding the electrolyte with excess moisture .

Vent tube 86 is provided to equalize the pressures between housing chamber 50 and the interior of tube 76 and thereby permits the water vapor to pass through the pores of the tube . Sight glass 69 indicates when refilling of humidifier tube 76 is required . To refill, a hypodermic syringe is pierced through plug 84 and the necessary quantities of potassium hydroxide solution While it is preferred to use an aqueous electrolyte solution in the humidifier column, so that the vapor pressures of the electrolyte and the humidifier are the same, oxygen-free water can also be used in lieu of the electrolyte solution.

In accordance with an alternative embodiment of this invention, and as shown in FIG . 4, an impervious ring 92 is wedged between the outer surface of porous glass tube 76 and the top of cathode 48. Ring 92 blocks the outlet end of central passage 50 to force all of the gas to pass through porous cathode 48 and thereby insures a more complete reduction of its oxygen content.

As further shown in FIG . 4, capillary rings 94 can be provided between the outer surface of porous tube 76 and cathode 48 to assist in the transfer of water from the humidifier to electrolyte matrix 48 to prevent a drying out of the matrix.

In accordance with a simplified embodiment of this invention, and as shown in FIGS. 6 and 7, housing 10 comprises a deep drawn cylindrical metal tube having a closed end 100 and an open end 102. A cap 104 fits over the open end 102 of the housing and includes an O-ring seal 106 that snap fits into channel 108 in the outer side walls of housing 10 to provide a sealed pressure type housing while permitting access to its interior.

All gas and electrical connections are provided in cap 104 to simplify construction, assembly, and repair of the detector . Hence, as embodied and as shown in FIG . 6, a gas inlet tube 110 and a gas outlet tube 112 are both secured to and extend through cap 104 into the interior of housing 10. The inner recess of cap 104 is filled with a sealing and insulating plug 114 to prevent leakage of gas around the tubes 110 and 112 and to revent the ca from shortin the ends of the electrodes . A similar insulating plug 116 is inserted into the closed end 100 of tube 10 to prevent shorting the other ends of the electrodes.

The tubular electrode assembly 40 of this embodiment is substantially similar to the assembly described in the previous embodiments and includes a cylindrical anode 42, a cylindrical and nonconductive electrolyte-retentive matrix 46, and a cylindrical porous cathode 48 that defines a central passage 50 for confining the flow of gas through housing 10.

An electrical lead 118, preferably of silver, is threaded through porous cathode 48 and passes out through plug 114 in cap 104 where it is connected to a terminal connector 60 that serves as the cathode terminal for the cell. Fitting 64, welded to cap 104, serves as the anode terminal as in the previous embodiments .

In this embodiment of the invention, an impervious cylindrical core 120 extends substantially through the entire length of central passage 50 in a manner similar to humidifier tube 76. Core 120 is closely spaced from the inner surface of porous cathode 48 and thus confines the flow of gas in passage 50 to insure more complete reduction of its oxygen content.

Preferably, the outer surface of core 120 is fluted to center it within the cathode and provide a plurality of passages for the flow of the gas .

In accordance with this invention, an axial conduit 122 is provided in core 120 for exhausting the gas after it has passed over cathode 48. As shown in FIG . 6, conduit 122 communicates with passage 50 through ports 124 in the upper end of core 120 and with gas outlet tube 112 at the lower end of core 120.

Operation of the embodiment of FIGS. 6 and 7 is substantially simil to the previous embodiments, gas entering housing 10 through inlet tube 110 surface of cathode 48 and forms an interface with the electrolyte in matrix 46. The oxygen depleted gas passes out of passage 50 and into conduit 122 in core 120 and then out of the housing through gas outlet tube 112.

The oxygen detector of this embodiment is contemplated for use with an external humidifier so that it can be kept compact and as simplified as possible, but it will be apparent that the device can easily be modified to accept internal humidifying means, if desired, without departing from the scope of this invention.

Preferably, and as schematically illustrated in connection with the embodiment of FIG . 6, a four-way valve 130 is used to control the flow of gas to the oxygen detectors . The valve, of course, can be used with any of the embodiments of this invention, but for convenience will be described as it relates to the embodiment of FIG. 6.

Valve 130 schematically includes a valve block 132, a sample gas inlet 138, a sample gas outlet 140, a vent 142, and passages 134 and 136 for connecting the valves ' inlet and outlet to the inlet and outlet means 12 and 14, respectively, of the oxygen detector. Thus, with valve block 132 in the extreme left or "OFF" position, the gas bypasses the detector and the inlet and outlet means of the cell are closed by the valve block to prevent the cell from being contaminated by ambient air.

To take a sample of the gas for analysis, valve . block 132 is simply moved to the extreme right or "ON" position, as shown in FIG . 6, so that the gas now passes through the cell where its oxygen content is measured as a function of the generated current . After a meaningful reading is obtained, the detector is then disconnected by moving the valve back to the "OFF" position to prevent unnecessary waste of the consumable metal anode .

* Thus, it will be apparent from the foregoing description that this invention provides a unique concentric construction for the electrode assembly of a galvanic oxygen detector that provides a high cathode surface area to gas volume ratio for maximum galvanic efficiency and a smooth gas flow for stable current outputs, leading to more constant and reliable results . Further, the concentric construction of the electrode assembly, and particularly in combination with the compact construction of the housing as shown in FIG . 6, permits the device to be readily assembled and disassembled for inspection, maintenance, replacement of parts, and repair.

By providing an outer metallic housing for the detector in accordanc with the invention, the housing can be used directly as the external electrical connection for the anode of the cell to minimize current losses through electrical resistance and, in combination with the gas-tight end seals, provides a pressurizable container for improving oxygen cathode diffusion.

In addition, the unique internal humidifier of this invention not only prevents a drying out of the electrolyte by maintaining a constant humidity within the cell, but its axial location confines the flow of gas, without disturbing its flow characteristics, to increase the cathode surface area to gas volume ratio "and insure more rapid and complete oxygen cathode diffusion. · The unique construction of the internal humidifier of this invention further permits monitoring and replenishing of the humidifying solution without disassembly of the detector device .

The invention in its broader aspects is not limited to the specific details shown and described and departures may be made from such details without departing from the principles of the invention and without sacrificing 34959/2

Claims (23)

1. Apparatus for detecting oxygen in a gas comprising* a) a tubular housing having gas inlet and outlet means; b) a tubular consumable anode incapable of evolving hydrogen upon being circuited with a cathode9 said anode being concentrically mounted adjacent the inner surface of the housing; c) a tubular porous and nonconductive electrolyte-retentive matrix concentrically mounted in intimate contact with the surface of the anode opposite to the surface adjacent the housings d) a tubular and porous reticulated cathode concentrically mounted in intimate contact with the surface of the electrolyte matrix opposite to the surface in contact with the anode, said tubular cathode defining a central passage for confining the gas flow through the housing over the inner surface of the cathode; e) a core impervious to the passage of gas located within the central passage in the tubular cathode to confine the flow of gas passing over the cathode into a thin tubular sheets and f) means for electrically connecting the anode and the cathode to an external electrical circuit for measuring the current across said anode and cathode.

2. · She apparatus of Claim lp in which the tubular housing is metallic and in electrical contact with ¾ie anode. 34959/2 ^ - 19 -

3. The apparatus of Claim 2, in which the tubular housing is of stainless steel.

4. The apparatus of any of the preceding claims, in which the housing, the anode, the electrolyte matrix, and the cathode are concentrically mounted cylinders.

5. The apparatus of any of the preceding claims, in which the anode contains an oxidizable metal selected from the group consisting of lead,, antimon 9 bismuth, cadmium, and mixtures thereof.

6. The apparatus of Claim 5, in which the anode comprises a cadmium impregnated nickel screen.

7. The apparatus of any of the preceding claims, in which the cathode contains an inert electrically conductive material selected from the group consisting of silve , goldP carbon, and elements of Group Θ of the Periodic Table having an tomic number of at least 44·

8. The apparatus of any of the preceding claims, in which the cathode is a graphite~impregnated cloth.

9. The apparatus of any of the preceding claims, in which the electrolyte matrix is a thin sheet of porous polymeric material.

10. The apparatus of Claim 9, in which the matrix is a nylon felt. 34959/2 - 20 -

11. The apparatus of any of the preceding claims,, in which the electrolyte is an aqueous solution of an alkali metal hydroxide,

12. The apparatus of Claim 11, in which the electrolyte is a 20 percent aqueous solution of potassium hydroxide.

13. The apparatus of any of the preceding claims, in which the gas inlet and outlet means are located at opposite ends of the tubular housing.

14. The apparatus of any of Claims 1 to 12, wherein the gas Inlet and outlet means are located at the same end of the housing, and the impervious core extends through the central passage in the tubular cathode from one end of the housing to the other and has an axial conduit communicating with the central passage opposite from said end of the housing so that the gas enters and passes over the cathode in one direction and then flows back through the axial conduit in the opposite direction to be exhausted through the outlet means.

15. The apparatus of Claim 14, in which the tubular housing is metallic and in electrical contact with the anode.

16. The apparatus of Claim 14, in which the housing is cup-shaped and the anode, matrix, and cathode are concentrically mounted cylinders within the housing, said gas inlet and outlet means teing located in a removable cap for the housing.

17. The apparatus of any of the preceding claims, including internal humidifying means for maintaining the proper humidity 34959/2 - 21 - within the tubular housing and preventing a drying out of the electrolyte.

18. The apparatus of Claim 17» in which the impervious core is the humidifying means and comprises a fine porous glass tube for confining a quantity of an aqueous solution, the tube permitting water vapor to pass through the pores of the tube and into the housing when necessary to maintain proper humidity in the housing·

19. T e apparatus of Claim 13, including means permitting refilling of the porous glass tube with the aqueous solution externally of the housing.

20. The apparatus of any cf the preceding claims including an impervious ring wedged between the outer surface of the impervious core and the top of the cathode to &rce all of the gas to pass through the cathode at its top.

21. The apparatus of Claim 1β wherein the outer s rface of the core is fluted to center it within the cathode and provide a plurality of passages for the flow of the gas.

22. The apparatus of Claim 18, including at least one capillary ring between the outer surface of the porous glass tube and the cathode to assist in the transfer of water from the humidifier to the electrolyte matrix to prevent a drying out of the matrix. ■2-3T> Apparatuo for dotootlng oxygon in a gao comprisiags 34959/3 - 22 -

23. Apparatus for detecting oxygen in a gas substantially as described above by w y of example with reference to the accompanying drawing. 24·: An internal humidifier for use in an oxygen detector cell according to Claim IT containing aa aqueous electrolyte-impregnated, matrix, said humidifier comprising a fine porous glass tube extending into the interior of the cell and in contac with the gas passing through the cell for confining a quantity of an aqueous solution and permitting water vapo to pass through the pores of the tube and into the matrix when necessary to maintain proper humidit in the cell. 34959/2 - 23 - 2ξ An internal humidifier for an oxygen detector cell substantially as described above by way of example v/ith reference to the accompanying drawing. For the Applicants DR.REIMOLD COHN AM) PARTNERS SG:BH