Patent Description:
<CIT> discloses a planar electrochemical gas sensor is provided with at least one working electrode, at least one counterelectrode, at least one electrolyte-filled planar electrolyte carrier, at least one planar housing upper part and at least one planar housing lower part. The electrodes are arranged such that they are in two-dimensional contact with the electrolyte carrier. The housing upper part and the housing lower part are connected with one another such that the electrodes and the electrolyte carrier are pressed against one another in such a way that they are secured against displacement. The housing upper part and the housing lower part are partially in direct two-dimensional contact with one another, and the connection of the housing upper part and the housing lower part in the area of the direct two-dimensional contact is present at least along a closed figure, which surrounds the electrodes and the electrolyte carrier.

For a more complete understanding of the present invention, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.

It should be understood at the outset that although illustrative implementations of one or more embodiments are illustrated below, the disclosed systems and methods may be implemented using any number of techniques, whether currently known or not yet in existence. The invention should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, but may be modified within the scope of the appended claims.

The oxygen sensor described herein relies upon the principle of an oxygen pump. In this type of sensor, oxygen is reduced at the sensing electrode and water is oxidized at the counter electrode according to the following half reactions:.

At the sensing electrode: O<NUM> + <NUM>+ + 4e- → <NUM><NUM>O     (Eq. <NUM>).

At the counter electrode: <NUM><NUM>O→ O<NUM> + <NUM>+ + 4e-     (Eq. <NUM>).

The overall reaction results in the consumption of oxygen at the sensing electrode with an equivalent production of oxygen at the counter electrode. The overall reaction is maintained by means of a reference electrode and a potentiostat, which lower the potential at the sensing electrode and allow the reaction to proceed. The resulting current between the sensing electrode and the counter electrode is proportional to the oxygen concentration of the ambient gas. In contrast to others sensors, there is no consuming reaction in which the electrodes themselves are consumed.

The result of the reactions at each electrode is an oxygen concentration gradient in the electrolyte. The concentration of the dissolved oxygen in the electrolyte varies with the composition of the electrolyte, the temperature of the electrolyte, the atmospheric pressure, and the position relative to the sensing electrode and the counter electrode. The oxygen concentration at or near the sensing electrode may be around <NUM>%, while the oxygen concentration in the electrolyte at or near the counter electrode may correspond to a concentration close to or above the ambient gas concentration. Within this gradient, the dissolved oxygen and/or nitrogen concentration may exceed a saturated concentration due to a temperature rise. As the temperature rises above the saturation concentration, a gas phase comprising oxygen can form, and the resulting gas phase bubbles can travel to the sensing electrode where they may react. The resulting spike in the concentration value can result in a false alarm.

In order to control the oxygen concentration in the electrolyte surrounding the sensing electrode, the air/electrolyte interface can be controlled so that the closest interface is positioned a suitable distance away from the sensing electrode. Specifically, a low oxygen zone can be created around the sensing electrode that is substantially sealed off from the air/electrolyte interface. This zone may limit the oxygen introduction to the sensing electrode to that occurring through the wetted separator. In order to control the inlet oxygen diffusion rate, the relative geometric parameters of the separator can be adjusted along with the relative distances between the sensing electrode, the reference electrode, and the counter electrode so that a flux of the oxygen to the sensing electrode is controlled. This may provide an oxygen sensor having an improved resistance to spiking failures across a broad range of temperatures. A sealed area around the sensing electrode can be used to limit the amount of oxygen reaching the sensing electrode to a diffusional flow. The sealed area may limit or prevent oxygen in a gas from contacting the separator adjacent to the sensing electrode, which may help to avoid any spiking failures.

<FIG> illustrates a cross-section of an oxygen sensor <NUM>, and <FIG> illustrates an isometric view of the oxygen sensor of <FIG> with the claimed layout of the separator and electrodes illustrated. The oxygen sensor <NUM> can comprise a multi-part housing including at least a body <NUM> defining a hollow interior space <NUM> for receiving and retaining an electrolyte (e.g., forming an electrolyte reservoir), a base <NUM>, and a cap <NUM>. The base <NUM> and the cap <NUM> can sealingly engage the body <NUM> to form an integral unit.

The body <NUM> may have a generally rectangular or square shape, though other shapes such as cylindrical, oval, oblong, or the like are also possible. The body <NUM>, the cap <NUM>, and the base <NUM> can all be formed from materials that are inert to the selected electrolyte. For example, the body <NUM>, the cap <NUM>, and/or the base <NUM> can be formed from one or more plastic or polymeric materials. The body <NUM>, the cap <NUM>, and/or the base <NUM> can be formed from a material including, but not limited to, acrylonitrile butadiene styrene (ABS), polyphenylene oxide (PPO), polystyrene (PS), polypropylene (PP), polyethylene (PE) (e.g., high density polyethylene (HDPE)), polyphenylene ether (PPE), or any combination or blend thereof.

One or more openings can be formed through the body to allow the ambient gas to enter the interior space <NUM> and/or allow any gases generated within the housing to escape. The oxygen sensor <NUM> may comprise at least one inlet opening <NUM> to allow the ambient gas to enter the housing, and at least one exhaust opening <NUM> to allow the oxygen generated by the counter electrode <NUM><NUM> to exhaust from the housing. The inlet opening <NUM> and/or the exhaust opening <NUM> can be disposed in the cap <NUM> when a cap is present and/or in a wall of the body <NUM>. The inlet opening <NUM> and/or the exhaust opening <NUM> can comprise a diffusion barrier to restrict the flow of gas (e.g., oxygen) to the sensing electrode <NUM>. The diffusion barrier can be created by forming the inlet opening <NUM> and/or the exhaust opening <NUM> as a capillary and/or a film or membrane can be used to control the mass flow rate through one or more of the openings <NUM>, <NUM>.

The inlet opening <NUM> and/or the exhaust opening <NUM> may serve as capillary openings to provide a rate limited exchange of the gases between the interior and exterior of the housing. The inlet opening <NUM> may have a diameter between about <NUM> and about <NUM>, where the opening can be formed using a convention drill for larger openings and a laser drill for smaller openings. The inlet opening <NUM> may have a length between about <NUM> and about <NUM>, depending on the thickness of the cap <NUM>. The exhaust opening <NUM> may have a diameter and length in the same ranges as the inlet opening <NUM>. Two or more openings may be present for the inlet gases and/or the exhaust gases. When a membrane is used to control the gas flow into and/or out of the housing, the opening diameter may be larger than the sizes listed above as the film can contribute to and/or may be responsible for controlling the flow rate of the gases into and out of the housing.

A porous membrane <NUM> can also be disposed within the sensor <NUM>, a portion of which can serve as a vent membrane to allow any gas forming within the sensor to pass through the membrane and out the exhaust opening <NUM> in the cap <NUM> to the atmosphere and/or to control the flow of the ambient gases into the housing through the inlet opening <NUM>. The membrane <NUM> may be porous to a gas, but can generally form a barrier to the passage of any liquids such as the electrolyte solution in order to form a liquid seal relative to the outside environment. The porous membrane <NUM> can be formed from polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), polyethylene (PE), polypropylene (PP), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET) polyaryletheretherketone (PEEK), perfluoroalkoxy (PFA), ethylene chlorotrifluoroethylene (E-CTFE), and any combination thereof. The porous membrane <NUM> can cover the inlet opening <NUM> and/or the exhaust opening <NUM>. The flow rate of the gas can then be controlled by the relative permeability of the porous membrane <NUM> to selected gases.

A higher density, lower porosity bulk flow membrane <NUM> can be disposed within the cap <NUM> to serve as a barrier to the bulk flow of gases into the sensor <NUM>, which can impart tolerance to local environmental pressure changes. The opening <NUM> through the cap <NUM> and/or the bulk flow membrane may provide a restrictive and/or tortuous diffusional path to allow the gases in the atmosphere to pass into the sensor <NUM> to react with the electrodes and electrolyte solution at a flowrate that does not cause undesirable sensor response characteristics, which can manifest as significant increased responses over an extended period such as minutes to hours depending upon the magnitude of the bulk flow gas volume that diffused through the capillary.

The inlet opening <NUM> may provide an opening into the central space within the housing. The resulting incoming oxygen may contact the electrolyte, for example, within the separator <NUM>. The exhaust opening <NUM> can be disposed adjacent to the counter electrode <NUM> and can serve to allow the oxygen generated at the counter electrode <NUM> to escape from the housing so that the oxygen does not accumulate within the housing and create false readings by flowing to the sensing electrode <NUM>.

Within the electrochemical gas sensor <NUM>, a separator <NUM> may be disposed between the body <NUM> and the cap <NUM>. The separator <NUM> can comprise a porous member that acts as a wick for the retention and transport of the electrolyte between the reservoir and the electrodes. The separator can comprise a nonwoven porous material (e.g., a porous felt member), a woven porous material, a porous polymer (e.g., an open cell foam, a solid porous plastic, etc.), or the like, and is generally chemically inert with respect to the electrolyte and the materials forming the electrodes. The separator <NUM> can be formed from various materials that are substantially chemically inert to the electrolyte including, but not limited to, glass (e.g., a glass mat), polymer (plastic discs), ceramics, or the like.

The electrolyte can comprise any aqueous electrolyte such as a solution of a salt, an acid, or a base depending on the target gas of interest. The electrolyte can comprise a hygroscopic acid such as sulfuric acid for use in an oxygen sensor. Other target gases may use the same or different electrolyte compositions. In addition to aqueous based electrolytes, ion liquid electrolytes can also be used to detect certain gases. The electrolyte can be maintained within the sensor <NUM> to allow the reactions to occur at the sensing electrode <NUM> and the counter electrode <NUM>. The electrolyte can be a liquid that is maintained in the separator <NUM>, which acts as an absorbent medium to retain the electrolyte in contact with the electrodes. The electrolyte can be present in the form of a gel.

The electrodes <NUM>, <NUM>, <NUM> within the electrochemical gas sensor <NUM> can be electrically connected to an external circuit through one or more electrical connections. The electrodes may have connector pins <NUM>, <NUM> extending through the base <NUM> and/or the body <NUM> that can be electrically coupled, directly or indirectly, with the electrodes <NUM>, <NUM>, <NUM>. While not shown in <FIG>, the sensing electrode <NUM> can have a connector pin disposed through the base <NUM> to contact the sensing electrode <NUM>. The external surfaces of the connector pins <NUM>, <NUM> can be electrically coupled to an external circuit. The connector pins <NUM>, <NUM> may sealingly engage the base <NUM> and/or the body <NUM> so that the connector pins <NUM>, <NUM> are substantially sealed from the interior space <NUM> of the electrochemical gas sensor <NUM>.

The connector pins <NUM>, <NUM> can be formed from an electrically conductive material, which may be plated or coated. The connector pins <NUM>, <NUM> can be formed from brass, nickel, copper, or the like. The connector pins <NUM>, <NUM> can be coated to reduce degradation due to the contact with the electrolyte. For example, the connector pins <NUM>, <NUM> can include a coating of precious metal such as gold, platinum, silver, or the like, or other base metals such as tin.

The external circuitry can be used to detect a current between the sensing electrode <NUM> and the counter electrode <NUM> to determine the oxygen concentration in an ambient gas in contact with the sensor <NUM>. A potentiostatic circuit can be used to maintain the potential of the sensing electrode <NUM> at a predetermined value relative to the reference electrode <NUM> independently from the counter electrode, whose potential remains uncontrolled and limited only by the electro catalytic properties of the electrode. The potential of the sensing electrode <NUM> relative to the reference electrode <NUM> can be set at a value of between about -<NUM> and -<NUM> mV for the oxygen sensor <NUM>. The sensing electrode <NUM> and the reference electrode <NUM> may comprise platinum when the potential of the sensing electrode <NUM> relative to the reference electrode <NUM> is set at a value of between about -<NUM> and -<NUM> mV for the oxygen sensor <NUM>.

The electrodes generally allow for various reactions to take place to allow a current or potential to develop in response to the presence of a target gas such as oxygen. The resulting signal may then allow for the concentration of the target gas to be determined. The electrodes can comprise a reactive material suitable for carrying out a desired reaction. For example, the electrodes can be formed of a mixture of electrically conductive catalyst particles in a binder such as polytetrafluoroethylene (PTFE). For an oxygen sensor, the electrode can comprise carbon (e.g., graphite) and/or one or more metals or metal oxides such as copper, silver, gold, nickel, palladium, platinum, ruthenium, iridium and/or oxides of these metals. The catalyst used can be a pure metal powder, a metal powder combined with carbon, a metal powder supported on electrically conductive medium such as carbon, or a combination of two or more metal powders either as a blend or as an alloy. The materials used for the individual electrodes can be the same or different.

The electrode can also comprise a backing material or substrate such as a membrane to support the catalyst mixture. The backing material or substrate can comprise a porous material to provide gas access to the electrode through the substrate. The backing material may also be hydrophobic to prevent the electrolyte from escaping from the housing.

The electrodes can be made by mixing the desired catalyst with a hydrophobic binder such as a PTFE emulsion and depositing the mixture on the backing material. The electrodes might be deposited onto the substrate by for example screen printing, filtering in selected areas from a suspension placed onto the substrate, by spray coating, or any other method suitable for producing a patterned deposition of solid material. Deposition might be of a single material or of more than one material sequentially in layers, so as for example to vary the properties of the electrode material through its thickness or to add a second layer of increased electrical conductivity above or below the layer which is the main site of gas reaction.

The oxygen concentration at the sensing electrode <NUM> results from both the oxygen diffusing to the sensing electrode <NUM> through the separator <NUM> as well as oxygen entering the separator <NUM> as a result of a gas/separator <NUM> interface at any point along the separator <NUM>. When the gas can contact the electrolyte in the separator <NUM> adjacent to the sensing electrode, a substantial portion of the oxygen contacting the sensing electrode <NUM> can result from the contact between the gas and the electrolyte in the separator <NUM> adjacent to the sensing electrode <NUM>. This can result in a localized oxygen concentration in the electrolyte that exceeds the saturation concentration or solubility at certain temperatures. A temperature rise could then result in higher rate of gaseous oxygen diffusion to the active catalyst of the sensing electrode, as a direct result of gas evolution from the electrolyte and subsequent contact with the sensing electrode to create a transient "spike" in the output current.

In order to avoid the potential for spiking, the oxygen concentration at or near the sensing electrode <NUM> can be controlled to a level less than the threshold oxygen solubility at the operational temperature. In general, the dissolved oxygen concentration in the electrolyte at or near (e.g., within several millimeters) the sensing electrode <NUM> should be as close to zero as possible, thereby ensuring that the majority of the measured sensor response results from the controlled diffusion rate of gaseous oxygen through the capillary gas entry hole, rather than the less controlled, internal diffusion rate of dissolved oxygen through the separator. Limiting or preventing the internal diffusion rate of oxygen can improve the correlation between the sensor response and the gaseous oxygen concentration of the external environment. Ideally, the rate at which the oxygen reaches the sensing electrode <NUM> from the environment in which the oxygen is to be detected should be equal or close to the consumption rate of the oxygen at the sensing electrode <NUM>. Typically, the consumption rate of oxygen at the sensing electrode (e.g., the reduction rate) may be greater than the diffusion rate of gaseous oxygen to the sensing electrode <NUM> from the environment in which the oxygen is to be detected. When additional internal diffusion of dissolved and/or gaseous oxygen occurs from the electrolyte near the sensing electrode, the current generated in the circuit may correlate poorly with the external oxygen gas concentration.

The threshold may be a saturation concentration at a design temperature. For example, the threshold may be the oxygen saturation concentration in the electrolyte at the upper operating temperature specified for the sensor <NUM>. The threshold may be a percentage of the saturation concentration at a specified temperature. This may allow for a safety factor to be included in the design of the oxygen sensor. For example, the oxygen concentration may be controlled to less than about <NUM>%, less than about <NUM>%, or less than about <NUM>% of the saturation concentration at a specified temperature.

The oxygen concentration can be controlled in a number of ways including providing a spacing between the counter electrode <NUM> and the sensing electrode <NUM>, limiting a gas/electrolyte contact at or near the sensing electrode <NUM>, and/or selecting a geometry for the separator <NUM> retaining the electrolyte to limit the flux of oxygen to the sensing electrode <NUM><NUM> to a rate that is less than a consumption rate of oxygen at the sensing electrode <NUM> (e.g., an oxygen reduction rate at the sensing electrode <NUM>).

A dissolved oxygen concentration gradient can be established in response to the operation of the oxygen sensor between the counter electrode <NUM> and the sensing electrode <NUM> under normal operational conditions. The dissolved oxygen concentration gradient may generally be expected to represent a concentration either at, or approaching, the solubility limit of dissolved oxygen in the electrolyte surrounding the counter electrode <NUM> and either low, or approximately zero, concentration of dissolved oxygen in the electrolyte around the sensing electrode <NUM> when the oxygen sensor <NUM> is used in typical operational environments. Since the oxygen concentration in the electrolyte decreases towards the sensing electrode <NUM>, the creation of a suitable distance between the counter electrode and the sensing electrode <NUM> can be used to limit the concentration of dissolved oxygen in the electrolyte near the sensing electrode <NUM><NUM> to less than the threshold.

The separation distance can be provided by forming an ionically conductive pathway between the electrodes having a desired length. The sensing electrode <NUM>, the reference electrode <NUM>, and the counter electrode <NUM> are disposed on the ionically conductive pathway, with a distance separating each electrode. The ionically conductive pathway can extend in any direction within the housing in order to achieve a desired spacing. The resulting separation may include a labyrinthian configuration so that the ionically conductive pathway is not in a straight line, which would result in an oxygen sensor having relatively large dimensions.

As shown in <FIG>, the separator <NUM> comprises a planar configuration, and each of the electrodes <NUM>, <NUM>, <NUM> is disposed in a planar configuration in contact with the electrolyte retained in the separator <NUM>. As shown, the separator <NUM> forms an ionically conductive pathway, hereafter referred to as a "conductive pathway," by virtue of the electrolyte being retained in the separator <NUM>. The separator <NUM> extends in a plane within the housing between the counter electrode <NUM>, the reference electrode <NUM>, and the sensing electrode <NUM>. Each of the electrodes <NUM>, <NUM>, <NUM> are disposed in a substantially planar arrangement to contact the electrolyte in the planar separator <NUM>. The conduction pathway does not extend in a straight line within the housing, which allows the conduction pathway to attain the desired length or separation between the electrodes while maintaining a compact sensor design. While the conduction pathway does not extend in a straight line, a single conduction pathway is formed in which the sensors are arranged in series on the pathway. For example, the middle electrode is disposed between the two end electrodes, and a shorter path is not present between the end electrodes than between either of the end electrodes and the middle electrode.

As shown in <FIG>, the sensing electrode <NUM> and the counter electrode <NUM> are disposed at the ends of the conduction pathway, and the reference electrode <NUM> is disposed between the sensing electrode <NUM> and the counter electrode <NUM>. This configuration locates the reference electrode <NUM> at a position where it is subject to relatively steady concentration gradients of dissolved oxygen and ions diffusing through the electrolyte, which may reduce the current variations generated by the potentiostatic driver circuitry that manifest as measurement "noise". The steady concentration gradients across the reference electrode in this configuration may be considered a more robust design solution for producing a stable reference potential than the highly variable and fluctuating concentrations of protons and dissolved oxygen that might be generated across the reference electrode <NUM> when it is positioned outside of the potential gradient, which can result in increased noise in the circuit.

The distance between the electrodes <NUM>, <NUM>, <NUM> on the conduction pathway may affect the potential for spiking to occur. The distance along the conduction pathway (e.g., along a centerline of the conduction pathway) between the counter electrode <NUM> and the reference electrode <NUM> may be between about <NUM> and about <NUM> or between about <NUM> and about <NUM>, where the reference electrode <NUM> is disposed between the counter electrode <NUM> and the sensing electrode <NUM>. The distance (e.g., along a centerline of the conduction pathway) between the sensing electrode <NUM> and the counter electrode <NUM> may be between about <NUM> and about <NUM>, or between about <NUM> and about <NUM>. The relative ratio between the electrodes <NUM>, <NUM>, <NUM> may also affect the oxygen concentration gradient. A ratio of a distance between the counter electrode <NUM> and the reference electrode <NUM> to a distance between the counter electrode <NUM> and the sensing electrode <NUM> can be between about <NUM>:<NUM> and about <NUM>:<NUM>, or between about <NUM>:<NUM> and about <NUM>:<NUM>.

A diffusion barrier may be provided within the sensor to prevent the cross-diffusion of oxygen to the sensing electrode <NUM>. This configuration may allow the conduction pathway to place the sensing electrode <NUM> relatively close to the counter electrode <NUM> without any oxygen generated at the counter electrode <NUM> reaching the sensing electrode <NUM>. The use of a diffusion barrier may also allow the separator <NUM> to be positioned within a compact sensor while providing the separation needed to control the spiking type errors.

As shown in <FIG>, the diffusion barrier can be formed around the sensing electrode <NUM> while the reference electrode <NUM> and the counter electrode <NUM> are disposed outside of the area defined by the diffusion barrier. The diffusion barrier can comprise a seal formed between the cap <NUM> and the body <NUM>. The seal can comprise a shoulder <NUM> formed on the body <NUM> and/or the cap <NUM>, with a corresponding recess <NUM> or mating structure on the other component. A compliant material may be disposed on the shoulder <NUM> and/or the recess <NUM> to form a seal between the body <NUM> and the cap <NUM>. The shoulder <NUM> can be formed from the same material as the body <NUM> and/or the cap <NUM>, and may comprise an integral structure with the body <NUM> or the cap <NUM>. The compliant seal, when present, may comprise any impermeable materials that is inert with respect to the electrolyte and has a sufficiently low oxygen permeation rate to ensure a low dissolved oxygen concentration is maintained in the electrolyte.

When the cap <NUM> and the body <NUM> are enclosed, the shoulder <NUM> may contact the recess <NUM>, and any compliant seal may be positioned therebetween so that a seal is formed in the area around the sensing electrode <NUM>. A joining process (e.g., ultrasonic welding, etc.) can be used to fuse the two components to enhance the seal between the body <NUM> and the cap <NUM>. A similar barrier may be formed around the perimeter of the body <NUM> and the cap <NUM> as part of the manufacturing process for the sensor to seal the housing and prevent the electrolyte from leaking.

A small gap may exist between the shoulder <NUM> and the recess <NUM>, or at an edge of the joint between the shoulder <NUM> and the recess <NUM>. When the material of the shoulder <NUM> and the recess <NUM> are hydrophilic, a small amount of the electrolyte may be retained in the gap due to capillary action, resulting a layer of the electrolyte being maintained at the seal between the cap <NUM> and the body <NUM>. The relatively small gap size along with the electrolyte disposed in the gap may form a diffusion barrier that has a greater diffusional resistance than the separator <NUM>, thereby effectively limiting the diffusion of any oxygen through the chamber edge as compared to the diffusional path through the separator <NUM>.

When the body <NUM> and the cap <NUM> are engaged, a chamber <NUM> can be formed by an inner surface of the body <NUM>, an inner surface of the cap <NUM>, and the inner surface of the shoulder <NUM>. The edge seal may also define a surface of the chamber <NUM>. The chamber <NUM> has an opening <NUM> through which the separator <NUM> retaining the electrolyte extends. Within the chamber <NUM>, the separator <NUM> is positioned to maintain contact between the electrolyte and the sensing electrode <NUM>. The separator <NUM> retaining the electrolyte substantially fills the opening <NUM> so that any gas within the sensor <NUM> is substantially prevented from entering the chamber <NUM> through a convective flow.

When the separator <NUM> substantially fills the opening <NUM>, the oxygen reaching sensing electrode <NUM> may originate from oxygen diffusing through the electrolyte retained in the separator <NUM> from outside of the chamber <NUM>, for example, as resulting from a gas/electrolyte interface outside of the chamber <NUM>. The distance between the opening <NUM> and the sensing electrode <NUM> may be small compared to the conduction pathway length between the counter electrode <NUM> and the sensing electrode <NUM>. The portion of the separator <NUM> contained within the chamber <NUM> may be configured to provide an oxygen concentration within the electrolyte within the separator <NUM> corresponding to less than about a <NUM> %, less than about a <NUM>%, less than about a <NUM>%, less than about a <NUM>%, less than about a <NUM>%, or less than about a <NUM>% oxygen concentration at the sensing electrode <NUM>. The distance between the opening <NUM> and the sensing electrode <NUM> may be between about <NUM> and about <NUM>, or between about <NUM> and about <NUM>. A similar distance may be provided around the sensing electrode <NUM> within the chamber in the event that any oxygen is able to enter the chamber <NUM> through the seal.

The positioning of the sensing electrode <NUM> within the chamber <NUM> may limit the area of the gas/electrolyte interface and reduce or prevent a gas/electrolyte interface within the chamber <NUM>, which can limit the potential for creating a high oxygen concentration at or near the sensing electrode <NUM>. Rather, any oxygen diffusing to the sensing electrode <NUM> must diffuse through the electrolyte in the separator <NUM> over a short distance between the exterior of the chamber <NUM> and the sensing electrode <NUM> within the chamber <NUM>. The resulting zone of decreased oxygen concentration within the electrolyte may help limit the potential for the oxygen concentration to exceed a saturation concentration in the electrolyte within the chamber <NUM>. Any gas evolving due to a temperature rise may then be prevented from reaching the sensing electrode <NUM> except through the electrolyte in the separator <NUM>.

The geometry of the separator <NUM> may affect the flux of oxygen to the sensing electrode <NUM> through the electrolyte in the separator <NUM>, and the geometry can be selected so that the rate of oxygen diffusion to the sensing electrode <NUM> is less than a consumption rate of oxygen at the sensing electrode <NUM> (e.g., an oxygen reduction rate at the sensing electrode <NUM>). The thickness of the separator <NUM> (e.g., a distance perpendicular to the plane of the separator <NUM>) near the chamber <NUM> may be determined by the available distance between the cap <NUM> and the body <NUM> when the sensor is assembled. The separator <NUM> may contact both the cap <NUM> and the body <NUM>. The thickness of the separator <NUM> may be between about <NUM> and about <NUM>. The width of the separator <NUM> at the opening <NUM> may be based on a total area available for the diffusion of oxygen into the chamber <NUM> through the electrolyte retained in the pores of the separator <NUM>. In general, the area for diffusion (e.g., the product of the width times the thickness along with the porosity of the separator <NUM>) may affect the total amount of oxygen diffusing into the chamber <NUM> through the electrolyte to contact the sensing electrode <NUM>. The width of the separator <NUM> at the opening <NUM> may be between about <NUM> and about <NUM>, between about <NUM> and about <NUM>, or between about <NUM> and about <NUM>. The selection of the material for the separator <NUM>, the selection of the electrolyte and electrolyte concentration, and/or the desired oxygen detection range can affect the selection of the available area of the separator <NUM> at the opening <NUM>.

In use, the sensor <NUM> can detect an oxygen concentration of a gas in the environment in which the sensor <NUM> is disposed. Referring to <FIG> and <FIG>, the gas in the environment around the sensor <NUM> can enter the housing through an opening <NUM> so that the oxygen can be received within the housing. As described herein, the housing can comprise the counter electrode <NUM>, the reference electrode <NUM>, and the sensing electrode <NUM>. The oxygen can contact the separator <NUM> retaining the electrolyte. The separator <NUM> with the electrolyte retained therein forms the ionically conductive pathway between each of the electrodes <NUM>, <NUM>, <NUM>, which are disposed in a planar alignment. A potentiostatic circuit can be used to maintain a potential of the sensing electrode <NUM> lower than the reference electrode <NUM><NUM>, and as a result, the oxygen may begin to be reduced at the sensing electrode <NUM> while water is oxidized at the counter electrode <NUM>. The oxygen can thus be consumed at the sensing electrode <NUM> and regenerated at the counter electrode <NUM>. The oxygen generated at the counter electrode <NUM> can pass through a diffusional barrier and pass out of the sensor through an outlet <NUM>. An oxygen concentration gradient can then be formed in the electrolyte in the separator <NUM> between the counter electrode <NUM> and the sensing electrode <NUM>. A current can be developed based on the reaction of the oxygen and water at the sensing electrode <NUM> and the counter electrode <NUM>, which may allow the oxygen concentration in the gas contacting the separator <NUM> to be determined.

During the detection process, the oxygen concentration in the electrolyte at or near the sensing electrode <NUM> can be limited to less than a threshold amount. In general, the oxygen concentration in the electrolyte in the separator <NUM> can be limited to less than a saturation concentration at a predetermined temperature, and in some examples, the oxygen concentration in the electrolyte in the separator <NUM> can be limited to less than a percentage of a saturation concentration at a predetermined temperature. The length of the separator <NUM> and the distance between the counter electrode <NUM> and the sensing electrode <NUM> can be selected so that the oxygen concentration along the oxygen concentration gradient is below the threshold at or near the sensing electrode <NUM>. For example, the oxygen concentration along the oxygen gradient may be below the saturation concentration in the electrolyte at a predetermined temperature, or below a percentage of a saturation concentration at the predetermined temperature, within about <NUM>, within about <NUM>, within about <NUM>, within about <NUM>, or within about <NUM> of the sensing electrode.

The sensing electrode <NUM> is disposed within the chamber <NUM> formed within the housing. The separator <NUM> extends into the chamber <NUM> to provide contact between the electrolyte in the separator <NUM> and the sensing electrode <NUM>. The separator <NUM> is positioned within the chamber <NUM> and the opening <NUM> to prevent or limit any gas/electrolyte contact within chamber <NUM>. Some amount of gas/electrolyte contact may occur within the chamber <NUM>, but the gas may not be able to be exchanged with a gas outside of the chamber <NUM>, thereby limiting the potential for the formation of a high-oxygen concentration gas contacting the electrolyte in the separator <NUM> near to the sensing electrode <NUM>. The use of the chamber <NUM> may limit the rate at which the oxygen can diffuse to the sensing electrode <NUM> during the detection process and thereby limit the oxygen concentration along the oxygen gradient near the sensing electrode <NUM> to less than the threshold amount.

Claim 1:
An oxygen sensor (<NUM>) comprising:
a housing defining an interior space (<NUM>);
a sensing electrode (<NUM>);
a reference electrode (<NUM>);
a counter electrode (<NUM>);
a separator (<NUM>) extending in a plane within the housing between the sensing electrode (<NUM>), the reference electrode (<NUM>), and the counter electrode, wherein the sensing electrode (<NUM>), the reference electrode (<NUM>), and the counter electrode (<NUM>) are disposed in a planar arrangement in contact with an electrolyte retained by the separator (<NUM>), wherein the electrolyte provides an ionically conductive pathway between each of the sensing electrode (<NUM>), the reference electrode (<NUM>), and the counter electrode (<NUM>), within the housing, wherein the sensing electrode (<NUM>), the reference electrode (<NUM>), and the counter electrode (<NUM>) are arranged in series on the conductive pathway, with the sensing electrode (<NUM>) and the counter electrode (<NUM>) at the ends of the conductive pathway; and
a chamber (<NUM>) within the housing, wherein the chamber (<NUM>) contains the sensing electrode (<NUM>), wherein the chamber (<NUM>) comprises an opening (<NUM>), and wherein the separator (<NUM>) extends into the chamber (<NUM>) and substantially fills the opening (<NUM>);
characterized in that the conductive pathway does not extend in a straight line, and in that a shorter path is not present between the sensing electrode (<NUM>) and the counter electrode (<NUM>) than between either of the sensing electrode (<NUM>) and the counter electrode (<NUM>) and the reference electrode (<NUM>).