ELECTROCHEMICAL SENSOR INCLUDING A MEASURING CELL AND AN OXIDATION COMPONENT AND PROCESS USING SUCH A SENSOR

An electrochemical sensor (100) is capable of detecting a gas component in a gas (G). A process uses such a sensor (100). The gas (G) flows through an inlet (Ö) on the measuring cell side in a housing (10) to a measuring cell (2, 4, 12, 15). A chemical reaction takes place at the measuring cell (2, 4, 12, 15), which depends on the concentration of the gas component and influences a measurable electrical quantity. An electrical measuring unit (15) measures this electrical quantity. Gas can also pass through a pressure equalizing outlet (16) into the housing (10). An oxidation component (6) between the pressure equalizing outlet (16) and the measuring cell (2, 4, 12, 15) oxidizes an oxidizable gas component in this gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 of German Application 10 2022 111 351.5, filed May 6, 2022, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The invention relates to an electrochemical sensor with a measuring cell, wherein the measuring cell is capable of detecting a predetermined gas component in a gas and/or is for measuring the concentration of the predetermined gas component, for example oxygen, in the gas. Furthermore, the invention relates to a process in which such a sensor is used.

BACKGROUND

EP 2002251 B1 describes an electrochemical oxygen sensor1. A housing12encloses a chamber in which a measuring cell with a measuring electrode6, a counter electrode5, a reference electrode9and an electrolyte is arranged. An impermeable insulating element11is arranged between the measuring electrode6and the counter electrode5. A gas to be analyzed for oxygen can reach the measuring electrode6through a first opening2. The gas can escape from the chamber again through a second opening3. Both openings2and3are arranged on the same side of the housing12.

SUMMARY

An object of the invention is to provide an electrochemical sensor and a process which are capable of detecting a predetermined gas component in a gas and/or of measuring the concentration of the gas component in the gas and which are less sensitive to environmental influences than known electrochemical sensors and processes.

The object is achieved by an electrochemical sensor having sensor features according to the invention and by a process having process features according to the invention. Advantageous embodiments of the electrochemical sensor according to the invention are, as far as useful, also advantageous embodiments of the process according to the invention and vice versa.

The electrochemical sensor according to the invention comprises a housing. This housing encloses an interior space. A measuring cell side inlet is able to establish at least temporarily a fluid communication (fluid connection) between the interior space and an environment of the electrochemical sensor. Furthermore, the interior space is at least temporarily in fluid communication (fluidically connected) with an environment via a pressure equalization outlet.

The electrochemical sensor according to the invention further comprises a measuring cell. The measuring cell is arranged between the measuring cell side inlet and the pressure equalization outlet. A gas to be analyzed may pass through the measuring cell side inlet to the measuring cell. This gas may have at least one predetermined gas component. This gas component is to be detected and is often also referred to as the target gas.

The measuring cell includesa measuring electrode,a counter electrode, anda layer between the two electrodes.

On the one hand, the layer between the two electrodes of the measuring cell is electrically insulating and, in particular, prevents a short circuit between the two electrodes. On the other hand, the layer is ionically conductive, thus allowing ions to flow from one electrode to the other.

A chemical reaction takes place or can take place at or in the measuring cell. This chemical reaction depends on the concentration of the gas component to be detected in the interior. If several gas components to be detected are present in the gas to be analyzed, the chemical reaction usually depends on the summed concentrations of these gas components.

The chemical reaction influences a measurable electrical quantity, for example the current intensity or the electrical charge (summed current intensity) or the electrical voltage. The electrochemical sensor according to the invention further comprises an electrical measuring unit. The electrical measuring unit is capable of measuring an indicator of this influenced or influenceable electrical quantity. An “electrochemical sensor” is understood to be a sensor with such a measuring cell.

Furthermore, the electrochemical sensor comprises an oxidation component. This oxidation component is arranged between the pressure equalization outlet and the measuring cell. A spatial distance occurs between the measuring cell and the oxidation component. The oxidation component is electrically conductive and electrically insulated from the measuring cell. Preferably, at least a portion or part of the oxidation component, for example an electrode, is electrically conductive. Preferably, the oxidation component is ionically conductively connected to the measuring cell.

The oxidation component is capable of oxidizing, ideally completely oxidizing, at least one, preferably every oxidizable (combustible) gas component in the gas, whereby this gas component flows or has flowed through the pressure equalization outlet into the interior, e.g. as part of the gas to be analyzed. Because the gas component is oxidized, it no longer reaches the measuring cell.

The process according to the invention is carried out using an electrochemical sensor according to the invention and comprises the following steps:A sample of the gas to be investigated flows through the measuring cell side inlet into the interior, in particular by diffusion and/or suction, and reaches the measuring cell.A chemical reaction takes place at and/or in the measuring cell. This chemical reaction depends on the concentration of the gas component to be detected in the interior. The chemical reaction influences a measurable electrical variable.The electrical measurement unit measures an indicator of this electrical quantity.It is possible for gas to pass through the pressure equalization outlet into the interior. The oxidation component oxidizes at least one, preferably any, oxidizable gas component, the or each oxidized gas component being part of the gas that passes through the pressure equalization outlet into the interior space.

The measuring unit of the measuring cell measures an indicator of the electrical quantity which is influenced by the chemical reaction in the measuring cell. This chemical reaction is caused and/or maintained by at least one gas component to be detected, which may be present in the gas in the environment to be tested. As a rule, no component of the sensor, in particular no component of the electrical measuring unit, is significantly consumed or used up in the course of an operation. Therefore, it is often not necessary to monitor the level of a substance and to refill this substance if necessary.

In many cases, the measuring cell consumes less electrical energy than a conceivable other design of an electrochemical sensor and as an optional signal processing evaluation unit and an optional display unit. Often, the measuring cell and the measuring unit consume no electrical energy at all, as long as there is no target gas to be detected. The advantage of low energy consumption is particularly relevant when the sensor according to the invention is used as a portable device and comprises its own power supply unit.

Gas to be analyzed can diffuse or otherwise flow through the measuring cell side inlet into the interior and there to the measuring cell, for example be sucked in. The chemical reaction at the measuring cell can lead to an overpressure in the interior. Thanks to the pressure equalization outlet, this overpressure can be relieved into the environment. As a rule, no fluid conveying unit is required to relieve this overpressure.

However, especially if there is no overpressure in the interior, gas to be analyzed or other gas can also pass through the pressure equalization outlet into the interior. When this gas reaches the measuring cell, i.e. from the other side, it can falsify a measurement of the measuring cell. A major reason for a possible falsification is that the gas changes a reference voltage of the measuring electrode, whereby the influenced electrical quantity depends on this reference voltage. This reference voltage would be distorted in particular if the reference voltage is determined by an optional reference electrode and the gas reaches the reference electrode through the pressure equalization outlet. In particular for this reason, gas under investigation should not reach the measuring cell through the pressure equalization outlet, but only through the measuring cell side inlet. In addition, gas entering the interior through the pressure equalization outlet could lead to unwanted deposits on an electrode of the measuring cell.

The oxidation component according to the invention creates a solution for this problem. The oxidation component is arranged in a fluid communication between the pressure equalization outlet and the measuring cell and oxidizes at least one, preferably all oxidizable components in a gas which flows through the pressure equalization outlet into the interior. In this way, the oxidation component completely or at least largely prevents an oxidizable gas component from reaching the measuring cell from the pressure equalization outlet. On the other hand, the oxidation component ideally has no influence on a gas that flows through the measuring cell side inlet into the interior and reaches the measuring cell.

It is possible, but thanks to the oxidation component not necessary, to close the pressure equalization outlet with a gas-impermeable closure as long as there is no overpressure in the interior, and to remove this closure only to relieve an overpressure. Such a closure could inadvertently be on the pressure equalization outlet even when an overpressure is present and should be relieved. It is possible to close the pressure equalization outlet with a gas-permeable closure, whereby overpressure can be relieved through this closure and whereby this closure protects the interior to a certain degree from mechanical damage from the outside. It is possible to close the pressure equalization outlet with a gas-impermeable closure when the electrochemical sensor is not in use.

According to the invention, the oxidation component is arranged between the pressure equalization outlet and the measuring cell. Because the oxidation component oxidizes oxidizable gas components and is arranged at a distance from the measuring cell, there is less risk of an oxidizable gas component changing a reference voltage of the measuring electrode and thereby falsifying a measurement result, compared to an arrangement without an oxidation component and/or without a distance. The distance makes it possible to arrange at least one barrier layer between the oxidation component and the measuring cell.

A sufficiently high electrical potential can be applied to the oxidation component. This high electrical potential increases the certainty that all oxidizable components in the interior are actually oxidized such that no relevant amount of an oxidizable component can reach the measuring cell. Because the oxidation component works electrically, it is not consumed or at least is consumed and/or used-up more slowly than other possible components which are capable of oxidizing a gas. In particular, it is possible that the oxidation component does not need to have a chemical substance, which is consumed in the course of use.

Because the oxidation component is preferably ionically conductively connected to the measuring cell, the measuring cell is able to determine (fix) an electrical reference potential and/or an electrical reference voltage of the oxidation component.

The oxidation component preferably has an electrical potential that is above the electrical potential of the measuring electrode. This embodiment reliably and quickly prevents an oxidizable gas component, in particular organic vapors, from passing through the pressure equalization outlet to the measuring cell. Instead, this gas component is reliably oxidized by the oxidation component.

In particular, when the sensor according to the invention is used in ambient air, the measuring cell can heat up considerably and could even be damaged if a lot of oxygen reaches the measuring cell. As is known, ambient air has an oxygen content of about 20%. In a preferred embodiment, a volume flow reducer is therefore arranged between the measuring cell side inlet and the measuring cell. This volume flow reducer is preferably a mechanical filter and very strongly reduces the volume flow of a gas to the measuring cell, which gas flows through the measuring cell side inlet into the interior, compared to a condition in which no volume flow reducer is arranged between the measuring cell side inlet and the measuring cell. “Very strongly” means: by at least 95%, preferably by at least 99%, particularly preferably by at least 99.9%, especially by at least 99.99%.

The volume flow reducer preferably comprises at least one barrier layer. This barrier layer preferably has a thickness of less than 0.1 mm, particularly preferably less than 20 μm. Thanks to the oxidation component, no such volume flow reducer needs to be arranged between the pressure equalization outlet and the measuring cell. Therefore, a possible overpressure in the interior is reduced faster than if the volume flow through the pressure equalization outlet would also be reduced by a further volume flow reducer.

In a preferred embodiment, the electrochemical sensor comprises a reference electrode. This reference electrode can be denoted as a component of the measuring cell. This reference electrode has a constant electrical potential and thereby determines (fixes) the electrical reference voltage of the measuring electrode. Preferably, the reference electrode is ionically conductively connected to the measuring electrode and thereby determines the electrical potential of the measuring electrode.

Preferably, the oxidation component is located between the reference electrode and the pressure equalization outlet. Particularly preferably, a distance occurs between the reference electrode and the oxidation component. This positioning further reduces the risk of a relevant amount of a combustible gas component reaching the reference electrode through the pressure equalization outlet and changing the reference voltage in an undesirable manner. The spacing allows at least one barrier layer to be provided.

The oxidation component preferably comprises at least one electrode, preferably two electrodes. Preferably, the oxidation component is not only ionically conductively connected to the measuring cell, but also to the reference electrode. In this way, the reference electrode also determines the electrical potential of the or at least one electrode of the oxidation component.

In one implementation, the oxidation component comprises two electrodes and a layer between the two electrodes, this layer on the one hand electrically insulating the two electrodes from one another and on the other hand connecting the two electrodes with each another in an ionically conductive manner. This layer can be constructed in the same way as the layer between the two electrodes of the measuring cell and, in particular, comprise an electrolyte.

The pressure equalization outlet can also be used as an inlet, in particular prior to a deployment of the sensor. Preferably, a liquid electrolyte can flow into the housing and there through the pressure equalization outlet and passing the oxidation component to the measuring cell. This embodiment makes it possible to fill the electrochemical sensor with liquid electrolyte prior to use and, in particular, to ensure that the filled liquid electrolyte reaches the oxidation component and the measuring cell and establishes the required or desired ionically conductive connections. Preferably, the sensor is moved to a position where the sensing element is vertically or obliquely below the pressure equalization outlet. Force of gravity causes the electrolyte in the interior to flow vertically or obliquely downward to the sensing element. Ideally, it is not necessary to refill liquid electrolyte after commissioning the sensor.

In one embodiment, at least one gas-impermeable barrier layer is arranged between the pressure equalization outlet and the measuring cell. At least one gas-permeable passage opening is embedded in the or each barrier layer, optionally at least two gas-permeable passage openings are embedded in a barrier layer. The configuration with the barrier layer further reduces the risk that gas which has flowed through the pressure equalization outlet into the interior reaches the measuring cell. Thanks to the passage opening, pressure equalization can still take place.

According to the invention, a gas from the environment can flow through the measuring cell side inlet into the interior space. In one embodiment, the gas flows from a spatial region directly adjacent to the housing of the electrochemical sensor into the interior, for example by diffusion and/or by suction. In one embodiment, an adapter can be placed on the measuring cell side inlet and preferably removed again. This adapter is connected in a fluid-tight manner to one end of a fluid guiding unit, this fluid guiding unit being arranged outside the electrochemical sensor. With the adapter in place, the measuring cell side inlet is connected to the environment only through the fluid guiding unit. With other words: A gas sample cannot bypass the adapter for entering the interior space. The gas flows through this fluid guiding unit and through the measuring cell side inlet into the interior. Preferably, the adapter does not cover the pressure equalization outlet, so that an overpressure in the interior can be quickly relieved even when the adapter is attached. Preferably, the sensor can be used either with the adapter attached or without the adapter. Preferably, the sensor or also the adapter comprises a fluid conveying unit that is able to draw in a gas through the fluid guiding unit.

A fluid guiding unit is a component that is capable of guiding a fluid along a given trajectory and ideally prevents the fluid from leaving this trajectory. A hose and a tube are two examples of a fluid guiding unit. A fluid conveying unit is capable of causing a flow of fluid. A pump, a fan, and a piston-cylinder unit are three examples of a fluid conveying unit.

The embodiment with the attachable adapter and the fluid guiding unit makes it possible to examine a gas in a spatial region that is spatially distant from the electrochemical sensor. This spaced area can in particular be an enclosed space, and the electrochemical sensor can be arranged outside this space. Substances in the enclosed space can only reach the interior of the sensor through the fluid guiding unit and the measuring cell side inlet, but not through the pressure equalization outlet.

The sensor according to the invention may be configured as a portable device and can have its own power supply unit. A user may carry the sensor according to the invention to investigate a gas in his or her environment. The sensor according to the invention may also be configured as a stationary device and may be connectable to a stationary voltage supply network. Optionally, the sensor according to the invention comprises a fluid conveying unit, such as a pump or fan, which is capable of drawing or sucking in gas from the environment. The sensor according to the invention may comprise its own output unit, which outputs a measurement result of the sensing element in a manner that can be perceived by a human. The sensor may comprise a dedicated alarm unit, which outputs an alarm in a human-perceivable manner when a measured value is outside a predetermined range. Preferably, the alarm is output in a tactilely perceptible form, for example by vibrations. It is also possible that the sensor according to the invention comprises a communication unit which is configured to transmit a measurement result or an alarm of the measuring cell to a spatially remote receiver.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, in an embodiment, the sensor according to the invention is used to detect whether or not a gas contains a predetermined gas component, and/or to measure the content (concentration) of this gas component in the gas. The gas component is, for example, oxygen in the form of O2or a gas mixture containing oxygen, for example N2O or H2O or CO, or hydrogen in the form of H2or organic vapors. In the following description, oxygen is mentioned by way of example as the gas component to be detected. In addition to the oxygen to be detected, oxidizable gas components may be present in the gas to be analyzed, for example SO2, NO2, alcohol, aldehydes and/or unsaturated hydrocarbons.

FIG.1shows schematically in a cross-sectional view how gas G can enter the interior of an electrochemical sensor100according to the invention.FIG.2schematically shows a cross-sectional view of the sensor100according to the invention ofFIG.1.

The sensor100includes a housing10having a cover11, the housing10surrounding an interior space I. In a plane perpendicular to the drawing planes ofFIG.1andFIG.2, the housing10preferably has a circular cross-sectional area.

When the sensor100is used, a gas G to be examined flows in the direction R.1through an opening Ö into the interior I, i.e. upward inFIG.1andFIG.2. This opening is hereinafter referred to as the measuring cell side opening and functions as the measuring cell side inlet of the embodiment. The opening is recessed in a wall W.1of the housing10.

When the sensor100is completed and/or before it is used, a liquid electrolyte E is transferred from a container20into the interior I. In the example ofFIG.1, the electrolyte E flows into the interior I from above. The liquid electrolyte E is introduced into the interior I, for example, when the cover11is removed or through an opening in the cover11which is not shown. An aqueous solution comprising sulfuric acid or phosphoric acid or perchloric acid with a water content of at least 5% is used as the electrolyte E, for example. The boiling point of the liquid electrolyte E is above the temperature at which the sensor100is used so that the electrolyte E does not evaporate.

A measuring electrode (working electrode)2and a counter electrode4are arranged in the interior I, cf.FIG.2. In the example shown, the measuring electrode2is in the form of a disk, and the counter electrode4is in the form of a circular ring (torus). In one embodiment, the following chemical reactions take place:Oxygen on the measuring electrode2is reduced to 2O2−. For example, the chemical reaction O2+2H2O+4e−→4OH−takes place.2O2−, which is contained in the electrolyte E, is oxidized to O2at the counter electrode4. For example, the reverse chemical reaction 2H2O→O2+4H++4e−takes place.

A reference electrode5determines (fixes) the electrical potential at the measuring electrode2. Preferably, the electrical potential of the measuring electrode2is several 100 mV lower than that of the reference electrode5. This causes the chemical reaction at the measuring electrode2. In particular, the electric potential of the measuring electrode2causes the chemical reactions described above. Ideally, the following situation is caused: The concentration of O2at the measuring electrode2is permanently zero due to the chemical reaction. On the other hand, O2is generated at the counter electrode4. One realization form of such a measuring cell can be called an “oxygen pump”.

An O2concentration gradient forms between the two electrodes2and4as a result of the chemical reactions. This O2gradient causes electrons to flow from the counter electrode4to the measuring electrode2. Therefore, an electric current flows through a wire14that connects the measuring electrode2to the counter electrode4and is arranged with a distance to the electrolyte provided with an insulating layer12. The electrodes2and4, the electrolyte12, and the wire14together form an electrical circuit. An electrical measuring resistor, which is not shown, is arranged in the wire14. A measuring unit in the form of a current intensity sensor (amperage sensor)15measures the intensity or also the quantity (electric charge) of the current flowing through the wire14. The current strength or also the electrical charge are a measure of the concentration or content of oxygen in the interior I in the sensor100.

In the embodiment example, the two electrodes2and4, the insulating layer12, the wire14and the current sensor15belong to the measuring cell of the sensor100.

The distance between the two electrodes2and4is so great, and the insulating layer12with the electrolyte E is so configured, that the following effect is achieved: O2is largely prevented from diffusing back from the counter electrode4to the measuring electrode2. The electrodes2and4are made of a metal, preferably a noble metal, which is resistant to materials of the electrolyte E as well as to CO, H2S and other possible components of the gas G. Particularly preferably, both electrodes2,4are made of platinum or of gold. In this material, the chemical reactions described above take place sufficiently rapidly. On the other hand, this material is chemically resistant to the electrolyte E. It is also possible that the measuring electrode2and/or the counter electrode4are made of gold.

The two electrodes2,4are electrically insulated from each other by an insulating layer12with at least one layer of glass fiber or a glass nonwoven, which is impregnated with the electrolyte E. At the same time, the electrolyte E enables the flow of ions.

The cover11is mounted on a support sleeve16, which is mechanically connected to a wall W.2of the housing10. The area enclosed by this support sleeve16functions as the pressure equalization outlet of the embodiment. Inside this supporting sleeve16, an elastic gas-permeable seal (plug)9is arranged. This seal9presses the electrodes and other layers together and causes the electrodes to be able to perform only a relatively small relative movement with respect to each other, and therefore the wire14permanently and reliably contacts the two electrodes2and4.

As is well known, air has an oxygen content of about 20%. Such a large amount of oxygen would lead to a large turnover of oxygen at the measuring electrode2and quickly damage the measuring electrode2. Therefore, a volume flow reducer in the form of a thin barrier layer1separates the measuring electrode2from the environment in a nearly fluid-tight manner. “Nearly fluid-tight” in the embodiment example means that less than 1%, in particular less than 1‰, preferably less than 0.1‰, of the gas G flowing from the environment in the direction R.1through the opening Ö into the interior I can penetrate the barrier layer1and reach the measuring electrode2. This small amount is sufficient for the measured current or electric charge through the wire14to correlate with sufficient reliability with the sought oxygen concentration.

In the embodiment example, the barrier layer1has a thickness of at least 5 μm and less than 50 μm, preferably less than 20 μm, particularly preferably a thickness between 10 μm and 15 μm. It is desired to prevent a crack or hole from appearing in the barrier layer1. In the example shown, the barrier layer1is arranged in the manner of a sandwich between two supporting membranes7and13. Each supporting membrane7,13is formed of PTFE, for example. The supporting membrane7between the opening Ö and the barrier layer1protects the barrier layer1from mechanical damage due to influences acting from the outside through the opening Ö. The supporting membrane13between the barrier layer1and the measuring electrode2protects the barrier layer1from possible mechanical damage caused by the wire14.

Viewed in the direction of flow R.1, the reference electrode5is arranged downstream from the counter electrode4. This reference electrode5provides a reference electrical potential and prevents the current intensity or the electrical charge measured by the current intensity sensor15from drifting or oscillating while the oxygen concentration remains constant. Because the reference electrode5is located a sufficiently large distance downstream from both electrodes2and4, the reference electrode5is outside the region where the O2concentration gradient described above occurs. Therefore, this O2gradient does not affect the reference electrode5and, ideally, does not affect the reference voltage that defines the reference electrode5.

Several parallel insulating layers3are arranged in the interior I, which layers are impregnated with the electrolyte E and comprise, for example, glass fleece (glass nonwoven). Through the insulating layer3, the electrolyte E reaches the insulating layer12and wets the surface of the measuring electrode2. Because the counter electrode4has the shape of a circular ring, the electrolyte E can flow through the hole in the circular ring to the measuring electrode2. The membrane13between the measuring electrode2and the barrier layer1prevents the electrolyte E from flowing further in the direction of the barrier layer1.

In one embodiment, the reference electrode5is surrounded on both sides by an insulating layer3with electrolyte E in the manner of a sandwich. At least one insulating layer3is arranged between the reference electrode5and the counter electrode4, in the example shown two insulating layers3are so arranged. As a result, the reference electrode5is electrically insulated from the counter electrode4but ionically conductively connected. Due to the distance described above and this electrical insulation, the reference electrode5is not influenced in any significant way by the O2gradient between the two electrodes2and4. The undesirable effect is largely prevented because the insulating layers3have an electrically insulating effect and because oxygen diffuses relatively slowly.

An overpressure can occur in the interior I as a result of a chemical reaction, in particular one of the reactions just described at the measuring cell. This overpressure must be relieved quickly, in particular because the overpressure can cause oxygen in the electrolyte E to dissolve and diffuse to the measuring electrode2, which can result in incorrect measurement results. Therefore, in one embodiment, the optional cover11is gas-permeable or permanently has an opening. The seal9is also gas permeable. The cover11protects the seal9from mechanical damage from the outside. The overpressure in the interior I can therefore be relieved through the seal9and the cover11. The seal9and the cover11are arranged in or on the pressure equalization outlet16of the embodiment.

As explained above, oxygen in the form of O2is generated at the counter electrode4. The barrier layer12with the electrolyte E prevents the undesirable event of this oxygen reaching the measuring electrode2. In particular, thanks to the oxygen generated at the counter electrode4not being able to penetrate the barrier layer12, an overpressure occurs in the region between the counter electrode4and the cover11. In the embodiment example, the counter electrode4is configured as a ring, and a gap occurs between the reference electrode5and the housing10. This implementation form facilitates the desired result that this overpressure is relieved in the direction R.1through the pressure equalization outlet16, i.e. upward inFIG.1andFIG.2.

With the cover11removed or through the cover11or through an opening in the cover11and through the elastic seal9, gas G can flow from the environment in the direction R.2, i.e. downwards inFIG.1andFIG.2, and thereby get into the interior I. This applies in particular when there is no overpressure in the interior I relative to the environment. This gas G can have at least one oxidizable gas component which can shift the reference potential of the reference electrode5or which can lead to deposits on an electrode2,4,5. Examples of such oxidizable gas components were mentioned above.

An oxidation component6is arranged between the seal9on the one hand and the electrodes5,4,2on the other. This oxidation component6largely prevents oxidizable gas components which have flowed through the pressure equalization outlet16into the interior I from reaching an electrode5,4,2. The oxidation component6comprises at least one electrode, in the example shown two electrodes6.1and6.2spaced apart from each other. In one embodiment, the oxidation component6comprises two electrodes6.1,6.2just like the measuring cell, as well as an ionically conductive connection between the electrodes6.1and6.2, for example a layer3saturated with the electrolyte E. An O2gradient also occurs between these two electrodes6.1and6.2.

Preferably, the or each electrode6.1,6.2of the oxidation component6is made of platinum or gold. The or at least one electrode6.1,6.2of the oxidation component6is held at an electrical potential that differs from the electrical potential of the reference electrode5and is between 0.1 V and 0.5 V, preferably between 0.2 V and 0.4 V. The oxidation component6therefore oxidizes all oxidizable components of the gas G which diffuse through the cover11and the seal9in the direction R.2into the interior I, and ideally completely.

At least one insulating layer3electrically insulates the oxidation component6from the reference electrode5, in the example shown two insulating layers3spaced apart from each other. The oxidation component6is ionically conductively connected to the reference electrode5, in the embodiment example by the electrolyte E in the insulating layer3. Thanks to the ionically conductive connection, the reference electrode5additionally determines the electrical potential of the oxidation component6or at least the electrical potential of an electrode6.1or6.2of the oxidation component6.

Optionally, two gas-impermeable barrier foils8.1and8.2are arranged between the oxidation component6and the counter electrode4. The two barrier foils8.1and8.2are made, for example, of a plastic that is impermeable to gas, such as a perfluoroalkoxy polymer (PFA) or a polyvinylidene fluoride (PVDF). The barrier foils8.1and8.2reduce the risk of O2diffusing from the counter electrode4to the reference electrode5or even to the oxidation component6, or of a gas flowing in the direction R.2reaching an electrode2,4. The two barrier foils8.1and8.2are resistant to the electrolyte E. In order that the electrolyte E can nevertheless reach the measuring electrode2, a sufficiently large gap Sp occurs between the barrier foil8.2and the housing10. The barrier foil8.1has the shape of a ring (torus) so that the electrolyte E can flow through the hole Lo in this ring to the measuring electrode2.

In the embodiment shown inFIG.1, the gas G diffuses into the interior I from a spatial region immediately adjacent to the opening Ö.FIG.3shows an alternative configuration. An adapter30is placed on the opening Ö. The adapter30is fluid-tightly connected to a hose31. The gas G can only reach the opening Ö through the hose31, but not by bypassing the adapter30. Apart from the hose31, the adapter30surrounds the opening Ö in a fluid-tight manner. The free end of the hose31is located in an area with gas to be investigated. A schematically shown pump32sucks the gas G through the hose31and directs it further through the opening Ö into the interior I. Just like an embodiment ofFIG.1, gas G can also reach the interior I through the pressure equalization outlet16.

The adapter30can again be removed from the opening Ö so that the gas G then again reaches the interior I directly through the opening Ö, as indicated inFIG.1.

List of reference characters1Inlet-side barrier layer, arranged between the supporting diaphragms 7 and13, acts as a volume flow reducer for the orifice Ö2Measuring electrode, belongs to the measuring cell3Insulating layer impregnated with electrolyte E4Annular counter electrode, belongs to the measuring cell5Reference electrode, belongs to the measuring cell6Oxidation component, comprises at least one electrode, preferably the twoelectrodes 6.1 and 6.2, arranged between the pressure equalization outlet 16and the measuring cell6.1, 6.2Electrodes of the oxidation component 67Gas-permeable supporting membrane between the barrier layer 1 and theopening Ö8.1Annular barrier foil between the reference electrode 5 and the measuringelectrode 2, has the hole Lo8.2Disc-shaped barrier foil between the oxidation component 6 and thereference electrode 5, 5, surrounded by the annular gap Sp9Elastic seal (plug) between the oxidation component 6 and the cover 11 11in the support sleeve 1610Housing, encloses the interior I, includes the opening Ö on the measuringcell side, is mechanically connected to the support sleeve 16,accommodates, among other things, the measuring cell, and the oxidationcomponent 611Cover for the housing 1012Insulating layer with electrolyte between electrodes 2 and 4 of themeasuring cell13Gas-permeable supporting membrane between the barrier layer 1 and themeasuring electrode 214Wire electrically connecting electrodes 2 and 4 with each other15Current intensity sensor, measures the current intensity or the electriccharge of the current flowing through the wire 1416Support sleeve around the elastic seal 9, is mechanically connected to thewall W.2 of the housing 10, receives the seal 9, can be closed by the cover11, acts as a pressure equalization outlet20Container for filling the electrolyte E30Adapter that can be detachably placed on the opening Ö31Hose connected to the adapter 3032Pump sucking the gas G through the hose 31100Sensor, includes electrodes 2, 4, 5, 6, housing 10, barrier layers 3, barrierfoils 8.1 and 8.2, seal 9 and cover 11ELiquid electrolyte, is filled from the container 20 from above into thesensor 100GGas to be investigated, can flow into the interior I in the directions R.1 andR.2IInterior of the sensor 100, enclosed by the housing 10LoHole in the annular barrier foil 8.1ÖOpening through which gas G diffuses through the barrier layer 1 into theinterior I acts as a measuring cell side inletR.1Direction in which gas G to be tested flows through opening Ö into interiorIR.2Direction in which gas G to be investigated flows through the supportsleeve 16 into the interior I is opposite to the direction R.1SpGap between the barrier film 8.2 and the housing 10W.1Wall of the housing 10 in which the inlet Ö on the measuring cell side isrecessedW.2Wall of the housing 10 to which the support sleeve 16 is connected