GAS SENSOR Field of the Invention The present invention relates to gas sensors that use an electrochemical cell. BACKGROUND OF THE INVENTION An electrochemical gas sensor for capturing an oxidizable or reducible gas (ie carbon monoxide) in the atmosphere generally contains a working or sensing electrode, a counter electrode and an inlet (generally a diffusion barrier) to allow that the atmosphere allows the sensing electrode. Both electrodes are in contact with an electrolyte in order, first, to produce an electrochemical reaction at the sensing electrode with the gas to be captured, and secondly to produce an electrochemical reaction at the counter electrode with oxygen in the atmosphere , electrolyte or any other source of gas. The current is carried through the solution by ions produced in a reaction and by electrons through an external circuit, where the circuit current indicates the concentration of the gas. A reference electrode can be used in combination with a potentiostat circuit to maintain the potential between the sensor electrode and the electrolyte of the cell in order to increase the stability of the operation. In terms of physical construction, the sensor usually comprises an external compartment that acts as a reservoir for the electrolyte, a wick or matrix to put the electrolyte in contact with the electrodes, and external electrical terminals that make electrical connection with the electrodes. Most current sensor cells use an array of stacked electrodes, such as in US-A-4,406,770, with electrodes close to each other and in contact with pieces of wick by pressure produced by a ring compression seal. This design has the disadvantage that a large number of components are typically needed as contacts for the electrodes, to separate the electrodes, and to provide access to the wick for the electrolyte from the reservoir to the electrodes. This produces high costs and long assembly time. A second disadvantage is that the seal tends to loosen over time, producing electrolyte leaks and thus faults in the cell. This second disadvantage is avoided by designs in which the components are sealed together, as for example in DE-A-3324682, where the stacked electrodes are held in place in a central body by a heat and pressure welding operation. Both cells of the types in US-A-4,406,770 and DE-A-3324682 use metallic strip electrode contacts which, due to the corrosive nature of the electrolyte, have to be of platinum or a similar noble metal, and therefore are a significant fraction of the material cost of the cell. US-A-5183550 discloses a gas sensor in which the sensing, counter and reference electrodes are smeared on a common plane in a common ceramic substrate, whose contact tips extend from the electrodes to the other surface of the substrate for connection electric However, the resulting sensor still has many parts and involves relatively complex manufacturing steps that add to the manufacturing cost. SUMMARY OF THE INVENTION The object of the present invention is to reduce the number of components and the complexity of assembling a gas sensor over current designs, and to avoid the use of expensive platinum connections, thereby reducing the manufacturing cost. The present invention provides a gas sensor that includes: A substrate, at least first and second electrodes formed as porous planar elements in the substrate, and wherein the substrate is porous in at least one region adjacent to the first electrode to allow the gas to permeate towards the electrode from the outside of the gas sensor, a compartment containing a reservoir of liquid electrolyte to make contact with the electrodes, external terminal devices mounted in or towards the compartment to make an external electrical connection with at least the first electrode, wherein a portion of the first electrode extends to a position adjacent to the terminal device, where the terminal device and the first electrode are held in electrical connection with one another, while the porosity of the electrode is blocked in order to prevent the electrolyte from seeping through the electrode into the region of the electrical connection. In a further aspect, the invention provides a method for assembling a gas sensor, wherein the method comprises: 1) Providing at least first and second electrodes as porous flat elements in substrate; 2) Provide a compartment containing a reserve for liquid electrolyte, and where the compartment has electrical terminal devices associated with it; 3) Placing the substrate in relation to the compartment, so that a portion of the first electrode is positioned adjacent to the electrical terminal device; and 4) attaching the substrate to the compartment, so that the first electrode is electrically connected to the electrical terminal device, while the porosity of the electrode is blocked in the region of the electrical connection, to prevent the electrolyte from leaking out. the electrical connection. Although the adhesive bond could be used to bond the substrate and seal the electrode, or a mechanical device (i.e. a snap lock) to exert the necessary compressive force, it is preferred to use a pressure and / or heat sealing process to join the substrate with the compartment and compress and seal the electrode; this has the additional advantage of causing the impregnation of the electrode structure with the material of the compartment to properly seal the electrode in the region of the electrical connection. Thus, in accordance with the invention a particularly simple, reliable and effective device is made to connect a gas sensor electrode to the external world, avoiding the use of expensive platinum tips. In addition, the substrate is joined simultaneously to the compartment where, by a simple assembly operation, the electrical connections are manufactured and the compartment is sealed on its contacts. The first electrode will usually be the working electrode or sensor to create the desired electrochemical reaction between the electrolyte and the incoming gas that must be captured. The second electrode will usually be the counter electrode required to perform a counterpart of electrochemical reaction with oxygen and to create the required ionic and electron fluxes through the electrolyte between the electrodes. As preferred, the second electrode, similar to the first electrode, extends to the external terminal device where excess is in electrical connection, while the porosity of the electrode is blocked. In a preferred construction, the compartment is formed as two parts, a base portion containing the reservoir and a cap member. Their substrates and electrodes are attached to the base part, applying pressure and / or heat sealing from the base part of the compartment to the substrate while compressing the substrate and electrodes, and desirably causing material flow from the compartment to the pores of the electrodes; in this way the flow of the electrolyte through the electrodes is prevented. The pins of the electrical terminal mounted on the base part make electrical connection with the compressed electrodes. In another preferred construction, the substrate forms an upper part of the compartment so that the plug member is omitted. The substrate has selected regions of porosity to allow diffusion of the atmosphere into the compartment. In a preferred construction, an intermediate quantity of electrically conductive polymer placed between the electrodes and the terminal pins, so that when pressure and heat are applied, the conductive polymer is molded around the base of the terminal pins and impregnated in the material of the electrodes, and in order to create a stable and secure electrical coupling. It is preferable that the bolt heads of the electrical terminal are mounted in notches, and the conductive polymer fills the notches. As preferred, the substrate is porous and flexible, so as to cooperate with the sealing process. The electrodes are preferably formed of a porous and electrically conductive material containing PTFE or a similar polymeric glue, preferably catalyst particles, and optionally and additionally catalyst support material and material for increasing conductivity. The electrodes can be deposited on the substrate for example by screen printing, filter it in selected areas from a suspension placed on the substrate, by prorrociado coating, or by any other suitable method to produce that the solid material is deposited according to a pattern. The deposit can be of a single material or of more than one material, sequentially and in layers, as for example to vary the properties of the electrode material through its thickness, or to add a second layer of higher electrical conductivity above or below the layer, which is the main place of gas reaction. The material of preferably porous substrates, allowing gas access to the electrode through the substrate, and impermeable to the electrolyte. In one version of the design, the substrate is highly porous and does not present any barrier to the diffusion of the gas therethrough, the imitation of diffusion of the cell response is provided by a separate diffusion barrier. In another version of the design, the substrate is itself a diffusion barrier, and no other separate barrier material is required. The electrode and substrate assembly preferably sealed to the upper surface of the compartment, where the sealing process effects an electrically conductive contact between a region of electrode material and a contact conductor for each electrode, and simultaneously sealing the substrate and areas remaining from the electrode material to the compartment, forming a seal that is impervious to the electrolyte. While the seal could be an adhesive seal, the seal is preferably formed by applying heat and pressure to a plastic compartment. The plastic compartment is made of a material with a lower melting point than the substrate; When heat and pressure are applied to the compartment through the substrate, the compartment material is forced upward and impregnates the substrate, thus forming a tough bond. If necessary, the joint can be cooled under pressure to prevent loosening before the material in the compartment solidifies. Simultaneously, the electrode material deforms around the contact conductor and forms a closely conforming contact with low electrical resistance and high resistance to electrolyte filtration. The material of the electrode is a composition that allows this to occur. Alternatively, if an intermediate layer of conductive polymeric compound is used, during the sealing process the compound will flow around the contact conductor and the electrode material, and possibly impregnate the electrode material, as described above. A feature of the cell compartment is that the notch in which the compound flows is of an optimal configuration for this to occur. The external terminal device or contact conductors located in the plastic compartment can optionally be covered with a layer of conductive polymeric compound and charged with carbon before the electrode substrate assembly is sealed to the compartment. The composite will then flow during sealing so that it will coat the contact conductor, either to form a tight contact in accordance with the material of the electrode, or to impregnate with it. This will increase the reliability of the contact and its resistance to filtration of the slow electrolyte filtration over the contact path, a problem recognized in previous cell designs. If a metallic conductor is used, coating it with the polymer compound will be particularly advantageous to prevent corrosion of the electrolyte that seeps through the electrode material. This would allow a cheap and non-noble metal to be used. In the design where the electrode substrate does not limit diffusion, a cover plate containing a device for limiting gas diffusion to the sensing electrode is attached on the side of the electrode assembly, which does not charge with the electrodes. The diffusion limiting device may be one or more capillaries, a porous membrane, or a combination of both. The cover plate prevents gas access to the electrodes that are not the sensing electrode. If the substrate itself is the diffusion barrier, the cover plate acts only to prevent gas access to the electrodes in addition to the sensing electrode.
Alternatively in this case, gas access can be prevented by treating the substrate to remove its permeability in regions on these electrodes, and the cover plate is omitted. The design is suitable for manufacturing more than one sensor on the same substrate by depositing more electrodes, and adding more cell components, either to form a cell with several sensor electrodes and other pieces in common, or more than a separate cell in a common substrate using adequately formed compartments and wicking components. Descriptions of the Drawings The preferred embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic view of a first embodiment of the invention. Figure 2 is a plan view of the first embodiment on line 2-2 of Figure 1; Figure 3 is a schematic view of a second embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Figures 1 and 2, there is shown an electrochemical gas sensor 2 comprising a two-part compartment, specifically a body part 4 which is cylindrical and with a hollow interior 6 for forming the reserve of electrolyte, and a disk-shaped plug member 8. Three cylindrical nickel or copper-tin terminal pins 10, have heads 14 located in notches 16 in the upper part of the body of the compartment
4, wherein the notches 16 have a rectangular shape in one plane, and a stepped form in sections to provide enlarged upper portions 18. A porous flexible substrate 20, in the form of a disk, is disposed on the upper surface of the body member 4. First, second and third electrodes 22, 24 and 25, formed from a mixture of electrically conductive catalyst particles in PTFE glue, are printed with screen or deposited by filter on the lower surface of the substrate in the form of segments as shown in figure 2 An amount of polymer / carbon conductor compound 26 is placed in the notches 16 on each contact pin head 14. The plug member 8 has holes 28 drilled therein towards a region of multiple surfaces with notches 30 to allow the atmospheric gas is diffused through the openings 28 and from there, through the area of multiple surfaces 30, through the substrate 20 to the electrode 22. The electrolyte with the notch of the electrolyte or reservoir 6 is kept in contact with the electrodes 22, 24 by means of a wick 31 formed by a porous felt member, brought into contact with the three electrodes by a plastic spring member of the electrode. U-32. The reservoir is closed at the base by a base member 34 with a pressure exhaust opening at 36 closed by the porous membrane 38. To assemble the structure shown in figures 1 and 2, the part of the base 4 has electrical terminal contact pins 10 positioned therein with conductive masses 26 positioned within the notches 16 on the heads 14. In an alternative method of assembly, the material 26 can be applied within the notches or applied to the contact pins before that these are inserted in the body.
In the next assembly step, the substrate is placed on top of the cylindrical body 4. Heat and pressure are applied to the areas A as shown by a pressure tool (not shown) in order to compress the substrate and the electrodes in the upper plastic surface, and the conductive masses 26 in order to join the assembly so that the substrate is securely fixed in the upper part of the compartment. The compression of the electrodes and the substrate in region A, together with the impregnation in the porous material of the plastic compartment and the conductive mass 26, ensure that the substrate and the electrodes are sealed to prevent the ingress of electrolyte into the regions of the electric connections. Simultaneously, the plastic mass 26 is molded likewise around the heads of the terminal pins, thus ensuring a good electrical connection between the contact pins and the electrodes. Subsequently, the cap member 8 is attached to the upper part of the substrate by adhesive. The wick 31 is placed in the reserve space 6 and holding its place by means of the U-32 spring. Electrolyte is added to the reservoir and the lower plug member 34 is sealed in place by ultrasonic bonding. In the operation of the sensor shown in FIGS. 1 and 2, atmospheric gas enters through the openings 28 towards the area of multiple surfaces 30. These openings 28 attenuate the gas flow in the chamber, and form a diffusion barrier for control the gas influx regime. The gas flows through the substrate, which in this mode essentially does not form a diffusion barrier to the gas, in contact with the electrode 22. The electrode 22 functions as a sensing electrode for a target gas present in the atmosphere, and where it is present, it acts as a catalyst to react the gas with water in the electrolyte to produce ions in solution and electrons. In the counter electrode 24 the electrolyte oxygen reacts with the ions released by the sensing electrode to complete an electrical circuit. The voltage generated by the electrochemical reactions appear on the contact pins 10, and a resulting current flowing through an external circuit connected to the pins is a measure of the concentration of the gas. In addition, the electrode 25 acts as a reference electrode in conjunction with an external potentiostat circuit to tilt the cell toward the desired voltage level. Referring to the figure, which shows a second embodiment of the present invention, the parts similar to those of figure 1 are denoted by the same reference number. In this modality, several differences are apparent. First, the contact pin heads 14 are mounted so as to partially protrude from the upper surface of the body part 4. The upper surface of the substrate 20 has a gas impermeable layer 40 coated therein, so that the gas can only enter. in a central region 42 on the electrode 22. In this embodiment, the upper plug member of the body is not present. The substrate 20 is of a low but controlled diffusivity permeability, in order to define a diffusion barrier for the incoming gas, in order to provide precise control over the gas ingress regime. The permeability can be uniform for the entire substrate, or modified in region 42 by, for example, tightening or impregnating a higher permeability substrate reducing permeability. In the embodiment of Figure 3, the substrate and the electrodes are sealed to the upper surface of the body of the compartment by a process that will be described with reference to Figure 1. However, in this embodiment the electrodes 22, 24 are thus molded same around the contact pin heads 14, and with them a direct electrical connection. In this embodiment, a porous matrix 44 is arranged within the reserve space 6 in order to preserve the electrolyte by capillary action. The upper surface of the matrix 44 is compressed against the electrodes, or, alternatively, a compressive incersion is used to ensure that the electrical contact is maintained with the electrodes. Other examples may be derived by combining characteristics of the two previous examples. While three electrodes are described as those on the same substrate, the number may be greater in the case of multiple sensing functions, or one or both of a reference electrode or counter electrode that may be arranged elsewhere in the configuration of the cell, also on the substrate. Additionally, if high concentrations of gas must be captured, a separate access for the oxygen to the counter and reference electrodes can be provided by passages included in the body or in the plug of the cell, where these passages carry a supply of air free of the captured gas. The liquid electrolyte can be replaced by a gel or a polymer, and sticking to the electrodes is required. - Revised the modalities previously described, the following advantages are apparent: (1) The assembly of the flat electrode simplifies the production - all the electrodes can be produced in a single process; (2) The contact method avoids the use of expensive metal contacts, and makes contact quickly and simultaneously with the assembly process of the cell; (3) The contact method means that electrolyte filtration around the contacts, a problem recognized in conventional cells, is avoided. The use of a conductive polymer compound, which coats the contact conductor during the sealing process, is particularly advantageous to ensure reliability if metallic conductors are used, since they are highly susceptible to corrosion if electrolyte filtration occurs. (4) The method of contacting the electrode material in a remote region of the electrolyte prevents changes in the resistance in contact that is used, which arises from the movement of the contact in relation to the electrode and the entrance of the electrolyte to the right, a problem recognized in conventional designs. (5) The sealing process produces a high strength cell that is resistant to leakage. (6) The small amount of components and processes needed for assembly means assembly is quick and cheap. The components are individually robust, so damage during assembly is unlikely. (7) The components of the cells are produced using conventional and simple processes. (8) The simple assembly process of the cell is capable of being automated. (9) The flat assembly of electrodes allows more than three electrodes to be deposited on the same substrate if this is necessary, for example, to provide more than one sensor, and for example, to have sensitivity to different gases, and in the same device.