THERMORESISTIVE GAS SENSOR

Description

[0001] The invention relates to a thermoresistive gas sensor.

[0002] A thermoresistive sensor or detector is a measuring element with an electrical resistance whose value changes reproducibly with temperature. Such resistors are also called thermistors, whereby a distinction is made between thermistors with a positive temperature coefficient (PTC) and thermistors with a negative temperature coefficient (NTC). PTC thermistors include, in particular, metals, preferably platinum and nickel, and non-metallic materials such as polycrystalline ceramics, e.g., based on barium titanate, and doped semiconductors, e.g., silicon, with impurity depletion. NTC thermistors include, in particular, pure or doped semiconductors outside the impurity depletion range, as well as polycrystalline semiconductors made of metal oxides.

[0003] Thermoresistive gas sensors are used, for example, as flow sensors or thermal conductivity detectors in gas analysis or in medical technology (e.g., anesthesia or ventilation devices).

[0004] Thermal flow sensors are used, for example, in gas analysis or in medical technology (e.g., anesthesia or ventilation devices). The underlying measuring principle is based on the use of a sensor element that is electrically heated and whose electrical resistance depends on the temperature. When a fluid flows around it, heat is transferred into the fluid, and this transfer changes with the flow velocity. By measuring the electrical parameters of the sensor element, the flow can be detected and its velocity or the mass flow rate of the fluid can be measured. Particularly for detecting alternating flows, the sensor element can be positioned in the flow together with another sensor element located upstream and / or downstream, so that crosstalk occurs in the form of a heat exchange between the sensor elements – which changes accordingly with periodically alternating flow.This can be measured in a manner known per se by means of an electrical measuring bridge in which the sensor elements are arranged in different bridge branches.

[0005] Thermal conductivity detectors are used to detect specific liquid or gaseous substances (fluids) based on their material-specific thermal conductivity and are particularly prevalent in gas chromatography. For this purpose, the substances to be detected are passed sequentially through a channel past an electrically heated element (e.g., a heating filament made of gold or platinum) after their chromatographic separation. Depending on the thermal conductivity of the flowing substance, more or less heat is transferred from the heating element to the channel wall, causing the element to cool down accordingly. This cooling of the heating element changes its electrical resistance, which is then detected. If the heating element is kept at a constant temperature, the changing electrical heating power is detected.The heating element is usually arranged in a measuring bridge, which contains further resistors and another heating element in a further channel through which a reference fluid flows.

[0006] Such a thermal conductivity detector with meandering heating filaments is known, for example, from Rastrello F. et al: "Thermal Conductivity Detector for gas-chromatography: Acquisition system and experimental measurements", 2013 IEEE INTERNATIONAL INSTRUMENTATION AND MEASUREMENT TECHNOLOGY CONFERENCE (I2MTC), IEEE, May 13, 2012 (2012-05-13), pages 1226-1230.

[0007] From WO 2009 / 153099 A1, a flow sensor designed as a silicon-based microflow sensor for gas analysis is known. The known flow sensor consists of two plate-shaped components, each with a window-like opening for the flowing gas and a silicon lattice structure extending over it. The two components are joined to form a plate assembly, with the two openings aligned and the lattice structures, oriented perpendicular to the flow direction, positioned one behind the other in the direction of the flow. The lattice structures are produced from crystalline silicon, for example, using silicon-on-insulator (SOI) wafer material, and are meandering in shape.

[0008] Compared to similar metal grids with electrical resistance values ​​of a few ohms, such as those known from DE 1 698 048 A1, DE 32 04 425 A1, or WO 00 / 59824 A1, the resistance of the silicon-based grid is in the range of a few kilohms. This higher resistance value leads to an improved signal-to-noise ratio. Furthermore, the higher resistance coefficient compared to metals leads to greater measurement sensitivity, i.e., to higher temperature-dependent resistance changes.

[0009] The grid structures of the flow sensor known from the aforementioned WO 2009 / 153099 A1 can consist of several adjacent sub-grid structures, which are also meandering and can be connected in series, for example, to further increase the resistance. In particular, four grids can be connected to form an electrical measuring bridge, whereby the grids diagonally opposite each other in the measuring bridge each form a grid pair with a common grid plane, and the two different grid pairs are located one behind the other in the flow to be measured. This results in a significantly higher measurement sensitivity than in measuring bridges where only two bridge resistances are formed by resistance grids in the flow to be measured and the other two bridge resistances consist of fixed resistors.

[0010] WO 2017 / 194319 A1 also discloses a flow sensor with four thermoresistive sensor elements connected to form an electrical measuring bridge. The sensor elements are arranged one after the other in the direction of flow and, for example, are designed as thin-film sensor elements with meandering resistance structures in a row next to one another on one side of a substrate. The flow to be measured therefore does not flow through the meandering resistance structures, but rather along them. The first two sensor elements in the row are located diagonally opposite each other in the measuring bridge; the same applies to the last two sensor elements.

[0011] US 2003 / 015034 A1 discloses a flow sensor comprising a heating element and a temperature sensor arranged one behind the other on a membrane in the flow direction of the fluid to be measured and configured as meandering resistance layers. Both the heating element layer and the thermometer layer can have a conductive path configuration in which individual resistance elements are connected in parallel.

[0012] In a flow sensor known from US Pat. No. 7,010,971 B2, a heating element and four temperature sensors are formed on a membrane, with the heating elements arranged in pairs upstream and downstream of the heating element in the direction of flow. The temperature sensors are designed as meandering resistance layers, which are nested within each pair and connected in diagonally opposite branches of an electrical measuring bridge.

[0013] In a similar flow sensor known from JP H08219836 A, the sensor elements in the measuring bridge, which are diagonally opposite each other, are not arranged one behind the other in the flow, but at the same height in the direction of flow, but on different sides of the substrate facing away from each other.

[0014] In another similar flow sensor known from WO 2007 / 101185 A1, the four sensor elements with their meandering resistance structures are again arranged on one side of the substrate. The sensor elements, which are diagonally opposite each other in the measuring bridge, are not arranged one behind the other in the direction of flow, but rather interdigitally, meaning that the meandering loops of the sensor elements interlock to form an interdigital structure.

[0015] WO 2009 / 095494 A1 discloses a thermal conductivity detector with an electrically heatable filament mounted in the center and along the length of a channel, allowing fluid to flow around it. The filament is held at both ends by two electrically conductive supports traversing the channel. To ensure long service life and inertness to chemically aggressive gas mixtures, the filament and supports are made of doped silicon. The doped silicon can be applied to a silicon substrate with an insulating layer of silicon dioxide interposed therebetween. In etching processes, the supports and filament are formed by structuring the silicon substrate, the silicon dioxide layer, and the layer of doped silicon, and the channel is formed in the support plate.In view of the brittleness of silicon, EP 3 096 133 A1 proposes to replace the heating filament clamped on both sides by an extremely thin cantilever beam made of doped silicon in order to increase mechanical stability.

[0016] US 2003 / 015034 A1 discloses a thin-film flow sensor comprising resistance elements arranged between two insulating films. The resistance elements are arranged in a plane that is essentially flatly swept across by a gas flow. Based on the resistance elements, a temperature difference can be determined along the direction of the gas flow, which in turn can be used to determine the flow velocity of the gas flow.

[0017] From US patent specification 7,010,971 B2, a flow sensor is known which has a heating resistor and a thermistor whose resistance value is measurable. The flow sensor includes temperature sensors comprising conductive traces arranged in meandering loops. The temperature sensors are arranged flat on a plane that is essentially swept by a gas flow. A temperature difference can be detected by means of the temperature sensors, from which the flow velocity of the gas flow can in turn be determined.

[0018] Furthermore, US 2002 / 038508 A1 discloses a method for manufacturing self-supporting microstructures that can be used in measuring devices for weak gas flows. The microstructures are designed in the form of meandering struts that are electrically connected in series.

[0019] Patent application US 2007 / 0204688 A1 discloses a Wheatstone bridge that can be used in a flow sensor. The flow sensor comprises heat-sensing means that include meandering conductors and are arranged in a plane. The plane is subjected to a substantially shallow flow angle for measuring the flow rate, with the heat-sensing means arranged one behind the other in the direction of flow.

[0020] Due to the high electrical resistance of the silicon heating filament, a higher detection sensitivity of the thermal conductivity detector is achieved compared to heating filaments made of metal (usually gold or platinum). However, this also results in a higher voltage drop across the heating element when it is heated to a desired temperature with a certain heating power, which may preclude intrinsically safe operation in potentially explosive atmospheres. EP 3 096 137 A1 therefore proposes dividing the silicon heating filament into two or more sections, which are physically arranged in series in the fluid flow and electrically connected in parallel.

[0021] A thermoresistive gas sensor with a flat grid through which the gas can flow, the grid webs of which are made of a semiconductor material and are electrically connected in parallel, are arranged parallel to one another in the grid plane and are designed in an S-shape, is the subject of the older, not yet published, European patent application with the official file number EP 18164748.8.

[0022] From US patent 2004 / 056321 A1, a planar heater is known in which a heating element is formed on a substrate, consisting of strands running parallel next to each other in an s-shape and connected electrically in parallel at the ends.

[0023] The invention is therefore based on the objective of providing a thermoresistive gas sensor that can be used equally as a flow sensor or thermal conductivity detector, is characterized by high measurement sensitivity and mechanical stability, and is easy and inexpensive to manufacture.

[0024] According to the invention, the problem is solved by the thermoresistive gas sensor specified in claim 1.

[0025] Advantageous further developments of the thermoresistive gas sensor according to the invention, a flow sensor containing at least two such gas sensors and a thermal conductivity detector based on the gas sensor according to the invention can be found in the further claims.

[0026] The subject of the invention is therefore a thermoresistive gas sensor with two identical, flat grids through which a gas can flow, which grids consist of a semiconductor material with a predetermined conductivity type and are connected in diagonally opposite branches of an electrical measuring bridge, wherein each grid has grid webs running parallel to one another and electrically connected in parallel at the ends, and the grid webs of the two grids alternately lie next to one another in a common grid plane and extend over a window opening in a carrier plate.

[0027] Since the two grids are identical and their grid bars alternate side-by-side in the same grid plane, both grids are exposed in practically the same way to a gas that reaches the grids through the window opening and, depending on the application, flows between the grid bars. The gas flows perpendicularly through the support plate at the window opening. Any change in the composition, flow velocity, or flow profile of the gas reaches both grids simultaneously and affects their temperature-dependent electrical resistance simultaneously and to the same extent. The sensitivity of the measuring bridge with the diagonally arranged grids is twice as high, while maintaining at least the same accuracy, as that of a gas sensor with only one grid and a fixed resistor diagonally opposite it in the measuring bridge.

[0028] In the gas sensor according to the invention, the grids are not meandering but consist of struts that are both physically and electrically parallel. The grid struts, with their semiconductor material, form temperature-dependent resistances that are high compared to metal and therefore, as mentioned above, enable high detection sensitivity of the sensor. However, since the struts, or rather the resistances they form, are connected in parallel, the total resistance of the respective grid, and thus the electrical voltage drop across the grid at a given heating power, is low. This allows for intrinsically safe operation of the gas sensor in potentially explosive atmospheres.

[0029] Doped semiconductor materials, such as silicon, are particularly suitable as materials for the semiconductor layer due to their high resistance compared to metals. Silicon is also characterized by its inertness to chemically aggressive gas mixtures. Depending on the doping, a positive temperature coefficient (PTC) or a negative temperature coefficient (NTC) can be achieved. The use of monocrystalline semiconductor materials results in good mechanical stability, which can also be enhanced by the directional dependence of the elastic modulus and the piezoresistive coefficient.

[0030] Although the meandering grids of the aforementioned known gas sensors also have parallel grid sections, these are electrically connected in series.

[0031] Semiconductor-based grids can be heated to higher temperatures than conventional metal grids. The grid struts in the grid plane can therefore be S-shaped, preventing them from deflecting uncontrollably or even buckling when heated and expanded. Instead, the spacing between the struts remains constant, and they cannot touch each other.

[0032] The grid bars can have varying cross-sectional areas across the width of the grid. For example, the outer grid bars can be designed with a larger cross-sectional area (width) than the inner grid bars to ensure a uniform temperature distribution across the width of the grid and to prevent the grid bars in the center of the grid from heating up more than those at the edge.

[0033] The grids can be produced in various ways, for example by etching, laser cutting, or 3D printing. Preferably, the semiconductor material is formed as a semiconductor layer on a semiconductor substrate forming the carrier plate, optionally with an insulating layer in between. The semiconductor layer extends across the window opening in the semiconductor substrate and is formed there in the shape of the two grids. Outside the window opening, the semiconductor layer has metallizations in four areas extending at least across the width of the grids. These metallizations electrically connect the grid struts at the four ends of the two grids. The metallizations can be connected to separately formed contact surfaces for contacting the gas sensor or can themselves form the contact surfaces. Furthermore, the semiconductor layer outside the window opening contains separating structures that insulate the four ends of the grids from each other.The separation structures can be formed by removing the semiconductor material in certain areas or by leaving it undoped. Where, for example, a metallization connects the lattice bars of one of the two gratings across the lattice bars of the other grating, the separation structure can be formed by insulating layers between the lattice bars of the other grating and the metallization.

[0034] The gas sensor according to the invention is advantageously used in a flow sensor, where it is arranged together with at least one other identical gas sensor in a gas flow to be measured such that the grids are arranged one behind the other and perpendicular to the flow direction, and wherein all four grids of the two gas sensors are connected to the electrical measuring bridge, in which the grids of each of the two gas sensors are diagonally opposite each other. The measuring bridge is thus formed entirely from the four grids of the flow sensor, so that no fixed resistors are required to complete the measuring bridge.Depending on the direction of the gas flow being measured, heat is transferred from the two grids of one gas sensor to the grids of the other gas sensor. This results in the grids of the two different gas sensors experiencing opposite temperature-dependent changes in resistance, while within each gas sensor, the two grids within that sensor experience the same change in resistance. Due to the diagonal arrangement of the grids in the measuring bridge, as described above, maximum sensitivity of the flow sensor is achieved.

[0035] For the fabrication of the flow sensor, it can be advantageous if the four metallizations mentioned above extend beyond the width of the grids by a predetermined amount in one direction to form contact surfaces, and if the semiconductor substrate (including the semiconductor layer and, if applicable, the insulating layer) contains through openings arranged symmetrically to the contact surfaces in the other direction. The two gas sensors can then be easily mounted one on top of the other, offset by 180°, with the contact surfaces of the lower gas sensor accessible through the openings of the gas sensor above it, thus allowing both gas sensors to be contacted from one side, the top.

[0036] The gas sensor according to the invention is also advantageously used in a thermal conductivity detector, where it can be arranged in a measuring gas flow with the grids aligned with the flow direction. For this purpose, the gas sensor can be held between two components, each containing a recess open towards the grids and aligned with the window opening in the support plate. The two recesses together form a measuring chamber in which the measuring gas flows around the grids, which are introduced into and led out of the measuring chamber via gas connections. The two gas connections can be designed such that they widen towards the measuring chamber to the width of the grids, so that the measuring gas flows over the entire area of ​​the grids. Each of the two components can contain one of the two gas connections.To simplify the design and use of the thermal conductivity detector, both gas connections can alternatively be located in one of the two components.

[0037] When used in a gas chromatograph in which the sample gas is dosed into a carrier gas stream and then passed through a separation column or separation column circuit to separate different components of the sample gas, it is advantageous to arrange another identical thermoresistive gas sensor in the reference gas flow with the grids oriented longitudinally to the flow direction, wherein all four grids of the two gas sensors are connected to the electrical measuring bridge in which the grids of each of the two gas sensors are diagonally opposite each other.

[0038] The thermoresistive gas sensor according to the invention is equally suitable for use in a flow sensor, for example, in a non-dispersive infrared (NDIR) analyzer or a gas analyzer operating according to the paramagnetic alternating pressure method, as well as in a thermal conductivity detector for gas chromatography or for the analysis of binary gas mixtures. The electrical wiring of the gas sensor, e.g., in a bridge circuit, is known per se and is not the subject of the invention.

[0039] For further explanation of the invention, reference is made below to the figures in the drawing; in detail they show: FIG. 1 an embodiment of the gas sensor according to the invention with two grids in top view, FIG. 2 a cross-section through the gas sensor along a line AA' in FIG. 1 , FIG. 3 a cross-section through the gas sensor along a line BB' in FIG. 1 , FIG. 4 a measuring bridge in which the grids of the gas sensor are connected as bridge resistors, FIGS. 5, 6 and 7 further embodiments of the gas sensor according to the invention in plan view, FIG. 8 an embodiment of a flow sensor with two gas sensors in a perspective exploded view, FIG. 9 the flow sensor according to Fig. 6 with assembled gas sensors, Figs. 10 and 11 show an embodiment of a thermal conductivity detector with a gas sensor in a section in the longitudinal direction and in the transverse direction to the grids.

[0040] The representations in the figures are schematic and not to scale, but can indicate qualitative size relationships.

[0041] The same or similar components in different figures are provided with the same reference numerals.

[0042] The invention is not limited in its embodiments to the preferred embodiments shown in the figures. Rather, a multitude of variants are conceivable, which, in the solution shown, make use of the invention even in fundamentally different embodiments, in accordance with the claims.

[0043] FIG. 1 shows an embodiment of the gas sensor 1 according to the invention in a top view.

[0044] FIG. 2 shows the same gas sensor 1 in a cross-section along line AA'.

[0045] FIG. 3 shows the gas sensor 1 in a cross-section along line BB'.

[0046] The gas sensor 1 has a substrate 2 in the form of a semiconductor substrate, for example made of silicon, on which a semiconductor material in the form of a semiconductor layer 4 is deposited beneath an insulating layer 3, e.g., silicon dioxide. The semiconductor material has a conductivity type, determined, for example, by doping, and is highly resistive compared to metal. The substrate 2 and the overlying insulating layer 3 contain a window-like opening 5, over which the semiconductor layer 4 extends and is structured there to form two identical lattices 6, 7. The two lattices 6, 7 each consist of a plurality of lattice struts 8 and 9, respectively, which run parallel in a common lattice plane and alternately lie next to each other. The window opening 5 and the lattices 6, 7 can be produced by an etching process.The grid bars 8, 9 form high-resistance components whose resistance values ​​are determined by the length and cross-sectional area of ​​the grid bars 8, 9.

[0047] The grid webs 8, 9 are connected to each other at the ends 10, 11, 12, 13 of the two grids 6, 7 and are thus electrically connected in parallel. For this purpose, the semiconductor layer 4 on the semiconductor substrate 2 is reduced on both sides of the window opening 5 to two rectangular island areas, each extending over the width of the grids 6, 7, between which the grid webs 8, 9 run across the window opening 5. The rectangular islands of the semiconductor material, which at these points can be doped to the point of degeneracy and thus be maximally conductive, are almost completely covered with metallizations 14, 15, 16, 17. As shown, the rectangular islands with the metallizations 14, 15, 16, 17 can extend in one direction by a predetermined amount beyond the width of the grids 6, 7 in order to form contact surfaces 18, 19, 20, 21 there.In the other direction, the semiconductor substrate 2 can contain through openings 22, 23, 24, 25 arranged in a mirror-symmetrical manner to the contact surfaces 18, 19, 20, 21.

[0048] The ends 10, 11, 12, 13 of the two grids 6, 7, i.e., the rectangular islands of semiconductor material with the metallizations 14, 15, 16, 17, are separated or insulated from one another by separating structures 26, in which the semiconductor material of the semiconductor layer 4 is removed down to the insulating layer 3. Where the metallizations, e.g., 16, of one grid, e.g., 7, extend over the grid bars 8 of the other grid 6, the separating structure 26 is formed by insulating layers 27, which surround the respective grid bars 8 and insulate them from the overlying metallization 16 and the semiconductor material of one grid 7. The insulating layers 27 can be made of silicon dioxide in the same way as the insulating layer 3.

[0049] As already mentioned, the grid bars 8, 9 made of semiconductor material form temperature-dependent resistors that are highly resistive compared to metal and therefore enable high detection sensitivity of the gas sensor 1. Since the grid bars 8, 9 are electrically connected in parallel for each of the two grids 6, 7, the total resistance of the respective grid 6, 7 and thus the electrical voltage drop across the grid 6, 7 at a given heating power is low, which enables intrinsically safe operation of the gas sensor 1 in potentially explosive atmospheres.

[0050] Since the two grids 6, 7 are identical and their grid webs 8, 9 lie alternately next to each other in one and the same grid plane, both grids 6, 7 are exposed in a virtually identical manner to a gas 28, which reaches the grids 6, 7 through the window opening 5 and flows between the grid webs 8, 9 or flows around the grid webs 8, 9. Any change in the composition, flow velocity, or flow profile of the gas 28 reaches both grids 6, 7 simultaneously and influences their temperature-dependent electrical resistance simultaneously and to the same extent.

[0051] FIG. 4 Figure 1 shows the electrical arrangement of the two grids 6, 7 of the gas sensor 1 together with further resistors (e.g. fixed resistors) 29, 30 in a measuring bridge (Wheatstone bridge) 31. The two grids 6, 7 are arranged in diagonally opposite branches of the measuring bridge 31, so that their sensitivity is twice as high as that of a conventional gas sensor with only one grid and a fixed resistor diagonally opposite it in the measuring bridge.

[0052] FIG. 5 Figure 1 shows a variant of the gas sensor 1 in which the struts 8, 9 of the grids 6, 7 are formed in an S-shape in the grid plane, so that they are not deflected uncontrollably or even buckled when heated and expanded. Instead, the distance between the struts 8, 9 remains constant, and they cannot touch each other.

[0053] FIG. 6 shows a variant of the gas sensor 1 in which the struts 8, 9 of the two grids 6, 7 are not individually, but alternately in pairs next to each other in the common grid plane above the window opening 5.

[0054] FIG. 7 shows a further variant of the gas sensor 1, in which the webs 8, 9 of the two grids 6, 7 are also arranged in groups of two, alternating next to each other, in the common grid plane above the window opening 5. Within each group of two, the two webs are not connected in parallel, but in series in the form of a U-loop, whereby the electrical resistance of the respective grid 6, 7 is doubled.

[0055] In the illustrated embodiments, the cross-section of the grid bars can be, for example, 1 µm x 1 µm to 100 µm x 100 µm, so that with a cross-section of 10 µm x 10 µm and a window opening 5 size of 1 mm x 1 mm, the two grids 6, 7 would each have up to 25 bars.

[0056] In order to obtain a uniform temperature distribution over the width of the grids 6, 7 and to prevent the grid webs 8 and 9 located in the middle of the respective grids 6, 7 from heating up more than those at the edge of the grid, the outer grid webs 8, 9 can be designed with a larger cross-sectional area (width) than that of the inner grid webs 8, 9.

[0057] FIG. 8 shows a perspective exploded view of a flow sensor 32 consisting of two identical gas sensors 1, 1' in a plate assembly.

[0058] FIG. 9 shows the same flow sensor 32 in assembled state.

[0059] The two gas sensors 1, 1' lie one above the other, possibly with a surface seal and / or a thermally insulating intermediate layer or the like in between, with their window openings 5, 5' aligned with each other. The grid pairs 6, 7 and 6', 7' of the gas sensors 1, 1' lie parallel one behind the other at a distance that is essentially determined by the thickness of the semiconductor substrate (carrier plate) 2. The distance can be precisely adjusted to a definable value using an intermediate layer. The gas sensors 1, 1' are offset by 180° from each other, so that the contact surfaces 18', 19', 20', 21' of the lower gas sensor 1' are accessible through the openings 24, 25, 22, 23 of the gas sensor 1 lying thereon, and the flow sensor 32 can be contacted from one side.

[0060] How FIG. 4 As shown, all four grids 6, 7, 6', 7' of the two gas sensors 1, 1' are connected to the electrical measuring bridge 31, in which the grids 6, 7 and 6', 7' of each of the two gas sensors 1 and 1' are diagonally opposite each other.

[0061] FIG. 10 und FIG. 11 show an embodiment of a thermal conductivity detector 33 containing the gas sensor 1, once in a section longitudinal to the grids 6, 7 or the grid webs 8, 9 and once in a section transverse thereto. The gas sensor 1 is arranged between two components, here a block-shaped base body 34 (e.g., made of aluminum) and a cover 35 (e.g., made of aluminum or polyetheretherketone (PEEK)), in a recess of the cover 35 and with the grids 6, 7 facing the base body 34. The components 34, 35 are connected to one another by means of screws 36 and with the interposition of a surface seal or sealing film (not shown here). Both the base body 34 and the cover 35 each contain a recess 37, 38 that is open toward the grids 6, 7 and aligned with the window opening 5 of the gas sensor 1.The two troughs 37, 38 form a measuring chamber in which a measuring gas 39 flows around the grids 6, 7. The measuring gas 39 is introduced into and out of the measuring chamber via two gas connections 40, 41. In principle, each of the two components 34, 35 can contain one of the two gas connections 40, 41. To simplify the structural design of the thermal conductivity detector 33 and its use, in the example shown, both gas connections 40, 41 are located in the base body 34 and open into the trough 37 in the areas of the ends of the grids 6, 7. The gas connections 40, 41 are formed, for example, by milling or laser processing in the form of channels in the base body 34, into which cannulas 42, 43 are inserted. Contact with the gas sensor 1 is also made on the side of the base body 34 by spring contact pins 44.

[0062] The initially tubular gas connections 40, 41 widen towards the trough 37 to the width of the grids 6, 7, so that a laminar spreading or fanning of the measuring gas 39 occurs and the measuring gas 39 flows over the entire surface of the grids 6, 7. To ensure that the measuring gas 39 does not merely brush against the bars 8, 9 of the grids 6, 7, but flows completely around them, the bottom of the trough 37 can be convexly curved in the longitudinal direction of the grids 6, 7 and the grid bars 8, 9. In addition, the bottom of the other trough 38 in the cover 35 can be correspondingly concavely curved in the longitudinal direction of the grid 6, 7, so that the distance between the bottoms of both troughs 37, 38 remains largely the same and there is no widening or narrowing of the measuring space in the direction of the flow of the measuring gas 39.

[0063] Grids 6 and 7 are connected in measuring bridge 31, as shown in FIG. 4 is shown.

[0064] When used in, e.g., a gas chromatograph, another identical gas sensor or thermal conductivity detector can be arranged in a reference gas flow with the grids oriented longitudinally to the flow direction, wherein all four grids of the two gas sensors are connected to the electrical measuring bridge 31 and in which the grids of each of the two gas sensors are diagonally opposite each other.

Claims

1. Thermoresistive gas sensor (1) with two identical, flat meshes (6, 7) through which gas (28, 29) can flow, and which consist of a semiconductor material (4) with a predetermined conductivity and are interconnected in sections of an electric measuring bridge (31) which are diametrically opposite one another, wherein each mesh (6, 7) has mesh webs (8, 9) which run in parallel adjacent to one another and are connected electrically in parallel at the ends (10, 11; 12, 13), characterised in that the mesh webs (8, 9) of the two meshes (6, 7) extend alternately adjacent to one another in a shared mesh plane horizontally across a window opening (5) in a carrier plate (2), wherein gas (28, 29) can flow through the flat meshes (6, 7) on the window opening (5) at a right angle to the carrier plate (2).

2. Thermoresistive gas sensor (1) according to claim 1, characterised in that the mesh webs (8, 9) are embodied to run in an s-shape in the mesh plane.

3. Thermoresistive gas sensor (1) according to claim 1 or 2, characterised in that the mesh webs (8, 9) have crosssectional surfaces which vary across the width of the meshes (6, 7).

4. Thermoresistive gas sensor (1) according to one of the preceding claims, characterised in that the semiconductor material is embodied in the form of a semiconductor layer (4) on a semiconductor substrate (2) forming the carrier plate, which extends across the window opening (5) in the semiconductor substrate (2) and is embodied there in the form of the two meshes (6, 7), that the semiconductor layer (4) outside of the window opening (5) carries metallisations (14, 15, 16, 17) in four areas extending at least across the width of the meshes (6, 7), said metallisations connecting the mesh webs (8, 9) electrically to one another at the four ends (10, 11, 12, 13) of the two meshes (6, 7), and that the semiconductor layer (4) contains separating structures (26, 27) outside of the window opening (5), which insulate four ends of the meshes (10, 11, 12, 13) from one another.

5. Thermoresistive gas sensor (1) according to claim 4, characterised in that an insulating layer (3) is embodied between the semiconductor substrate (2) and the semiconductor layer (4).

6. Thermoresistive gas sensor (1) according to claim 4 or 5, characterised in that the four metallisations (14, 15, 16, 17) extend in a direction about a predetermined degree beyond the width of the meshes (6, 7) in order to form there contact surfaces (18, 19, 20, 21) and that the semiconductor substrate (2) in the other direction contains through openings (22, 23, 24, 25) arranged in mirror symmetry with the contact surfaces (18, 19, 20, 21).

7. Flow sensor (32) with two thermoresistive gas sensors (1, 1') of the same construction arranged one behind the other in a gas flow to be measured and with its meshes (6, 7; 6', 7') at right angles to the flow direction according to one of claims 1 to 5, wherein all four meshes (6, 7;, 6', 7') of the two gas sensors (1, 1') are interconnected to form the electric measuring bridge (31), in which the meshes (6, 7; 6',7') of each of the two gas sensors (1, 1') are diametrically opposite one another in each case.

8. Flow sensor (32) with two thermoresistive gas sensors (1, 1') of the same construction arranged one behind the other in a gas flow to be measured and with its meshes (6, 7; 6', 7') at right angles to the flow direction according to claim 6, characterised in that all four meshes (6, 7; 6', 7') are interconnected to form the electric measuring bridge (31), in which the meshes (6, 7 6', 7') of each of the two gas sensors (1, 1') are diametrically opposite one another in each case and that the two gas sensors (1, 1') are offset by 180° relative to one another so that the contact surfaces (18', 19', 20', 21') of the lower gas sensor (1') are accessible through the openings (24, 25, 22, 23) of the gas sensor (1) disposed thereupon.

9. Thermal conductivity detector (33) with a thermoresistive gas sensor (1) arranged in a measuring gas flow and with the meshes (6, 7) longitudinally relative to the flow direction according to one of claims 1 to 6.

10. Thermal conductivity detector (33) according to claim 9, characterised in that the gas sensor (1) is held between two components (34, 35), each of which contains a cavity (37, 38) which opens toward the meshes (6, 7) and is aligned with the window opening (5) in the carrier plate (2), wherein each of the two cavities in each case has a gas connection or one of the two cavities (37, 38) has two gas connections (40, 41) in the regions of the ends of the meshes (6, 7) and the gas connections (40, 41) extend in the direction of the respective cavity (37) over the width of the meshes (6, 7).

11. Thermal conductivity detector (33) according to claim 9 or 10, characterised in that a further thermoresistive gas sensor of the same construction is arranged in a reference gas flow and with the meshes longitudinally to the flow direction and that all four meshes of the two gas sensors are interconnected to form the electric measuring bridge (31), in which the meshes of each of the two gas sensors are diametrically opposite one another in each case.