SENSOR FOR DETECTING AT LEAST ONE PROPERTY OF A FLUID MEDIUM IN A MEASURING SPACE

Description

State of the art

[0001] A variety of sensors, sensor elements, and methods for detecting at least one property of a fluid medium in a measuring chamber are known from the prior art. These can, in principle, be any properties of a gaseous or liquid fluid medium, whereby one or more properties can be detected. The invention is described below, without limiting further embodiments and applications, in particular with reference to sensor elements for detecting a gas, especially the H₂ content in a measuring gas.

[0002] Sensor elements of the type described here are used in a wide variety of fields, such as automotive engineering, process engineering, chemistry, and mechanical engineering, particularly for determining gas concentrations. For example, determining hydrogen concentrations, such as in an air-hydrogen mixture, plays a crucial role in the application of hydrogen fuel cell systems. Safety-relevant applications are also relevant here. An air-hydrogen mixture becomes ignitable at a hydrogen content of approximately 4%. Sensor elements for detecting hydrogen can be used, for example, in hydrogen fuel cell vehicles to detect hydrogen leakage due to damage or defects and, by coupling them to appropriate systems, to trigger warning signals and / or protective measures.Therefore, several hydrogen sensors are needed per fuel cell vehicle, which are either installed in the exhaust system (exhaust) or operate under atmospheric conditions (ambient).

[0003] A variety of measurement principles can be used for such hydrogen sensors. These include, among others, the following: thermal conductivity, catalytic pellistor, electrochemical cell, semiconducting metal oxide, chemiresistor, and field-effect transistor. Sensor elements for measuring thermal conductivity are known, for example, from DE 10 2005 058 832 A1 and DE 10 2014 202 169 A1.

[0004] DE112014002928T5 discloses a gas sensor in an exhaust pipe, comprising a thermal conductivity sensor element, a branch element with flow pipe and interior, and an aperture.

[0005] Despite the advantages of existing sensor elements for detecting at least one property of a fluid medium, there is still room for improvement. If thermal conductivity is to be measured with a sensor element consisting of a thin membrane, convection of the exhaust gas past the membrane must be avoided, as this distorts the measurement signal. Instead, the fluid medium must be as still as possible at the membrane. However, in the exhaust stream of a fuel cell, gas flow velocities of up to 100 m / s are present. Additionally, a significant amount of water is produced, some of which is present as droplets, which can also distort the measurement. Disclosure of the invention

[0006] Within the scope of the present invention, a sensor for detecting at least one property of a fluid medium in a measuring space is therefore proposed, which at least largely avoids the disadvantages of known sensors for detecting at least one property of a fluid medium in a measuring space, and which offers sufficient sensitivity, measuring range, response time and selectivity with regard to the requirements in automotive engineering.

[0007] A sensor according to the invention for detecting at least one property of a fluid medium in at least one measuring chamber, in particular for detecting an H 2 content in a measuring gas, comprises at least one sensor element designed to detect a thermal conductivity of the fluid medium and to output a measurement signal.

[0008] The measuring chamber can, in principle, be any open or closed space in which the fluid medium, in particular the measuring gas, is contained, and / or through which the fluid medium, in particular the measuring gas, flows.

[0009] The sensor element is designed, for example, as a sensor chip with a heated measuring membrane. The sensor chip comprises, for example, a chip surface. The chip surface has a measuring surface that is exposed to the fluid medium and a solid surface. Conductive traces of a sensor circuit with at least one heating element are applied to the measuring surface. Conductive traces of the sensor circuit with at least one temperature sensor are applied to the solid surface.

[0010] The sensor chip can be mounted on a socket, in particular a glass or silicon socket. A connection between the sensor chip and the socket can be achieved using anodic bonding.

[0011] An access channel can also be formed in the base. For example, the access channel can be implemented as a bore. The access channel can be created by scribing, sawing, or etching processes. With this design, it is possible to create a space on the side of the measuring surface facing the base, i.e., the underside of the measuring surface. This space can, for example, be shaped like a cavern.

[0012] Within the scope of the present invention, a membrane can be understood as a thin structure which, like a skin or film, has a large surface area relative to its thickness.

[0013] The sensor also includes a branching element that defines an interior space. This branching element is designed to divert a portion of the fluid medium from the measuring chamber into this interior space.

[0014] The sensor also includes at least one aperture. The sensor element is fluidly connected to the interior via the aperture.

[0015] In the context of the present invention, an orifice can be understood as a component that causes a cross-sectional narrowing. For this purpose, the orifice has an opening through which a fluid medium can flow. The opening has a significantly smaller cross-sectional area than the areas adjacent to the orifice.

[0016] In the sensor according to the invention, the branching element captures a portion of the fluid medium, which may be in a flowing motion, and directs it towards the sensor element. As the fluid medium passes through the aperture, it exhibits convective and diffusive components. Along this flow path, the convective flow is calmed by one or more apertures.

[0017] The orifice can be designed to reduce turbulence in the fluid medium. This calms the flow of the fluid medium.

[0018] The orifice can be designed to reduce the convective components of the fluid medium. This calms the flow of the fluid medium and, in particular, reduces convective components, thus significantly reducing distortion of the measurement signal.

[0019] The aperture can be designed such that the fluid medium reaches the sensor element primarily by diffusion. This results in such a low movement of the fluid medium at the sensor element that no signal error is caused by the remaining convective component. Accordingly, the fluid medium reaches the sensor element predominantly by diffusion. The term "primarily by diffusion" can be understood to mean that the velocity of the fluid medium from the measuring chamber is reduced so significantly that, in the area around the sensor element, it is only 0.2%, preferably only 0.1%, and even more preferably only 0.05% of the velocity in the measuring chamber.

[0020] The aperture can be designed to essentially prevent heat transfer from the sensor element to the fluid medium. This prevents distortion of the thermal conductivity measurement, and the remaining convective component eliminates any signal error. The phrase "essentially preventing heat transfer from the sensor element to the fluid medium" can be understood to mean that heat transfer is negligible. In other words, the flow at the sensor element is so low that heat dissipation from it by a convective component of the fluid medium is so minimal that the measurement of the thermal conductivity of the fluid medium is only minimally affected, and the measurement error relative to the maximum signal of the sensor element is therefore no greater than 5%, preferably no greater than 3%, and even more preferably no greater than 2%.However, it is understood that heat transfer also takes place without convection, namely through heat conduction, since this is the measuring principle of the sensor.

[0021] The orifice(s) is / are preferably designed such that their opening area reduces the cross-sectional area to 1 / 10 of the flow cross-section of the adjacent space.

[0022] The branching element is designed to reduce the flow velocity of the fluid medium as it branches off from the measuring chamber into the interior. The branching element thus captures a portion of the fluid medium, which may be in motion, and directs it as quickly and as short a distance as possible towards the sensor element. This increases the exchange of the fluid medium and the response time of the sensor element.

[0023] The branching element can be designed such that the portion of the fluid medium that can be branched from the measuring chamber into the interior is no more than 10%, preferably no more than 5%, and even more preferably no more than 3% of the volume flow of the fluid medium in the measuring chamber. This ensures that only a small but representative proportion of the main flow of the fluid medium passes through the sensor, and any water droplets or other solid particles present cannot reach the sensor element due to their inertia.

[0024] The sensor can further comprise multiple apertures, with the apertures arranged sequentially in the flow direction from the measuring chamber to the sensor element such that the convective components of the fluid medium decrease in the flow direction. By connecting several apertures in series, the flow of the fluid medium can be calmed to such an extent that the movement of the fluid medium at the sensor element is so minimal that no signal error is caused by the remaining convective component.

[0025] The branching element is designed as a Venturi nozzle with a collection tube. Due to its design as a Venturi nozzle, large water droplets, due to their inertia, can pass through the sensor and are thus kept away from the sensor element. Very small droplets may follow the flow, but their small size makes them insignificant for measurement purposes.

[0026] In the context of the present invention, a Venturi nozzle is understood to be a component in the form of a pipe section, as defined in the characterizing part of claim 1. The operating principle is based on the fact that when a fluid flows through a Venturi nozzle, the dynamic pressure (stagnation pressure) is at its maximum and the hydrostatic pressure at its minimum at the narrowest point of the pipe. The fluid velocity increases proportionally to the cross-sectional area as it flows into the narrower section because the same mass flows through the entire pipe per unit time (continuity law). This causes the pressure in the intake pipe located in the narrow section to decrease. This creates a differential pressure, which can be used in measuring instruments or for the suction of liquids or gases. For ideal liquids (incompressible and frictionless), the pressure difference is given by Bernoulli's equation. For ideal gases, the extended Bernoulli equation applies.

[0027] The branching element can be designed to essentially prevent liquid droplets and / or particles from reaching the sensor element. The term "essentially prevent" means that no droplets and / or particles above a predetermined size can reach the sensor element. Only very small droplets or particles, i.e., below the predetermined size, may follow the flow, but due to their small size, they are not critical for the measurement.

[0028] The measuring chamber can be a flow pipe, in particular an exhaust pipe of a fuel cell. Accordingly, the sensor is also suitable for areas where comparatively high flow velocities are present. Brief description of the drawings: Further details and features of the invention will become apparent from the following description of preferred embodiments, which are shown schematically in the figures.

[0029] It shows: Figure 1 a cross-sectional view of a sensor according to an embodiment of the present invention. Embodiments of the invention

[0030] Figure 1Figure 10 is a cross-sectional view of a sensor 10 for detecting at least one property of a fluid medium 12 in at least one measuring chamber 14, in particular for detecting the hydrogen content in a measuring gas 16. The sensor 10 can be configured for use in a hydrogen fuel cell vehicle. However, other applications are also possible. The sensor 10 can, in particular, comprise one or more additional functional elements not shown in the figures, such as electrodes, electrode leads and contacts, multiple layers, or other elements. Accordingly, the sensor 10 can be installed in the exhaust system of the hydrogen fuel cell vehicle (exhaust) or operate under atmospheric conditions (ambient). Consequently, the measuring chamber can be an exhaust system or the interior of the hydrogen fuel cell vehicle.In the illustrated embodiment, the measuring chamber 14 is a flow pipe in the form of an exhaust pipe 18.

[0031] The sensor 10 is attached to the exhaust pipe 18, for example by screwing, plugging in, or welding. The fluid medium, in the form of a gas or a gas mixture containing hydrogen, flows in the exhaust pipe 18 in a main flow direction 20. The flow velocity can be up to 100 m / s.

[0032] Sensor 10 comprises a sensor element 22. The sensor element 22 is designed to detect the thermal conductivity of the fluid medium and to output a measurement signal. For this purpose, the sensor element 22 is designed as a sensor chip 24 with a heatable membrane 26. The membrane 26 is heated by means of a heating element (not shown) located on the membrane 26. The measurement signal is output by means of a temperature sensor conductor (not shown) located on a surface of the sensor chip 24.

[0033] The sensor 10 also has a branch element 28. The branch element 28 defines an interior space 30. The branch element 28 is designed to divert a portion of the fluid medium 12 from the measuring chamber 14 into the interior space 30. For this purpose, the branch element 28 has at least one opening 32. Furthermore, the branch element 28 can be connected to the measuring chamber 14. Figure 1Figure 28 shows the branching element 28 in its connected state. The branching element 28 is designed to reduce the flow velocity of the fluid medium 12 as it branches off from the measuring chamber 14 into the interior space 30. In other words, the branching element 28 is designed such that the flow velocity of the fluid medium 12 decreases as it enters the interior space 30. Furthermore, the branching element 28 is designed such that the portion of the fluid medium 12 that can be branched off from the measuring chamber 14 into the interior space 30 is no more than 10%, preferably no more than 5%, and even more preferably no more than 3% of the volume flow of the fluid medium 12 in the measuring chamber 14. As a result, only a small but representative proportion of the main flow of the fluid medium 12 passes through the sensor 10, and any water droplets or other solid particles present cannot reach the sensor element 22 due to their inertia.In other words, most of the fluid medium from the measuring chamber 14 flows past the sensor 10, and of the part that enters the branching element 28, only a part flows into the interior 30.

[0034] As in Figure 1As shown, the branch element 28 is designed as a Venturi nozzle 34 with a discharge pipe 36. The branch element 28 thus has a flow pipe 38 that extends parallel to the main flow direction 20 of the fluid medium in the exhaust pipe 18. The fluid medium 12 can flow through the flow pipe 38. The flow pipe 38 has at least a first pipe section 40 with a first cross-sectional area 42 and a second pipe section 44 with a second cross-sectional area 46. The first pipe section 40 faces the main flow direction 20, and the second pipe section 44 faces away from the main flow direction 20. In other words, the first pipe section 40 is located at the upstream end of the flow pipe 38, and the second pipe section 44 is located at the downstream end of the flow pipe 38. The first cross-sectional area 42 is larger than the second cross-sectional area 46.It is understood that the flow pipe 38 can have further pipe sections. For example, the flow pipe 38 can have a third pipe section located further downstream than the second pipe section 44, and which has a third cross-sectional area. The third cross-sectional area is larger than the second cross-sectional area and can, for example, be the same size as the first cross-sectional area.

[0035] The sampling tube 36 defines the interior space 30. In the illustrated embodiment, the sampling tube 36 is oriented essentially perpendicular to the flow tube 38. However, it is explicitly emphasized that other orientations of the sampling tube 36 are also possible. The orientation of the sampling tube 36 relative to the flow tube 38 is not crucial for the measuring principle of the sensor 10.

[0036] The intake pipe 36 is bent. The intake pipe 36 is connected to the flow pipe 38 in the region of the cross-sectional constriction of the flow pipe 38. The intake pipe 36 is U-shaped. Thus, the intake pipe 36 has a first opening 48, which is fluidly connected to the first pipe section 40, and a second opening 50, which is fluidly connected to the second pipe section 44.

[0037] The sensor 10 further comprises at least one aperture 52. The sensor element 22 is fluidly connected to the interior 30 by means of the aperture 52. Accordingly, the sensor element 22 is not directly connected to the measuring chamber 14, but rather via the interior 30 of the branching element 28. The aperture 52 is designed to reduce turbulence in the fluid medium 12. The aperture 52 is designed to reduce convective components of the fluid medium 12. In particular, the aperture 52 is designed such that the fluid medium 12 reaches the sensor element 22 primarily by diffusion. Furthermore, the aperture 52 is designed to essentially prevent heat transfer from the sensor element 22 to the fluid medium 12. For this purpose, the aperture 52 is designed with an aperture opening 54 that has a comparatively small cross-section.

[0038] As in Figure 1As shown, the sensor 10 in the illustrated embodiment has several apertures 52. The apertures 52 are arranged one behind the other in the flow direction from the measuring chamber 14 to the sensor element 22 such that the convective components of the fluid medium 12 decrease in the flow direction. More precisely, the sensor 10 in the illustrated embodiment has a first aperture 56 with a first aperture opening 58, which divides the interior 30 into a first interior section 60 and a second interior section 62. The first interior section 60 borders the flow pipe 38 or the exhaust pipe 18. Furthermore, the sensor 10 has a second aperture 64 with a second aperture opening 66. The second aperture 64 is spaced apart from the first aperture 56 and borders the second interior section 62. The sensor element 22 is fluidly connected to the second interior section 62 via the second aperture opening 66.The second aperture 66 has a smaller cross-sectional area than the first aperture 54.

[0039] The operating mode of sensor 10 is described below. The fluid medium 12 flows in the exhaust pipe 18, which constitutes the measuring chamber 14, at a relatively high velocity of up to 100 m / s. A portion of the fluid medium also flows through the flow pipe 38 of the branching element 28. As described above, the discharge pipe 36 is U-shaped in the area of ​​the interior 30, and more precisely in the first interior section 60. This causes the fluid medium 12 to be partially diverted from the measuring chamber 14, or rather from the flow pipe 38, and to enter the interior 30, or initially the first interior section 60, through the first opening 48. A portion of the diverted fluid medium 12 is deflected by approximately 180° and exits back into the flow pipe 38 through the second opening 50, as indicated by arrow 68.Smaller water droplets 70 or particles can follow this flow deflection, but also exit back into the flow tube 38 or have no influence on the measurement signal of the sensor element 22. Larger water droplets 72 or particles cannot follow the flow deflection and continue to flow through the flow tube 38 with the fluid medium 12 in the main flow direction 20. The branching element 28 is designed in this way to essentially prevent droplets and / or particles from reaching the sensor element 22.

[0040] Another portion of the diverted fluid medium 12 enters the second interior section 62 through the first aperture 58 of the first aperture 56 by means of diffusion and convection, as indicated by arrow 74. From the second interior section 62, the fluid medium 12 reaches the sensor element through the second aperture 66 of the second aperture 64, primarily by diffusion, as indicated by arrow 76. Convective flow components of the fluid medium 12 at contact with the membrane 26 are either absent or negligible, allowing the sensor element 22 to perform an unadulterated measurement of the thermal conductivity of the fluid medium 12. Accordingly, a successive reduction in the flow velocity of the fluid medium 12 occurs, starting from the measuring chamber 14, through the first interior section 60 and the second interior section 62, and finally in the area adjacent to the sensor element 22.

Claims

1. Sensor (10) for detecting at least one property of a fluid medium (12) in a measuring chamber (14), in particular for detecting an H2 component in a measurement gas (16), comprising: at least one sensor element (22) designed to detect a thermal conductivity of the fluid medium (12) and to output a measurement signal, the sensor (10) furthermore defining a branching element (28) that defines an interior (30), the branching element (28) being designed to branch off a portion of the fluid medium (12) from the measuring chamber (14) into the interior (30), at least one diaphragm (52, 56, 64), the sensor element (22) being fluid-connected to the interior (30) by means of the diaphragm (52, 56, 64), and an exhaust pipe (12), in which the fluid medium (12) flows in a main direction of flow (20), characterized in that the branching element (28) is in the form of a venturi nozzle (34) with a U-shaped outlet pipe (36), the branching element (28) having a flow pipe (38) that extends parallel to the main direction of flow of the fluid medium in the exhaust pipe (18), the flow pipe (38) having a first pipe section (40), facing the main direction of flow (20), with a first cross-sectional area (42) and a second pipe section (44), facing away from the main direction of flow (20), with a second cross-sectional area (46), the first cross-sectional area (42) being larger than the second cross-sectional area (46), the outlet pipe (36) being connected to the flow pipe (38) in the region of a cross-sectional narrowing of the flow pipe (38), the outlet pipe (36) having a first opening, which is fluid-connected to the first pipe section (40), and a second opening, which is fluid-connected to the second pipe section (44).

2. Sensor (10) according to the preceding claim, the diaphragm (52, 56, 64) being designed to reduce turbulence in the fluid medium (12).

3. Sensor (10) according to either of the preceding claims, the diaphragm (52, 56, 64) being designed to reduce convective portions of the fluid medium (12).

4. Sensor (10) according to one of the preceding claims, the diaphragm (52, 56, 64) being designed in such a way that the fluid medium reaches the sensor element (22) substantially by means of diffusion.

5. Sensor (10) according to the preceding claim, the diaphragm (52, 56, 64) being designed to substantially prevent a transfer of heat from the sensor element (22) to the fluid medium.

6. Sensor (10) according to one of the preceding claims, the branching element (28) being designed to reduce a flow velocity of the fluid medium (12) when branching it off from the measuring chamber (14) into the interior (30).

7. Sensor (10) according to one of the preceding claims, furthermore comprising multiple diaphragms (52, 56, 64), the diaphragms (52, 56, 64) being arranged in succession in a direction of flow as seen from the measuring chamber (14) to the sensor element (22), in such a way that convective components of the fluid medium (12) as seen in the direction of flow decrease.

8. Sensor (10) according to one of the preceding claims, the branching element (28) being designed to substantially prevent drops of liquid and / or particles from entering the sensor element (22).

9. Sensor (10) according to one of the preceding claims, the measuring chamber (14) being a flow pipe, in particular an exhaust pipe (18) of a fuel cell.