Flow sensor and photoacoustic detector

Abstract

The invention relates to a flow sensor (1) comprising a substrate (2) with a groove (4), wherein a resistance meander (3) made of several platinum thin-film layers is arranged on the substrate (2), the layers having webs (6) that cross the groove (4), wherein a cover (16) is arranged on the substrate (2) covering the resistance meander (3), the cover having a groove (4) aligned with the groove (4) in the substrate (2), so that the grooves (4) form a flow channel for the medium, wherein one or more openings (5) are arranged in the substrate (2) and / or in the cover (16) for the lateral flow of a medium into the groove (4).The invention further relates to a photoacoustic detector (8) comprising a gas-tight housing (9) enclosing a detector chamber (10) and a buffer chamber (11) separated from the detector chamber (10) by a partition (14) with a bore (17), wherein a transparent inlet window (12) is arranged in the housing (9) between an environment (13) and the detector chamber (10), through which the detector chamber (10) and a medium located therein can be irradiated, wherein the buffer chamber (11) is designed as a Helmholtz resonator, wherein a flow sensor (1) is arranged in or on the partition (14) and is positioned such that a pressure change of the medium in the detector chamber (10) as a result of irradiation leads to a compensating flow via the flow sensor (1) into the buffer chamber (11).

Description

[0001] The invention relates to a flow sensor and a photoacoustic detector.

[0002] Flow sensors based on the anemometer principle are widespread and easy to use in many applications. The principle is based on the effect that a heated body or surface can release energy into the environment. The surrounding media can be gaseous, liquid, or solid. The energy release depends on the surface area and physical properties of the medium, as well as the flow velocity of the passing medium and the type of flow (turbulent or laminar). At zero flow, the energy release is determined solely by convection, the physical properties of the medium, and the surface of the heated body.

[0003] Flow sensors that operate according to the anemometer principle are described in numerous patent applications and patents. DE 42 24 518 C2 describes a silicon arrangement in which a meandering resistance structure is partially mounted on a dielectric support structure that has a flow channel extending through the silicon body. DE 10 2013 209 951 A1 describes arrangements in which thermocouple pairs are used for temperature measurement. DE 10 2009 029 169 A1 describes an arrangement in which vortex bodies are mounted. DE 10 2010 042 307 A1 describes an arrangement in which heaters and a temperature sensor are mounted separately.

[0004] Furthermore, flow sensors are known that are capable of detecting gas flows in a detector chamber in a miniaturized photoacoustic detector. These flows are generated by energy absorption due to the expansion of the preferably gaseous medium. However, known approaches using pressure sensors are usually too insensitive. The use of miniature microphones is limited by the high lower frequency limit of approximately 50 Hz to 100 Hz. The flow sensors used in large, conventional photoacoustic detectors are too large and cannot be used in the proposed detector chambers.

[0005] AT 404 758 B describes a miniaturized semiconductor probe for detecting thermophysical characteristics in fluid media, comprising at least one measuring cell arranged in a probe body and containing the medium, in which at least one electrical / thermal emitter / absorber / sensor probe element is arranged that comes into contact with the medium, and devices for supplying / removing medium to / from the measuring cell and energy to the thermal emitter / absorber / sensor elements, as well as devices for controlling, regulating, and acquiring and / or processing measurement data. The probe element is designed as a preferably bridge-like web element arranged in the measuring cell, freely surrounded by the fluid on all sides. It is provided that the web element is formed with at least one thermal emitter / absorber / sensor element based on a semiconductor material, applied in a layered manner to at least one layered carrier made of an insulating material.

[0006] US 4,542,650 A describes a flowmeter particularly suitable for monitoring flow during semiconductor manufacturing. The flowmeter measures the mass flow rate of liquids and is fabricated by depositing spaced ridges on a silicon substrate and etching liquid flow grooves into a substrate surface that extend beneath the spaced ridges. The ridges comprise a low thermal conductivity layer deposited in spaced, aligned portions of the substrate and electrically resistive paths deposited on the layer portions. The ridges function as temperature sensors and / or heaters. Heat is applied to the flowing liquid, and a differential temperature is measured in a bridge circuit, as is known in the art for flow rate measurement.A cover has etched grooves that align with the substrate grooves to form a fluid passageway, where the land bisects the passageway, forming an "airfoil" bridge element that extends completely over (or cantilevers) the flat surfaces of the substrate on either side of the groove. Typically, multiple rows of lands and multiple grooves are used to provide redundancy of the flowmeter channels in case contaminants clog a particular channel.

[0007] DE 41 16 280 A1 describes a gas analyzer with a pulsed selective radiation source, a measuring chamber having a beam-permeable end face and supply and discharge lines for the measuring gas, and a pneumatic / electrical converter with a pressure-conducting connection to the measuring chamber, wherein a resistance flow sensor operating according to the anemometer principle is used as the converter, which is connected on the one hand to the measuring chamber and on the other hand to a gas-filled compensation chamber, and wherein a protective gas source is connected to the measuring chamber and the compensation chamber via two line branches each containing a flow resistance, wherein the resistance flow sensor is contained in a further line connecting both line branches.

[0008] US 5,869,749 A describes a microstructured integrated optofluidic or optoacoustic sensor comprising a rapidly changing intensity or pulsed light source, an interference filter, a gas cavity, which may or may not be an optical and / or acoustic resonator tuned to a specific light wavelength or sound frequency, into which the detected gas can diffuse via a filter, and a fluid or pressure sensor for detecting the heating and cooling and the resulting expansion and contraction of the gas due to the absorption of light at the specific wavelength by the detected specific gas. The presence of other gases is inferred from the detected gas.

[0009] The invention is based on the object of providing a novel flow sensor and a novel photoacoustic detector.

[0010] The object is achieved according to the invention by a flow sensor having the features of claim 1 and by a photoacoustic detector having the features of claim 7.

[0011] Advantageous embodiments of the invention are the subject of the subclaims.

[0012] A flow sensor is proposed comprising a substrate with a groove, wherein a resistance meander consisting of several platinum thin-film tracks is arranged on the substrate, each track having webs that cross the groove. The groove can be closed at its ends. In particular, the groove can initially be introduced continuously in the substrate and later closed during assembly of the flow sensor in order to change the flow direction.

[0013] According to the invention, a cover is arranged on the substrate, covering the resistance meander. The cover has a groove aligned with the groove in the substrate, so that the grooves form a flow channel for the medium. The groove can be closed at its ends. In particular, the groove in the cover can initially be introduced continuously and later closed during assembly of the flow sensor.

[0014] According to the invention, one or more openings are arranged in the substrate and / or in the cover for the lateral flow of a medium into the groove.

[0015] According to the invention, an opening in the cover and an opening in the substrate are arranged in such a way that a medium entering the groove through one of the openings passes over several of the webs or all of the webs before reaching the other opening and exiting there.

[0016] In one embodiment, a temperature sensor is arranged on the substrate, which can also be used as a heater.

[0017] In one embodiment, the webs are approximately 10 µm wide and / or approximately 1 µm thick, and / or five to ten webs are provided. Web widths of 5 µm and a thickness of approximately 0.8 µm are technologically feasible. The number of webs can be up to 100 or more.

[0018] In one embodiment, the substrate is formed as a ceramic substrate.

[0019] In one embodiment, the substrate has dimensions of 2 mm by 2 mm by 1 mm, although 1 mm by 1 mm by 0.5 mm are also useful.

[0020] In one embodiment, the webs cross the groove at a right angle or approximately at a right angle. The openings can, for example, open into the groove at a right angle or approximately at a right angle. The webs can be connected in parallel or in series, or a combination of both.

[0021] According to one aspect of the present invention, a photoacoustic detector is proposed, comprising a closed, optically tight and gas-tight housing (the tightness should be maintained over the lifetime of the detector) which encloses a detector chamber and a buffer chamber separated from the detector chamber by an optically tight partition wall which has a bore, wherein a transparent entrance window is arranged in the housing between an environment and the detector chamber, through which window the detector chamber and a medium located therein can be irradiated, wherein the buffer chamber is designed as a Helmholtz resonator, wherein a flow sensor as described above is arranged in or on the partition wall and positioned such that a pressure change of the medium in the detector chamber as a result of irradiation leads to a compensating flow via the flow sensor into the buffer chamber. The irradiation is, for example, pulsed at a frequency of approx.5 Hz to 10 Hz.

[0022] In one embodiment, the detector chamber and / or the buffer chamber have / have a closed volume of less than 1 cm 3 on.

[0023] The present invention proposes a flow sensor capable of detecting gas flows between a detector chamber and a buffer chamber in a miniaturized photoacoustic (PA) detector, which are generated due to the expansion of the medium by energy absorption.

[0024] The flow sensor can be designed in such a way that, with dimensions of, for example, 2 mm x 2 mm x 1 mm, it is capable of detecting flows with cut-off frequencies of 0 to 50 Hz, in particular 5 Hz to 10 Hz. The measurement sensitivity can be such that the energy input into a closed volume of less than 1 cm 3The resulting pressure changes lead to a measurable flow at the sensor, which can be converted into a corresponding output voltage that correlates with the energy consumption of the PA sensor. For this purpose, the measuring current can be pulsed.

[0025] Embodiments of the invention are explained in more detail below with reference to drawings.

[0026] Showing: Fig. 1 a schematic view of a flow sensor, and Fig. 2 a schematic view of a photoacoustic detector.

[0027] Corresponding parts are provided with the same reference numerals in all figures.

[0028] Fig. 1 is a schematic view of a flow sensor 1.

[0029] For example, it can be provided that the flow sensor 1 has a substrate 2, for example a ceramic substrate 2. Arranged on the substrate 2 is a resistance meander 3 consisting of several freely suspended platinum thin-film tracks (for example, produced in a batch process). The meanders have webs 6 that run at an angle of, for example, 90° over a groove 4 in the substrate 2 and are therefore cooled by a medium flowing through the groove 4, also called a flow channel. In order to generate a turbulent flow in the medium, one or more openings 5 ​​are arranged in the substrate 2, for example at right angles to the groove 4, through which the flow can approach the groove 4, so that vortices can arise at a separation edge 15, which are amplified at the webs 6 and lead to additional vortices and turbulence, thus enabling improved energy transfer from the medium to the webs 6.The separation edge 15 is the edge at which the flow is deflected by 90 degrees. This can also be created by a constriction in the groove 4. A cover 16 can be arranged on the substrate 2, which has a groove 4 aligned with the groove 4, so that the grooves 4 form a flow channel for the medium. An opening 5 can also be arranged in the cover 16, for example at a right angle to the groove 4, through which the flow can approach the groove 4, so that vortices can arise at a separation edge 15, which are amplified at the webs 6 and lead to additional vortices and turbulence, thus enabling improved energy transfer from the medium to the webs 6. The opening 5 in the cover 16 and the opening 5 in the substrate 2 can be arranged such that a medium entering the groove 4 through one of the openings 5 ​​sweeps over several of the webs 6 or all of the webs 6 before reaching the other opening 5.

[0030] To suppress and / or compensate for temperature influences, a temperature sensor 7 is formed on the flow sensor 1, in particular the substrate 2, which can also be used as a heater, so that the temperature of the flow sensor 1 can be kept constant. The temperature sensor 7 can be made of platinum and covered by a glass layer or glass passivation, thus protecting it.

[0031] For example, the webs 6 of the resistance meander 3 can be approximately 10 µm wide and approximately 1 µm thick. For example, five to ten or more webs 6 can be provided.

[0032] The groove 4 is in Fig. 1 is shown open at its ends. This may be due to manufacturing constraints if the flow sensor 1 is manufactured in a batch process. For the use of the flow sensor 1, the ends of the groove can be closed. Alternatively, the flow sensor 1 could be manufactured with a groove closed at the ends.

[0033] Fig. Figure 2 is a schematic view of a photoacoustic detector 8, comprising a particularly gas-tight housing 9, which encloses a detector chamber 10 and a buffer chamber 11 separated from the detector chamber 10 by a partition 14 with a bore 17. Furthermore, an entrance window 12 is arranged in the housing 9 between an environment 13 and the detector chamber 10, through which the detector chamber 10 and a medium located therein can be irradiated. The entrance window 12 is permeable to infrared radiation, which can be absorbed by the irradiated medium. The buffer chamber 11 is designed as a Helmholtz resonator and is shielded from the radiation by the partition 14. The flow sensor 1 made of Fig.1. The medium can flow from the detector chamber 10 via the bore 17 into the aperture 5 in the substrate 2, from there into the groove 4 in the substrate 2 and cover 16, and along the webs 6 to the aperture 5 in the substrate 2 and cover 16, and from there into the buffer chamber 11, or in the reverse direction.

[0034] The flow sensor 1 is capable of detecting in the (miniaturized) photoacoustic detector 8 flows of the medium, in particular a gas, in the detector chamber 10, which are generated due to the expansion of the medium by energy absorption (irradiation).

[0035] The flow sensor 1 can be designed such that, with dimensions of, for example, 2 mm by 2 mm by 1 mm, it is capable of detecting flows with cut-off frequencies of 0 to 50 Hz. The measurement sensitivity can be such that the energy input into a closed volume of the detector chamber 10 and the buffer chamber 11 of, for example, less than 1 cm 3 resulting pressure changes lead to a measurable flow at the flow sensor 1, which is measurable as a change in resistance of the flow sensor 1, which is converted into a voltage by a bridge circuit with the reference sensor, which correlates with the energy absorption of the photoacoustic detector 8.

[0036] The temperature sensor 7, which can also be used as a heater, can be used to keep the average temperature of the medium in the detector chamber 10 and / or the buffer chamber 11 constant.

[0037] The flow sensor 1 is positioned in the housing 9 such that the pressure change in the detector chamber 10 generated by the energy absorption of the medium can flow through the flow sensor 1 and the partition wall 14 with bore 17 into the non-irradiated buffer chamber 11. Electrical connections 18 of the flow sensor 1 for the resistance meander 3 and / or the temperature sensor 7 are routed out of the housing 9 for this purpose. LIST OF REFERENCE SYMBOLS 1 flow sensor 2 Substrate, ceramic substrate 3 resistance meanders 4 grooves 5 Breakthrough 6 bridge 7 Temperature sensor 8 photoacoustic detector 9 housings 10 Detector chamber 11 Buffer chamber 12 entrance windows 13 Surroundings 14 Partition wall 15 Tear-off edge 16 lids 17 Hole 18 electrical connections

Claims

[1] Flow sensor (1), comprising a substrate (2) with a groove (4), wherein a resistance meander (3) made of several platinum thin-film tracks is arranged on the substrate (2), which have webs (6) that cross the groove (4), wherein a cover (16) is arranged on the substrate (2) covering the resistance meander (3), which cover has a groove (4) that is aligned with the groove (4) in the substrate (2), so that the grooves (4) form a flow channel for the medium, wherein one or more openings (5) are arranged in the substrate (2) and / or in the cover (16) for the lateral flow of a medium into the groove (4), characterized by that an opening (5) in the cover (16) and an opening (5) in the substrate (2) are arranged in such a way that a medium entering the groove (4) through one of the openings (5) passes over several of the webs (6) or all of the webs (6) before it reaches the other opening (5). [2] Flow sensor (1) according to claim 1, wherein a temperature sensor (7) is arranged on the substrate (2), which can also be used as a heater. [3] Flow sensor (1) according to one of the preceding claims, wherein the webs (6) are 5 µm to 10 µm wide and / or 0.8 µm to 1 µm thick and / or wherein five to ten or up to 100 webs (6) are provided. [4] Flow sensor (1) according to one of the preceding claims, wherein the substrate (2) is formed as a ceramic substrate (2). [5] Flow sensor (1) according to one of the preceding claims, wherein the substrate (2) has dimensions of 2 mm by 2 mm by 1 mm or 1 mm by 1 mm by 0.5 mm. [6] Flow sensor (1) according to one of the preceding claims, wherein the webs (6) cross the groove (4) at a right angle or approximately at a right angle and / or wherein the openings (5) open into the groove (4) at a right angle or approximately at a right angle. [7] Photoacoustic detector (8), comprising a gas-tight housing (9) which encloses a detector chamber (10) and a buffer chamber (11) separated from the detector chamber (10) by a partition wall (14) with a bore (17), wherein a transparent entrance window (12) is arranged in the housing (9) between an environment (13) and the detector chamber (10), through which window the detector chamber (10) and a medium located therein can be irradiated, wherein the buffer chamber (11) is designed as a Helmholtz resonator, wherein a flow sensor (1) according to one of the preceding claims is arranged in or on the partition wall (14) with a bore (17) and is positioned such that a pressure change of the medium in the detector chamber (10) as a result of irradiation leads to a compensating flow via the flow sensor (1) into the buffer chamber (11). [8] Photoacoustic detector (8) according to claim 7, wherein the detector chamber (10) and / or the buffer chamber (11) have a closed volume of less than 1 cm 3 have.

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