Method for determining fluid flow independent of thermal characteristics of fluid

Abstract

The invention relates to a method for determining the flow of a fluid (1) independent of the thermal characteristics of the fluid, comprising: placing a thermal flow sensor (2) in thermal contact with a fluid flow (3); measuring the flow; placing a thermal characteristic sensor (4) in a measurement chamber (5) in fluid contact with the fluid flow wherein the measurement chamber is completely open to the fluid flow; receiving a portion (6) of the fluid in the measurement chamber of the thermal characteristic sensor such that the portion of the fluid is substantially stationary in the measurement chamber; measuring at least one thermal property of the fluid (# imgabs0 #); and compensating the measured flow rate according to the at least one measured thermal characteristic.

Description

Technical Field

[0001] The present invention generally relates to a method for determining the flow rate of a fluid independently of the thermal properties of the fluid; a method for determining the thermal conductivity (κ) and heat capacity (c p ) method; a thermal characteristic sensor for use with a thermal flow sensor, wherein, during use, a fluid whose flow rate is to be determined flows through the thermal flow sensor; a thermal flow sensor for measuring fluid flow; and a method for manufacturing a thermal characteristic sensor. Background Art

[0002] Thermal flow sensors are used to measure the flow of gases and liquids. There are three main types of thermal flow sensors: anemometer, calorimetric, and time-of-flight. Today, these sensors can be miniature devices.

[0003] All three mentioned thermal flow sensor types typically consist of a heater and a temperature sensor and follow a similar operating principle: power is supplied to the heater to raise its temperature, and the change in temperature distribution across the sensor structure is measured as a measure of flow rate. Several variations are possible, such as where power is held constant and the heater simultaneously serves as a temperature sensor.

[0004] Thermal flow sensors have a simple operating principle and are relatively inexpensive to manufacture. However, they are dependent on the type of flowing medium, more specifically, the thermal properties of gas or liquid. This means that the fluid and its properties must be known for accurate flow measurement.

[0005] In "Modelling and simulation of a thermal flow sensor for determining the flow speed and thermal properties of binary gas mixtures" by YQ Zhu (EUROSENSORS 2016, pp. 1028–1031), a thermal flow sensor consisting of a heater and a downstream temperature sensor is used to measure the thermal conductivity of the gas. The temperature of the downstream temperature sensor within a specific flow region depends solely on the thermal conductivity (κ).

[0006] Christoph J. Hepp et al., “Multi-parameter monitoring of binary gas mixtures: concentration and flow rate by DC excitation of thermal sensor arrays” (Sensors and Actuators A, pp. 32-39, 2017), disclose a thermal sensor capable of simultaneously determining the gas concentration and flow rate of a binary gas mixture under flow conditions. The thermal capacity is determined within a flow range where the flow sensor is insensitive to flow, which limits the sensor's applicability (i.e., the sensor must initially be placed in a relatively "high flow range").

[0007] In "Measurement and simulation of the frequency response of a thermal flowsensor at different flow speeds" (Sensors and Actuators A, 203, pp. 225-233, 2013), DF Reyes used AC excitation to measure the voltage of a sensor near a heater. They exploited the fact that increasing the frequency thins the thermal boundary layer and compresses it close to the wall. Consequently, due to the no-slip condition at the wall, the heat exchange between the heater and the sensor is independent of the flow rate, and the heat transfer is affected only by the physical properties of the gas or fluid, not by the flow.

[0008] U.S. Patent 4,685,331 B1 discloses a thermal mass flow meter and controller comprising a series of flow channels and closed channels micro-etched from a silicon substrate, leaving a monolithic thin film suspended across the channels. The sensor of the thermal mass flow meter comprises a stationary gas channel extending parallel to the etched flow channels (also referred to as "closed channels"). The heat transfer equations described in U.S. Patent 4,685,331 B1 are independent of the sensor flow rate because the measurement is performed in the closed, stationary channel. Therefore, if the composition of the fluid in the flow channel changes, it will take a considerable amount of time to diffuse into the closed channel. Furthermore, different fluids can have different diffusion rates into the closed channel. This field requires immediate response to changes in gas (mixture) composition, making the use of an in-line sensor highly preferred.

[0009] Therefore, fluid-independent, in particular gas-independent, flow measurement remains a significant challenge. Therefore, the object of the present invention is to provide a method which is able to measure flow independently of fluid, in particular independently of gas. Summary of the Invention

[0010] The present invention provides a method for determining the flow rate of a fluid independently of the thermal properties of the fluid, comprising: - placing a thermal flow sensor in thermal contact with the fluid flow; -Measure flow rate; - placing a thermal property sensor in a measurement cavity that is in communication with the fluid flow and preferably adjacent to the fluid flow; - receiving a portion of the fluid in the measurement cavity of the thermal characteristic sensor such that the portion of the fluid is substantially stationary in the measurement cavity, wherein the measurement cavity is fully open to the fluid flow; - measuring at least one thermal property of the fluid ( ), or a combination of the fluid thermal properties ( );as well as - Compensating the measured flow rate according to the at least one measured thermal characteristic.

[0011] The significant advantage of the above method is that the thermal characteristic sensor is arranged in a "dead volume" so that at least one thermal characteristic of the fluid can be measured. ) or a combination of thermal properties ( The cavity is intentionally completely open to the fluid. By knowing such thermal characteristics from the measurement, a flow measurement that is independent of the fluid can be established.

[0012] The above-described technology is particularly suitable for thermal flow sensors within channels, particularly microelectromechanical thermal flow sensors within micromachined channels, and other "fluidic chip" or "lab-on-a-chip" applications. However, the technology can also be used with heaters and / or sensor elements fixed, for example, to the outer walls of the flow channel, where thermal contact with the fluid is established "indirectly." Depending on the sensor type, the thermal contact between the thermal flow sensor and the fluid flow can be direct or indirect; for example, a hot wire sensor would have direct contact, while the windings of a capillary sensor would have indirect contact.

[0013] In the context of this patent application, "substantially stationary" means that the fluid is essentially "at rest," but a small degree of diffusion and movement of fluid particles within the measurement chamber is permitted, even if such diffusion and movement is necessary to reflect changes in fluid composition. In the present invention, this is achieved without compromising measurement accuracy. Grooves, particularly V- and U-shaped grooves, have been found to be particularly suitable for this purpose. Most advantageously, they can be used facing or away from the flow.

[0014] An embodiment of the aforementioned method includes: - using said thermal flow sensor in said fluid flow to additionally measure at least one thermal property.

[0015] Regarding one embodiment, a second thermal flow sensor is added in the fluid stream.

[0016] Preferably, the thermal characteristic measured using the thermal flow sensor is heat capacity (c p ) or density ( ) or volumetric heat capacity ( ).

[0017] In one embodiment regarding the aforementioned method, the thermal property measured on the substantially stationary fluid is thermal conductivity (κ).

[0018] In another embodiment of the above method, a combination of thermal properties is measured simultaneously in the fluid flow and on the substantially stationary fluid, wherein the thermal property measured in the flow is heat capacity (c p ),density( ) or volumetric heat capacity ( ), the thermal property measured on the substantially stationary fluid is thermal conductivity (κ). The volumetric heat capacity ( In addition, it is possible to determine ( ).

[0019] In one embodiment of the aforementioned method, the pressure sensor additionally measures pressure and / or differential pressure to derive viscosity from the measured flow rate and the measured pressure and / or differential pressure. Those skilled in the art can refer to, for example, John van Baar's "Distributed thermal micro sensors for fluid flow," Chapter 2, 2 (2002 ISBN 90-36518288), and / or JJ van Baar et al.'s "Micro-machined structures for thermal measurements of fluid and flow parameters," Journal of Micromechanics and Microengineering, volume 11, issue 4 (July), pp. 311-318, 2001. The latter, like U.S. Patent No. 4,685,331 B1, discloses a sensor in a "dead volume" that necessarily contains a stationary fluid. In this setup, changes in the fluid flow in the channel will also take considerable time to diffuse into the enclosed channel. Furthermore, different fluids can have different diffusion rates into the enclosed channel. Van Baar (2001) also disclosed a larger thermal conductivity sensor measuring 1 mm long, 25 µm wide, and 1 µm thick, as well as a TCR ("thermal coefficient of electrical resistance") sensor using a thick silicon beam measuring 1 mm long, 150 µm wide, and 100 µm thick. The larger the volume, the greater the heat capacity of the structure, and therefore, the slower the sensor's response.

[0020] In one embodiment of the above method, the thermal property sensor includes a heating wire (e.g., a pair of heating wires), a probe, or a winding. Such a heating wire is relatively thin (compared to the volume of the measurement chamber) and, therefore, does not significantly affect the thermal properties of the substantially stationary fluid. Optionally, the thermal property sensor includes two or more heating wires, which can be used as backups for each other or to check for deviations. A particular advantage of guide wires is their fast response, typically <1 ms, which the present invention uses to increase measurement speed. Those skilled in the art will understand that other elements with low thermal mass can be used instead of heating wires.

[0021] In another embodiment of the foregoing method, the thermal property sensor comprises a probe, preferably having a sensor element made of platinum.

[0022] Regarding an embodiment of the aforementioned method, wherein the thermal conductivity (κ) is measured by: - heating the heating wire with a constant current (DC) or very low frequency alternating current (AC) for heating the portion of the fluid, and - During heating of the portion of the fluid, measuring the voltage of the heating wire using a voltage measuring device connected to the heating wire, and correlating the measured voltage with thermal conductivity.

[0023] In one embodiment of the aforementioned method, the heat capacity (c p ) by the following measurements: - activating the heating wire with alternating current (AC); and - During heating of the portion of the fluid, measuring the phase and amplitude of the third harmonic (3ω) of the measured alternating current (AC) voltage of the heating wire using a voltage measuring device connected to the heating wire, and correlating the measured phase and amplitude of the third harmonic of the measured AC voltage to heat capacity.

[0024] Therefore, AC excitation can be advantageously used to measure the voltage of the thermal property sensor. The insight employed is that by increasing the frequency, the thermal boundary layer thickness becomes thinner and can be compressed close to the wall. Therefore, due to the no-slip condition at the wall, the heat exchange between the heater and the sensor does not depend on the flow rate, so the heat transfer is affected only by the physical properties of the gas or fluid, not by the flow. Two physical parameters, thermal conductivity (κ) and volumetric heat capacity ( ), is derived from the phase and amplitude of the third harmonic of the measured AC voltage. A person skilled in the art will consider the type of sensor and other environmental factors when selecting the appropriate frequency. The flow rate itself can be measured by DC excitation, which depends on κ and It will be clear to those skilled in the art that very low frequency AC excitation can be used as an alternative to DC excitation if the frequency is low enough so that the measurement can be completed within a small fraction of the cycle, such as half a cycle or less. Those skilled in the art will of course consider the type of fluid and any additional measurements used for compensation. Thus, by knowing these characteristics from the AC measurement, flow measurement independent of the fluid / gas can be achieved.

[0025] Another aspect of the present invention relates to a method for determining the thermal conductivity (κ) and / or specific heat capacity (c p ) methods, including: - placing a thermal flow sensor in thermal contact with the fluid flow; -Measure flow rate; - placing a thermal property sensor in a measurement cavity, the measurement cavity being in communication with the fluid flow and preferably adjacent to the fluid flow, wherein the measurement cavity is completely open to the fluid flow, such as facing (or facing away from) the flow, for example being arranged in (the flow path of) the fluid flow; - receiving the portion of the fluid in the measurement cavity of the thermal property sensor such that the portion of the fluid is substantially stationary in the measurement cavity, - heating a heating wire of the thermal characteristic sensor with a constant current (DC) or an extremely low frequency alternating current (AC) for heating a portion of the fluid, and - during heating of the portion of the fluid, measuring the voltage of the heating wire using a voltage measuring device connected to the heating wire, and correlating the measured voltage with thermal conductivity; and / or - heating the heating wire of the thermal characteristic sensor using alternating current (AC); and - During the heating of the portion of the fluid, measuring the phase and amplitude of the third harmonic of the measured alternating current (AC) voltage of the heating wire using a voltage measuring device connected to the heating wire, and comparing the measured phase and amplitude of the third harmonic of the measured AC voltage with the heat capacity (preferably volumetric heat capacity, ) are associated.

[0026] In one embodiment, the thermal flow sensor is placed in thermal contact with the fluid flow, including thermal contact through an outer wall of the flow channel.

[0027] Another aspect of the present invention relates to a thermal property sensor for use with a thermal flow sensor, wherein, during use, a fluid whose flow rate is to be determined flows through the thermal flow sensor, comprising: a heating wire configured to be placed in a measurement chamber for receiving a portion of the fluid, wherein the portion of the fluid is substantially stationary in the measurement chamber; the heating wire further configured to be heated by: - a constant current (DC) or a very low frequency alternating current (AC) for heating the portion of the fluid; wherein, during the heating of the portion of the fluid, a voltage across the heating wire is measured using a voltage measuring device connected to the heating wire, wherein the measured voltage is correlated to the thermal conductivity; and / or - alternating current (AC), wherein, during heating of the measured portion of the fluid, the phase and amplitude of the third harmonic of the alternating current (AC) voltage of the heating wire are measured using a voltage measuring device connected to the heating wire, wherein the measured phase and amplitude of the third harmonic of the measured AC voltage are correlated with the heat capacity (preferably volumetric heat capacity, ) are associated.

[0028] In one embodiment of the thermal characteristic sensor, the measurement cavity is a groove having a U-shaped or V-shaped cross-section, wherein the heating wire is suspended within the measurement cavity having the U-shaped or V-shaped cross-section. This embodiment is particularly suitable for maintaining a fluid substantially stationary while allowing for diffusion necessary for fluid exchange.

[0029] In one embodiment of the above-mentioned thermal characteristic sensor, the cross-section of the heating wire is less than 10 µm, such as about 9 µm, about 8 µm, 7 µm, about 6 µm, 5 µm, about 4 µm, 3 µm, about 2 µm or about 1 µm, so as to achieve a small volume and thus obtain a small heat capacity, thereby achieving a fast response.

[0030] In one embodiment of the thermal characteristic sensor, the cross-section of the heating filament is flat on one side, such as in a square, triangular, semicircular, or most preferably rectangular shape. The flat portion of the flat cross-section preferably has a width of less than 10 µm, such as approximately 9 µm, approximately 8 µm, 7 µm, approximately 6 µm, 5 µm, approximately 4 µm, 3 µm, approximately 2 µm, or approximately 1 µm. Furthermore, depending on the stability of the material and conditions, the cross-section preferably has a thickness of 0.1 to 1 µm. Small sensor volumes allow for extremely high speeds. This has been evaluated for silicon: in silicon, the time constant is linearly proportional to volume. Therefore, if all other variables remain constant, reducing the volume by half would double the sensor's speed. In the field of flow sensors, a twice-faster response speed is desirable, and even greater improvements could be achieved by reducing the volume by more than half.

[0031] In one embodiment of the above-mentioned thermal characteristic sensor, the length of the measuring cavity and / or the heating wire with a U-shaped or V-shaped cross-section is 1 to 3 mm, such as 1.5 to 2.5 mm, and / or the width of the measuring cavity with a U-shaped or V-shaped cross-section is 20 to 60 µm.

[0032] Regarding an embodiment of the thermal characteristic sensor, the thermal characteristic sensor is placed to shorten the distance between the thermal characteristic sensor and the thermal flow sensor, that is, the thermal characteristic sensor is not removed from the cavity.

[0033] Another aspect of the present invention relates to a thermal flow sensor for measuring fluid flow, comprising: - a measuring cavity in fluid connection with the fluid flow, and preferably adjacent to the fluid flow, wherein the measuring cavity is completely open to the fluid flow; The aforementioned thermal characteristic sensor, wherein the heating wire is placed in the measuring chamber for accommodating a portion of the fluid, wherein the portion of the fluid is substantially stationary in the measuring chamber.

[0034] Regarding an embodiment of the above thermal flow sensor, wherein the thermal flow sensor is releasably insertable into a flow channel, wherein a fluid flow exists in the flow channel during use, or is fixed to an outer wall of the flow channel when (indirect) thermal contact is achieved through the outer wall of the flow channel.

[0035] Regarding an embodiment of the above-mentioned thermal flow meter or controller, the thermal flow meter or controller includes the above-mentioned thermal characteristic sensor and / or thermal flow sensor.

[0036] Regarding an embodiment of the thermal flow meter or controller, at least one of the thermal characteristic sensor or the thermal flow sensor is a micro-electromechanical (MEMS) device.

[0037] Regarding an embodiment of the above-mentioned thermal flow meter or controller, it also includes a viscosity sensor, an (external) humidity sensor, a CO2 sensor, an (external) temperature sensor, a dielectric or dielectric constant sensor, an additional pressure sensor, a fluid composition sensor or a multi-parameter sensor.

[0038] Another aspect of the present invention relates to use of the thermal flow meter or controller described above in a medical device.

[0039] This application can be used in breathing equipment.

[0040] Another aspect of the present invention relates to a method for manufacturing the above-mentioned thermal sensor flow sensor, comprising the following steps: (1) Depositing support layers on both sides of the wafer, (2) depositing a metal layer on one side of the wafer, (3) patterning the metal layer, (4) patterning the support to form openings for etching the underlying Si, (5) Repeat steps 2 to 4 on the other side of the wafer. (6) Etching the Si wafer to realize a V-groove and a flow sensing element cavity inside the wafer.

[0041] In one embodiment of the aforementioned preparation method: (4) Etching the support layer to form an opening for etching the underlying Si wafer.

[0042] In one embodiment of the aforementioned preparation method, the metal layer includes a Cr / Pt layer.

[0043] In one embodiment of the aforementioned preparation method, the support layer is a SiRN support layer.

[0044] In one embodiment of the aforementioned method: (1) The thickness of the support layer is 1 μm.

[0045] In one embodiment of the aforementioned method: (2) The thickness of the Cr / Pt layer is 20nm / 200nm, In one embodiment of the aforementioned method: (3) Patterning the support layer, which includes etching the Cr / Pt layer.

[0046] In one embodiment of the aforementioned method: (2) The Cr / Pt layer is deposited by sputtering.

[0047] In one embodiment, step 1 uses low pressure chemical vapor deposition (LPCVD).

[0048] In one embodiment, step 6 uses KOH for etching, preferably using a 1:3 KOH in distilled water mixture.

[0049] In one embodiment of the aforementioned thermal flow sensor, the thermal flow sensor includes: a sensor body having a flow section through which, during use, a fluid, the flow of which is to be determined, flows in a flow direction, - A flow sensor device comprising one or more flow sensing elements arranged at one or more locations in said flow section.

[0050] In one embodiment of the aforementioned thermal flow sensor, the thermal flow sensor includes: - Main body; - a first body portion extending from said main body portion; - a second body portion extending from the body portion; Wherein, the flow section is formed between the main body portion, the first main body portion and the second main body portion.

[0051] An embodiment of the aforementioned thermal flow sensor, wherein the measuring cavity is arranged in the main body portion.

[0052] An embodiment of the aforementioned thermal flow sensor, wherein the measuring cavity extends along one side of the flow section.

[0053] An embodiment of the aforementioned thermal flow sensor, wherein the flow sensor device includes a plurality of flow sensing elements arranged at a plurality of positions in the flow section.

[0054] An embodiment of the aforementioned thermal flow sensor, wherein the flow section is open at a side of the flow section that is not defined by the main body portion, the first main body portion, and / or the second main body portion.

[0055] An embodiment of the thermal flow sensor, wherein the first body portion and the second body portion are parallel to each other.

[0056] An embodiment of the aforementioned thermal flow sensor, wherein the flow section has a square or rectangular shape in a plane transverse to the flow direction.

[0057] An embodiment of the aforementioned thermal flow sensor, wherein the plurality of flow sensing elements extend between the first body portion and the second body portion.

[0058] An embodiment of the aforementioned thermal flow sensor, wherein the plurality of flow sensing elements are spaced apart in the flow section in a uniform manner.

[0059] An embodiment of the aforementioned thermal flow sensor, wherein the plurality of flow sensing elements includes three or more flow sensing elements.

[0060] An embodiment of the thermal flow sensor, wherein each flow sensing element includes a pair of flow sensing wires.

[0061] An embodiment of the thermal flow sensor, wherein the mutual distance between the flow sensing wire pairs is 300 to 500 μm, preferably 350 to 450 μm, and more preferably 375 to 425 μm.

[0062] An embodiment of the aforementioned thermal flow sensor, wherein a pair of flow sensing wires of the thermal flow sensor extends between the first body portion and the second body portion.

[0063] An embodiment of the aforementioned thermal flow sensor, wherein the sensor body is formed as a chip.

[0064] An embodiment of the aforementioned thermal flow sensor, wherein the sensor body is attached to or arranged on a printed circuit board (PCB).

[0065] An embodiment of the aforementioned thermal flow sensor, wherein the sensor, preferably at least one of the flow sensing wires, forms part of a Wheatstone bridge.

[0066] An embodiment of the aforementioned thermal flow sensor wherein each of one or more pairs of sensors, preferably flow sensing wires, forms one half of a Wheatstone bridge.

[0067] An embodiment of the aforementioned thermal flow sensor, wherein a fixed resistor disposed on the sensor body forms the other half of the Wheatstone bridge.

[0068] An embodiment of the aforementioned thermal flow sensor, wherein the thermal flow sensor is a micro-electromechanical system (MEMS) component. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The present invention will be explained by means of exemplary embodiments described in the accompanying drawings and the following detailed description of the drawings.

[0070] Figure 1 shows a perspective view of an example embodiment of a thermal flow sensor; Figure 2 A perspective view of an example embodiment of a thermal properties sensor having a measurement cavity is shown; Figure 3 An example embodiment of a heater wire of a thermal property sensor connected to a voltage measuring device is shown; Figure 4 shows a cross section of an example embodiment of a heating wire arranged in a measuring cavity having a V-shaped cross section; Figure 5 A circuit diagram of a Wheatstone bridge is shown; Figure 6 The manufacturing steps for producing a V-shaped measuring cavity in which a heating wire is arranged are shown; Figure 7 Another example embodiment of a flow channel having a thermal flow sensor and a thermal property sensor is shown. DETAILED DESCRIPTION

[0071] Figure 1 A perspective view of an example embodiment of a thermal flow sensor 2 for measuring a fluid flow 3 is shown. The thermal flow sensor 2 includes a measurement chamber 5 in fluid connection with, and preferably adjacent to, the fluid flow 3. Furthermore, the measurement chamber 5 is completely open to the fluid flow 3. The measurement chamber 5 can be disposed within, for example, immersed in, the fluid flow 3. The measurement chamber 5 can be oriented toward (or away from) the fluid flow 3. A thermal property sensor 4 includes the measurement chamber 5. A heating wire 8 is positioned within the measurement chamber 5 for receiving a portion of the fluid 6, such that the portion of the fluid 6 remains substantially stationary within the measurement chamber 5. The thermal flow sensor 2 can be releasably inserted into a flow channel 10 in which the fluid flow 3 is present during use. Figure 1 An embodiment of a chip with pads is shown. However, the skilled person will appreciate that other designs or embodiments are also conceivable.

[0072] The thermal flow sensor 2 can include a sensor body 11 having a flow section 12, and during use, a fluid 1 whose flow rate is to be determined flows through the flow section 12 in a flow direction. A flow sensor device 13 can be provided, including one or more flow sensing elements 14 arranged at one or more positions in the flow section 12. The thermal flow sensor 2 can include a main body portion 15, a first main body portion 16 extending from the main body portion 15, and a second main body portion 17 extending from the main body portion 15. The flow section 12 can be formed between the main body portion 15, the first main body portion 16, and the second main body portion 17. The measuring cavity 5 can be arranged in the main body portion 15. The measuring cavity 5 can extend along one side of the flow section 12. The flow sensor device 13 can include a plurality of flow sensing elements 14 arranged at multiple positions in the flow section 12, such as Figure 1 As shown, there may be three, for example. The flow segment 12 may be open on a side of the flow segment 12 not bounded by the main body portion 15, the first main body portion 16, and / or the second main body portion 17. The first main body portion 16 and the second main body portion 17 may be parallel to each other. The flow segment 12 may have a square or rectangular shape in a plane transverse to the direction of flow 3. A plurality of flow sensing elements 14 may extend between the first and second main body portions 16, 17. The plurality of flow sensing elements 14 may be uniformly spaced within the flow segment 12. Each flow sensing element 14 may include a pair of flow sensing wires 18. The mutual distance between the pairs of flow sensing wires 14 may be 300 to 500 µm, preferably 350 to 450 µm, and more preferably 375 to 425 µm. The pair of flow sensing wires 18 of the thermal flow sensor 2 may extend between the first and second main body portions 16, 17. Such wires 18 have been found to be surprisingly stable and insensitive to small particles (e.g., dust) that may occasionally contaminate the fluid. However, appropriate measures should be taken to prevent overheating. The sensor body 11 may be formed as a chip 19. The sensor body 11 may be attached to or arranged on a printed circuit board (PCB) 20. Figure 5 As shown, each of one or more pairs of flow sensing wires 18 can form one half (R2, R3) of a Wheatstone bridge 21. Those skilled in the art will understand that, to measure a differential flow signal, the arrow shown by R3 should point downward. Fixed resistors R1 and R4, arranged on the sensor body 11, can form the other half of the Wheatstone bridge 21. The thermal flow sensor 2 can be a microelectromechanical system (MEMS) component. In the absence of flow, R1 and R4, and R2 and R3, have the same value, so the output signal of the Wheatstone bridge 21 is zero. In the presence of flow, heat is transferred from the upstream wire to the downstream wire. Consequently, due to the temperature difference between the two wires R2 and R3, a positive output voltage signal or a negative output voltage signal (depending on the flow direction) is generated.

[0073] One or more flow sensing elements 14 may also be arranged at (outside) an outer wall (not shown) of the schematically indicated flow channel 10 in order to establish “indirect” thermal contact with the fluid flow 3 .

[0074] Figure 2 A perspective view of an example embodiment of a thermal property sensor 4 having a measurement cavity 5 is shown in greater detail.

[0075] As in Figure 3 As more clearly shown in FIG, the heating wire 8 can be configured to be heated using a constant current (DC) or an extremely low frequency alternating current (AC) for heating the portion 6 of the fluid 1, wherein, during the heating of the portion 6 of the fluid, a voltage of the heating wire 8 is measured using a voltage measuring device 9 connected to the heating wire 8, wherein the measured voltage is correlated with the thermal conductivity; and / or the heating wire 8 can be configured to be heated using an alternating current (AC), wherein, during the heating of the measured portion 6 of the fluid 1, a phase and amplitude of a third harmonic of the alternating current (AC) voltage of the heating wire 8 is measured using a voltage measuring device 9 connected to the heating wire 8, wherein the phase and amplitude of the third harmonic of the measured AC voltage is correlated with the thermal conductivity c p associated.

[0076] like Figure 4 As shown, the measuring cavity 5 can have a V-shaped cross section, wherein the heating wire 8 is suspended in the measuring cavity 5 having the V-shaped cross section. In a MEMS embodiment, the length of the measuring cavity 5 having the V-shaped cross section can be 1 to 3 mm, and the width (W Vgrove ) can be 20 to 60µm. The temperature of the heating wire 8 is determined by the thermal conductivity κ of the fluid / gas 6 in the measurement chamber 5 and is largely independent of the velocity of the fluid flow 3. Therefore, κ can be detected by monitoring the voltage drop across the heating wire 8 at a constant heating current. Figure 4 The angle α shown may be 50 to 60 degrees.

[0077] According to the present invention, a method for determining the flow rate of a fluid 1 independently of the thermal properties of the fluid 1 is provided, comprising: - bringing the thermal flow sensor 2 into contact with the fluid flow 3 or generally placing the thermal flow sensor 2 in thermal contact with the fluid flow 3; -Measure flow rate; - placing the thermal property sensor 4 in a measurement chamber 5 in fluid connection with the fluid flow 3 and preferably adjacent to the fluid flow 3, wherein the measurement chamber 5 is completely open to the fluid flow 3; - receiving a portion 6 of the fluid in the measurement chamber 5 of the thermal property sensor 4 such that the portion 6 of the fluid is substantially stationary in the measurement chamber 5; - measuring at least one thermal property of said fluid 1 ( );as well as - compensating said measured flow rate according to said at least one measured thermal characteristic.

[0078] The method may further comprise: - Using said thermal flow sensor 2 in said fluid flow 3 to additionally measure at least one thermal property.

[0079] The thermal property measured in the fluid stream 3 can be the heat capacity (c p ) or density ( ).

[0080] The thermal property measured on the substantially stationary fluid 6 is the thermal conductivity (κ).

[0081] The highly schematically shown pressure sensor 7 can additionally measure the pressure to deduce the viscosity from the thermal properties and the measured pressure (or the measured pressure difference). As shown by John van Baar, viscosity can also be measured "thermally".

[0082] Such as Figure 7 The highly schematically shown additional sensor 26 can be added to the device. The additional sensor 26 can be a viscosity sensor, an (external) humidity sensor, a CO2 sensor, an (external) temperature sensor, a dielectric or permittivity sensor, a fluid composition sensor or a multi-parameter sensor.

[0083] Thermal conductivity (κ) can be measured by: - heating the heating wire 8 with a constant current (DC) or very low frequency alternating current (AC) for heating the portion 6 of the fluid 1, and During the heating of the portion 6 of the fluid, the voltage of the heating wire 8 is measured by means of a voltage measuring device 9 connected to the heating wire 8 and the measured voltage is correlated with the thermal conductivity.

[0084] Heat capacity (c p ) can be measured by: - activating the heating wire 8 with alternating current (AC); and - During the heating of the portion 6 of the fluid, the phase and amplitude of the third harmonic of the measured alternating current (AC) voltage of the heating wire 8 are measured by means of a voltage measuring device 9 connected to the heating wire 8, and the measured phase and amplitude of the third harmonic of the measured AC voltage are compared with the heat capacity (preferably the volumetric heat capacity, ) are associated.

[0085] According to the present invention, a method for determining the thermal conductivity (κ) and / or specific heat capacity (c p ) methods, including: - bringing the thermal flow sensor 2 into contact with the fluid flow 3; -Measure flow rate; - placing the thermal property sensor 4 in a measurement chamber 5 in fluid connection with the fluid flow, preferably adjacent to the fluid flow, wherein the measurement chamber 5 is completely open to the fluid flow, for example arranged in the fluid flow; - receiving the portion 6 of the fluid in the measurement chamber 5 of the thermal property sensor 4 such that the portion 6 of the fluid is substantially stationary in the measurement chamber 5, and / or - heating the heating wire 8 of the thermal property sensor 4 with a constant current (DC) or an extremely low frequency alternating current (AC) for heating a portion 6 of the fluid, and - during the heating of the portion 6 of the fluid, measuring the voltage of the heating wire 8 by means of a voltage measuring device 9 connected to the heating wire 8 and correlating the measured voltage with the thermal conductivity; - heating the heating wire 8 of the thermal characteristic sensor 4 with alternating current (AC); and - During the heating of the portion 6 of the fluid, the phase and amplitude of the third harmonic of the measured alternating current (AC) voltage of the heating wire 8 are measured by means of a voltage measuring device 9 connected to the heating wire, and the measured phase and amplitude of the third harmonic of the measured AC voltage are compared with the heat capacity (preferably the volumetric heat capacity, ) are associated.

[0086] like Figure 6 As shown, the present invention also relates to a method for manufacturing the above-mentioned thermal characteristic sensor 4, wherein first, a support layer 22, preferably a support layer 22 of 1 µm SiRN, is deposited on a Si wafer 25 by, for example, LPCVD (1). Then, a 20 nm Cr adhesion layer and a 200 nm Pt layer 23 are deposited and etched by sputtering and IBE etching respectively to pattern the wires and metal traces (2, 3). The combination of these thicknesses of Cr and Pt gives excellent results, but other thicknesses and combinations of metals are also possible. The IBE etching step is performed twice using two different masks. The first step is used to transfer the metal pattern and the second step is used to narrow the beam width and define the pattern in the SiRN support layer 22. In (4), the SiRN is etched by plasma etching to form an opening for etching the Si. In order to have wires 8, 14 on both sides (5-7) of the wafer 25, all the above steps need to be repeated on the back side of the wafer 25. Finally, the Si is etched by KOH (KOH 1:3 DI-water) to realize the cavities 5 , 24 inside the wafer 25 between / around the conductive lines 8 , 14 .

[0087] Figure 7An exemplary embodiment of a flow channel 10 is shown, which includes a thermal flow sensor 2 and a thermal property sensor 4, wherein the flow sensing element 14 can include a probe. A pressure sensor 7 and an additional sensor 26 can be provided to measure differential pressure. The flow sensing element 14 in the form of a probe can be arranged at spaced locations in the flow 3. Also shown is a measuring chamber 5 with a heating wire 8.

[0088] Reference Signs List 1. Fluid 2. Thermal flow sensor 3. Fluid Flow 4. Thermal characteristic sensor 5. Measurement chamber 6. Static portion of the fluid 7. Pressure sensor 8. Heating wire 9. Voltage measuring device 10. Flow channel 11. Sensor body 12. Flow Section 13. Flow sensing device 14. Flow sensing element 15. Main body 16. First main part 17. Second Main Part 18. Flow sensing wire 19. Chip 20. PCB 21. Wheatstone Bridge 22. Support layer (of SiRN) 23. Cr / Pt layer 24. Flow sensing element cavity 25. Si wafer 26. Additional / Other Sensors.

Claims

1. A method for determining the flow rate of a fluid (1) independently of the thermal properties of the fluid, comprising: - placing a thermal flow sensor (2) in thermal contact with the fluid flow (3); -Measure flow rate; - placing a thermal property sensor (4) in a measurement chamber (5) in fluid contact with the fluid flow; - receiving a portion (6) of the fluid in the measurement cavity of the thermal property sensor such that the portion of the fluid is substantially stationary in the measurement cavity, wherein the measurement cavity is fully open to the fluid flow; - measuring at least one thermal property of the fluid ( ), or a combination of the thermal properties of the fluid ( );as well as - Compensating the measured flow rate according to the at least one measured thermal characteristic.

2. The method according to claim 1, comprising: - using the thermal flow sensor (2) in the fluid flow (3) to additionally measure at least one thermal property.

3. The method according to claim 2, wherein: The thermal property measured using the thermal flow sensor is heat capacity (c p ) or density ( ) or volumetric heat capacity ( ).

4. A method according to any one of the preceding claims, wherein The thermal property measured in the fluid (6) at substantially rest is thermal conductivity (κ).

5. A method according to any one of the preceding claims, wherein The pressure sensor (7) additionally measures the pressure and / or the pressure difference in order to deduce the viscosity from the measured flow rate and the measured pressure and / or the pressure difference.

6. A method according to any one of the preceding claims, wherein The thermal characteristic sensor (4) or the thermal flow sensor comprises a heating wire (8) or a probe or a winding.

7. The method according to claims 4 and 6, wherein: Thermal conductivity (κ) is measured by: - heating the heating wire (8) with a constant current direct current (DC) or very low frequency alternating current (AC) for heating the portion (6) of the fluid, and - During the heating of the portion of the fluid, the voltage of the heating wire is measured using a voltage measuring device (9) connected to the heating wire, and the measured voltage is correlated with the thermal conductivity.

8. The method according to claims 3 and 6, wherein: Heat capacity (c p ) by the following measurements: - activating the heating wire (8) with alternating current (AC); and - During heating of the portion (6) of the fluid, measuring the phase and amplitude of the third harmonic of the measured alternating current (AC) voltage of the heating wire using a voltage measuring device (9) connected to the heating wire, and correlating the measured phase and amplitude of the third harmonic of the measured alternating current (AC) voltage to a heat capacity.

9. A method for determining the thermal conductivity (κ) and heat capacity (c p ) methods, including: - placing a thermal flow sensor (2) in thermal contact with the fluid flow (3); -Measure flow rate; - placing a thermal property sensor (4) in a measurement cavity (5) in fluid connection with the fluid flow, wherein the measurement cavity is completely open to the fluid flow; - receiving the portion (6) of the fluid in the measurement chamber of the thermal property sensor such that the portion of the fluid is substantially stationary in the measurement chamber, - heating the heating wire (8) of the thermal property sensor with a constant current direct current (DC) or very low frequency alternating current (AC) for heating the portion of the fluid, and - During the heating of the portion of the fluid, measuring the voltage of the heating wire using a voltage measuring device (9) connected to the heating wire, and correlating the measured voltage with thermal conductivity; - heating the heating wire of the thermal characteristic sensor using alternating current (AC); and - During heating of the portion of the fluid, measuring the phase and amplitude of the third harmonic of the measured alternating current (AC) voltage of the heating wire using a voltage measuring device (9) connected to the heating wire, and correlating the measured phase and amplitude of the third harmonic of the measured alternating current (AC) voltage to heat capacity.

10. A method according to any one of the preceding claims, wherein Placing the thermal flow sensor (2) in thermal contact with the fluid flow (3) includes making thermal contact through an outer wall of the flow channel (10).

11. A thermal characteristic sensor (4) for use with a thermal flow sensor (2), wherein: During use, a fluid (1) whose flow rate is to be determined flows through the thermal flow sensor, comprising: a heating wire (8) configured to be placed in a measuring chamber (5) for receiving a portion (6) of the fluid, such that the portion of the fluid is substantially stationary in the measuring chamber, the heating wire being further configured to be heated by: - using a constant current direct current (DC) or very low frequency alternating current (AC) for heating said portion of said fluid, wherein during the heating of said portion of said fluid, a voltage across said heating wire is measured by means of a voltage measuring device (9) connected to said heating wire, wherein said measured voltage is correlated with thermal conductivity; and / or - using alternating current (AC), wherein, during the heating of the portion of the fluid, the phase and amplitude of the third harmonic of the alternating current (AC) voltage of the heating wire are measured by means of a voltage measuring device (9) connected to the heating wire, wherein the measured phase and the measured amplitude of the third harmonic of the alternating current (AC) voltage are correlated with the heat capacity.

12. The thermal characteristic sensor (4) according to claim 11, wherein: The measuring cavity (5) is a groove and has a U-shaped or V-shaped cross section, wherein the heating wire (8) is suspended in the measuring cavity having the U-shaped or V-shaped cross section.

13. The thermal characteristic sensor (4) according to claim 12, wherein: The length of the measuring cavity (5) with a V-shaped cross section and / or the heating wire (8) is 1-3 mm, for example 1.5-2.5 mm, and / or the width (W V凹槽 ) is 20-60 µm.

14. A thermal flow sensor (2) for measuring a fluid flow (3), comprising: a measuring chamber (5) in fluid connection with the fluid flow, wherein the measuring chamber is completely open to the fluid flow; - The thermal property sensor (4) according to any one of claims 11 to 13, wherein the heating wire (8) is placed in the measurement cavity for accommodating a portion (6) of the fluid, wherein the portion of the fluid is substantially stationary in the measurement cavity.

15. The thermal flow sensor (2) according to claim 14, wherein: The thermal flow sensor is releasably insertable into a flow channel (10) in which a fluid flow (3) is present during use, or, when referring to claim 10, is fixed to an outer wall of the flow channel (10).

16. A thermal flow meter or controller, comprising the thermal characteristic sensor (4) according to claims 11 to 13 and / or the thermal flow sensor (2) according to claims 14 to 15.

17. A thermal flow meter or controller according to claim 16, wherein: At least one of the thermal characteristic sensor (4) or the thermal flow sensor (2) is a micro-electromechanical device.

18. A thermal flow meter or controller according to claim 16 or 17, further comprising a viscosity sensor, an (external) humidity sensor, a CO2 sensor, an (external) temperature sensor, a dielectric or permittivity sensor, a fluid composition sensor or a multi-parameter sensor.

19. Use of the thermal flow meter or controller according to any one of claims 16 to 18 in a medical device.

20. Use of a thermal flow meter or controller according to claim 19 in a respiratory apparatus.

21. A method for manufacturing a thermal flow sensor (2) according to any one of claims 14 to 15, comprising the following steps: (1) depositing a support layer (22) on both sides of the wafer (25), (2) depositing a metal layer (23) on one side of the wafer, (3) patterning the metal layer, (4) patterning the support to form an opening for etching the underlying Si, (5) Repeat steps 2 to 4 on the other side of the wafer. (6) Etching the Si wafer to realize the V-grooves (5) and the flow sensing element cavity (24) within the wafer.

22. The method of claim 21, wherein: (4) Etching the SiRN support layer (22) to form an opening for etching the underlying Si wafer (25).

23. The method according to claim 21 or 22, wherein The metal layer includes a Cr / Pt layer.

24. The method according to any one of claims 21 to 23, wherein The support layer is a SiRN support layer (22).

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