Thermal flow sensor for determining the flow rate of a fluid

Abstract

A thermal flow sensor (2) determines the flow rate of the fluid (1); a sensor body (11) having a flow cross section (12) through which a fluid flows in a flow direction (3) in use; a flow sensor arrangement (13) including a plurality of flow sensing elements (14) arranged at a plurality of positions within the cross-section of the flow path to measure the flow velocity at different positions within the cross-section of the flow path; The plurality of flow rate detection elements (14) are arranged parallel to one another in a plane parallel to the cross section of the flow path, and a single flow rate detection element (14) among the plurality of flow rate detection elements (14) can be read out.

Description

[Technical Field]

[0001] The present invention relates generally to a thermal flow sensor for determining the flow rate of a fluid. [Background technology]

[0002] Thermal flow sensors are used to measure the flow rate of gases and liquids. There are three main types of thermal flow sensors: anemometers, thermal sensors, and time-of-flight sensors.

[0003] Power is applied to the heater to increase its temperature, and the change in temperature distribution across the sensor structure is measured as a measure of flow rate. Many variations are possible, such as when the power is kept constant and the heater also functions as a temperature sensor.

[0004] Thermal flow sensors have a simple operating principle and relatively low manufacturing costs. Furthermore, they are suitable for adaptation to microelectromechanical systems (MEMS) devices. However, they depend on the thermal properties of the flowing medium, more specifically, gas or liquid. This means that these sensors require calibration every time the medium is changed.

[0005] Alternative techniques include using different structures to reduce the flow rate dependence of the wire temperature, or implementing sensors in the dead volume to measure thermal conductivity.

[0006] DE 4224518A discloses a flow sensor for a gas analyzer flow meter, including a silicon body with a temperature-sensitive resistive structure. The resistive structure is attached to at least the edge of a sensor area on a dielectric support structure. Two sensor areas are connected by a fluid flow path. A single sensor traverses the flow path multiple times.

[0007] However, this technique is only suitable for flow sensors in microfabricated channels. Summary of the Invention [Problem to be solved by the invention]

[0008] It is therefore an object of the present invention to provide a thermal flow sensor that can be used to measure flow in larger tubes. It is also an object of the present invention to improve the speed of such a sensor and to provide a bidirectional flow sensor. [Means for solving the problem]

[0009] The present invention provides a thermal flow sensor for determining the flow rate of a fluid, and includes the following features. - A sensor body having a flow path cross section through which the fluid flows in the flow direction during use. - A flow sensor arrangement comprising a plurality of (e.g., two, three, four or more) flow sensing elements arranged at a plurality of positions within a cross-section of a flow path to measure flow velocity at different positions within the cross-section of the flow path. The plurality of flow rate detection elements are arranged parallel to one another in a plane parallel to the cross section of the flow path, for example, a plane coinciding with the cross section of the flow path. A single flow rate detection element among the plurality of flow rate detection elements can be read.

[0010] Advantageously, by placing multiple flow rate sensing elements at different locations across the flow path, it is possible to measure flow rates in large tubing, such as tubing used for ventilation, e.g., medical ventilation. Furthermore, it is possible to measure and observe flow profiles. The sensitivity differences between the multiple flow sensing elements can be used to create flow profiles or to select a single flow sensing element for readout to extend the range.

[0011] As used herein, "essentially stationary" means that the fluid is essentially "still," but allows for relatively small-scale diffusion and movement of fluid particles within the measurement cavity (such small-scale diffusion and movement may even be necessary to reflect changing fluid composition). This has been found to be possible without interfering with the accuracy of the measurement.

[0012] The flow sensing element must be strong enough to withstand the fluid pressures associated with the intended flow range.

[0013] An embodiment relates to the thermal flow sensor described above, and comprises: -Main body. a first base portion extending from the body portion; a second base portion extending from the main body portion; In this case, the flow path cross section is formed between the main body, the first base, and the second base.

[0014] An embodiment relates to the thermal flow sensor described above, wherein the first base portion and the second base portion are parallel to each other.

[0015] One embodiment relates to the aforementioned thermal flow sensor, wherein the plurality of flow sensing elements extend between the first base and the second base.

[0016] An embodiment relates to the thermal flow sensor described above, wherein the flow channel cross section has a square or rectangular shape in a plane transverse to the flow direction.

[0017] One embodiment relates to the thermal flow sensor described above, wherein a plurality of parallel flow sensing elements are spaced across the flow path in an even manner.

[0018] An embodiment relates to the thermal flow sensor described above, wherein the plurality of flow sensing elements comprises 2, 3, 4, 5 or more flow sensing elements.

[0019] An embodiment relates to the thermal flow sensor described above, wherein each of the plurality of flow sensing elements includes a pair of flow sensing wires or a combination of three flow sensing wires.

[0020] One embodiment relates to the aforementioned thermal property sensor, wherein the wire has a cross section of less than about 10 μm, for example, about 9 μm, about 8 μm, about 7 μm, about 6 μm, about 5 μm, about 4 μm, about 3 μm, about 2 μm, or about 1 μm.

[0021] An embodiment relates to the thermal property sensor described above, wherein the wire is silicon (Si), silicon oxide, silicon nitride, or other silicon compounds.

[0022] One embodiment relates to the aforementioned thermal property sensor, in which the wire has a cross section that is flattened on one side, such as a square, triangular, semicircular, or, most preferably, rectangular shape. The flat portion of the flattened cross section preferably has a width of less than 10 μm, such as about 9 μm, about 8 μm, about 7 μm, about 6 μm, about 5 μm, about 4 μm, about 3 μm, about 2 μm, or about 1 μm. Furthermore, the cross section preferably has a thickness of 0.1 to 1 μm, depending on the robustness and condition of the material. In this case, the sensor's small volume allows for extremely fast operation. Because the time constant is proportional to the volume, halving the volume roughly doubles the sensor's speed, with other variables remaining constant. In the field of flow sensors, a response that is twice as fast is desirable, and reducing the volume by half or less can achieve even faster response speeds. Alternatively, a set of probes, such as those made of platinum, can be used instead of the flow sensing wire, or the set of wires can be replaced with probes, resulting in a combination of wire and probe.

[0023] The flow rate detection wire is preferably as thin as possible and has a cross section such as that described above, but it is also possible to use, for example, a carbon nanotube.

[0024] One embodiment relates to the thermal flow sensor described above, wherein the pair of flow sensing wires or the combination of three flow sensing wires are spaced apart in the direction of flow.

[0025] An embodiment relates to the thermal flow sensor described above, wherein the sensor body is a microelectromechanical device.

[0026] One embodiment relates to a thermal flow sensor as previously described, wherein the flow sensor structure is an integral part of the sensor body.

[0027] One embodiment relates to the thermal flow sensor described above, in which a pair of flow sensing wires are arranged parallel to each other.

[0028] The mutual distance between the pair of flow sensing wires is 300 μm to 500 μm, preferably 350 μm to 450 μm, and more preferably 375 μm to 425 μm.

[0029] The mutual distance between the pair of flow sensing wires is 1 / 3 to 1 / 20 of the diameter of the flow tube in which the thermal flow sensor is disposed, for example, 1 / 4 to 1 / 20, or 1 / 5 to 1 / 20 of the diameter of the flow tube.

[0030] One embodiment relates to the thermal flow sensor described above, wherein each of the one or more pairs of flow sensing wires forms a Wheatstone bridge or a portion (eg, half) of a Wheatstone bridge.

[0031] A fixed resistor may be disposed in the sensor body to form the other part (eg, the other half) of the Wheatstone bridge.

[0032] An embodiment relates to the aforementioned thermal flow sensor. a measurement cavity configured to receive a portion of a fluid such that the portion of the fluid is substantially stationary, and a thermal property sensor including a heated wire; Equipped with. the heating wire is heated by a direct current (DC) or a very low frequency alternating current (AC) current to heat a portion of the fluid, and a voltage across the heating wire during heating is measured using a voltage measuring means connected to the heating wire, and the measured voltage is related to thermal conductivity. and / or The heating wire is heated by a high frequency alternating current (AC) current to heat a portion of the fluid, and the phase and amplitude of the third harmonic of the AC voltage of the heating wire during heating are measured using a voltage measuring means connected to the heating wire, and the measured phase and amplitude of the third harmonic are related to the heat capacity.

[0033] An embodiment relates to the thermal flow sensor described above, wherein the measurement cavity has a U-shaped or V-shaped cross section, and the heating wire is suspended within the measurement cavity having a U-shaped or V-shaped cross section.

[0034] An embodiment relates to the thermal flow sensor described above. The measurement cavity has a U-shaped or V-shaped cross section and is 1 mm to 3 mm long and 20 μm to 60 μm wide (W groov e) The V-shape may be slightly knotted or have other imperfections.

[0035] One embodiment relates to the thermal flow sensor described above, which, in use, is releasably inserted into a flow path through which fluid flow exists.

[0036] Another aspect of the invention relates to a method for determining the flow rate of a fluid without relying on the thermal properties of the fluid, comprising the steps of: - placing said thermal flow sensor in the fluid flow stream; - placing a thermal property sensor in a measurement cavity in fluid communication with, preferably adjacent to, said fluid flow; - Receiving a portion of the fluid into a measurement cavity of the thermal property sensor such that the portion of the fluid is substantially stationary. - at least one thermal property of the fluid (κ, ρ, c p ) measuring step. - correcting the measured flow rate for the measured thermal properties.

[0037] Another aspect of the invention is to measure the thermal conductivity (κ) and / or heat capacity (c) of the fluid whose flow rate is to be measured. p ) and includes the following steps: - placing said thermal flow sensor in the fluid flow and measuring the flow rate. - placing a thermal property sensor in a measurement cavity in fluid communication with, preferably adjacent to, said fluid flow; - Receiving a portion of the fluid into a measurement cavity of the thermal property sensor such that the portion of the fluid is substantially stationary. - heating a heating wire of a thermal property sensor with a direct current (DC) or a very low frequency alternating current (AC) current to heat a portion of the fluid, measuring the voltage of said heating wire during heating using a voltage measuring means connected to said heating wire, and relating the measured voltage to the thermal conductivity. and / or - heating a heating wire of a thermal property sensor with a high frequency alternating current (AC) current to heat a portion of the fluid, measuring the phase and amplitude of the third harmonic of the AC voltage of said heating wire during heating using a voltage measuring means (9) connected to said heating wire, and relating the phase and amplitude of the measured third harmonic to the heat capacity.

[0038] Another aspect of the present invention relates to a thermal flow meter or control device including the thermal flow sensor described above.

[0039] Another aspect of the invention relates to a medical device, particularly a respiratory device, that includes a thermal flow meter or control device as described above.

[0040] In yet another embodiment, certain wires can be heated to different temperatures by heating them with a different current than other wires, thereby allowing for characterization of fluids at different temperatures.

[0041] Another aspect of the present invention is a method for manufacturing the aforementioned thermal flow sensor, comprising the following steps. (1) Depositing a support layer 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 layer to open windows for etching Si underneath. (5) Repeat steps (2) through (4) on the other side of the wafer. (6) Etching the Si wafer to create a U-groove or V-groove and a flow sensing element cavity inside the wafer.

[0042] One embodiment relates to the aforementioned manufacturing method, wherein step (4) includes etching the support layer to open a window for etching the Si wafer underneath.

[0043] An embodiment relates to the aforementioned manufacturing method, wherein the metal layer is a Cr / Pt layer.

[0044] Certain embodiments relate to the aforementioned manufacturing method, wherein the support layer is a SiRN support layer.

[0045] An embodiment of the above-described manufacturing method has the following features. (1) The support layer has a thickness of 1 μm.

[0046] An embodiment of the above-described manufacturing method has the following features. (2) Cr / Pt has a thickness of 20 nm / 200 nm.

[0047] An embodiment of the above-described manufacturing method has the following features. (3) Patterning of the support layer involves etching the Cr / Pt layer.

[0048] An embodiment of the above-described manufacturing method has the following features. (2) The Cr / Pt layer is deposited by sputtering.

[0049] In one embodiment, step (1) uses low pressure chemical vapor deposition (LPCVD).

[0050] In an embodiment, step (6) uses KOH for etching, preferably 1:3 with distilled water. [Brief explanation of the drawings]

[0051] The invention is explained by means of exemplary embodiments illustrated in the accompanying drawings and the detailed description of the figures below. [Figure 1] 1 is an exemplary perspective view of a thermal flow sensor according to an embodiment; [Figure 2] 1 is an exemplary perspective view of a thermal property sensor having a measurement cavity according to an embodiment; [Figure 3] FIG. 10 illustrates a heating wire of a thermal property sensor connected to a voltage measurement means according to an embodiment. [Figure 4] 1 is a cross-sectional view of a heating wire disposed in a measurement cavity having a U-shaped or V-shaped cross section according to an embodiment. [Figure 5] FIG. 1 is a circuit diagram of a Wheatstone bridge. [Figure 6] FIG. 10 shows the steps of creating a U-shaped or V-shaped measurement cavity with a heating wire. [Figure 7] FIG. 10 is a diagram showing a flow path equipped with a thermal flow sensor and a thermal property sensor according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0052] 1 is a perspective view of an exemplary embodiment of a thermal flow sensor 2 for measuring a fluid flow 3, particularly a laminar fluid flow 3. The thermal flow sensor 2 includes a measurement cavity 5 in fluid communication with, and preferably adjacent to, the fluid flow 3. The measurement cavity 5 includes a thermal property sensor 4. A heating wire 8 is disposed within the measurement cavity 5 for receiving a portion of the fluid 6 such that the portion of the fluid 6 is substantially stationary within the measurement cavity 5. During use, the thermal flow sensor 2 is releasably insertable into a flow path 10 in which the fluid flow 3 resides.

[0053] The thermal flow sensor 2 may include a sensor body 11 having a flow cross-section 12 through which a fluid 1 whose flow rate is to be determined flows in a flow direction during use. According to the present invention, a flow sensor assembly 13 may be provided, including a plurality of flow sensing elements 14 arranged at multiple locations on the flow cross-section 12. The thermal flow sensor 2 may include a body 15, a first base 16 extending from the body 15, and a second base 17 extending from the body 15. The flow cross-section 12 may be formed between the body 15, the first base 16, and the second base 17. The measurement cavity 5 may be disposed in the body 15. The body 15 includes multiple bonding pads 27 for reading data generated by the plurality of flow sensing elements 14. Both ends of the plurality of flow sensing elements 14 are connected to the respective bonding pads 27 to facilitate individual reading of each single flow sensing element 14. Of course, this can be done simultaneously. The measurement cavity 5 may extend along a side of the flow cross-section 12. As shown in FIG. 1, the flow sensor assembly 13 may include a plurality of flow rate detection elements 14 arranged at a plurality of positions in the flow path cross section 12, for example, at three positions. The flow path cross section 12 may be open at a side of the flow path cross section 12 that is not defined by the main body 15, the first base 16, and / or the second base 17. The first base 16 and the second base 17 may be parallel to each other. The flow path cross section 12 may have a square or rectangular shape in a plane that intersects the direction of the flow 3. The plurality of flow rate detection elements 14 may extend between the first base 16 and the second base 17. The plurality of flow rate detection elements 14 may be arranged at equal intervals within the flow path cross section 12. Each flow rate detection element 14 may include a pair of flow rate detection wires 18. The mutual distance between the pair of flow rate detection wires 18 may be 300 μm to 500 μm, preferably 350 μm to 450 μm, and more preferably 375 μm to 425 μm. A pair of flow rate detection wires 18 of the thermal flow sensor 2 may extend between the first base 16 and the second base 17. The sensor body 11 may be formed as a chip 19. The sensor body 11 may be attached to a printed circuit board (PCB) 20 or may be disposed on the printed circuit board (PCB) 20.Each of the one or more pairs of flow sensing wires 18 may form one half (R2, R3) of a Wheatstone bridge 21, as shown in FIG. 5. The individual flow sensing wires 18 of a pair or three pairs of flow sensing wires 18 can be read separately. This can also be done simultaneously, as shown in FIG. 1, which shows the bonding pads 27 for separate connections. Those skilled in the art will understand that if a differential flow signal is measured, the arrow shown through R3 should point downward. Fixed resistors R1 and R4 disposed on the sensor body 11 may form the other half of the Wheatstone bridge 21. The thermal flow sensor 2 may be a microelectromechanical system (MEMS) component. Under no-flow conditions, R1 and R4, and R2 and R3, have the same value, so the output signal of the Wheatstone bridge 21 is zero. When current flows, heat is transferred from the upstream wire to the downstream wire. Therefore, the temperature difference between the two wires R2 and R3 results in a positive or negative output voltage signal (depending on the flow direction).

[0054] FIG. 2 is a perspective view showing an exemplary embodiment of a thermal property sensor 4 with a measurement cavity 5 in more detail.

[0055] As shown more clearly in Figure 3, the heating wire 8 can be configured to be heated with a constant current (DC) or a very low frequency alternating current (AC) current to heat the portion 6 of the fluid 1. Then, during heating of the portion 6 of the fluid 1, the voltage of the heating wire 8 is measured by a voltage measuring means 9 connected to the heating wire 8. The measured voltage is related to the thermal conductivity κ; and / or The heating wire 8 is configured to be heated with an alternating current (AC) current to heat the portion 6 of the fluid 1. During heating of the portion 6 of the fluid 1, the phase and amplitude of the third harmonic of the AC voltage of the heating wire 8 are measured by a voltage measuring means 9 connected to the heating wire 8. The phase and amplitude of the measured third harmonic of the AC voltage are used to calculate the heat capacity c p is associated with.

[0056] As shown in FIG. 4, the measurement cavity 5 may have a U-shaped or V-shaped cross section. In this case, the heating wire 8 is suspended within the measurement cavity 5 having a U-shaped or V-shaped cross section. The measurement cavity 5 having a U-shaped or V-shaped cross section has a length l of 1 mm to 3 mm and a width (W) of 20 μm to 60 μm in a MEMS embodiment. groove ) The temperature of the heating wire 8 depends on the thermal conductivity κ of the fluid / gas 6 in the measurement cavity 5, and is almost independent of the fluid flow rate. Therefore, κ can be detected by monitoring the voltage drop across the heating wire 8 under a constant heating current. The angle α shown in Figure 4 is 50 to 60 degrees.

[0057] In another embodiment, there is provided a method for determining the flow rate of fluid 1 without relying on the thermal properties of fluid 1. The method comprises: - bringing the thermal flow sensor 2 into contact with the fluid flow 3; - Measuring the flow rate. - placing a thermal property sensor 4 in a measurement cavity 5 in fluid communication with, and preferably adjacent to, the fluid flow 3; - Receiving a portion 6 of fluid into the measurement cavity 5 of the thermal property sensor 4, such that the portion 6 of fluid is substantially stationary within the measurement cavity 5. - at least one thermal property of fluid 1 (κ, ρ, c p ) measuring step. - correcting the measured flow rate for at least one measured thermal property.

[0058] The method may further include: - additionally measuring at least one thermal property in the fluid flow 3 using a thermal flow sensor 2;

[0059] The thermal properties measured in the fluid flow 3 are the heat capacity (c p ) or density (ρ).

[0060] The thermal property measured on an essentially stationary fluid 6 is the thermal conductivity (κ).

[0061] The pressure sensor 7 shown diagrammatically may additionally measure pressure and / or pressure differential in order to derive viscosity from the thermal properties and the measured pressure and / or pressure differential.

[0062] Additional sensors 26, shown in Figure 7, may be added to the device. The additional sensors 26 may be a viscosity sensor, an (external) humidity sensor, a CO2 sensor, an (external) temperature sensor, a permittivity or dielectric constant sensor, a fluid composition sensor, or a multi-parameter sensor.

[0063] The thermal conductivity (κ) can be measured by performing the following steps. - Heating the heating wire 8 with a direct current (DC) or a very low frequency alternating current (AC) current to heat the portion 6 of the fluid 1. - measuring the voltage of the heating wire 8 during heating of the portion 6 of fluid using a voltage measuring means 9 connected to the heating wire 8 and relating the measured voltage to the thermal conductivity.

[0064] Heat capacity (c p ) can be measured by performing the following steps: - activating the heating wire 8 with an alternating current (AC) current. - measuring the phase and amplitude of the third harmonic of the alternating current (AC) voltage of the heating wire 8 during heating of the portion 6 of the fluid using a voltage measuring means 9 connected to the heating wire 8, and relating the phase and amplitude of the measured third harmonic of the AC voltage to the heat capacity.

[0065] In another embodiment, the thermal conductivity (κ) and heat capacity (c p A method is provided for determining the .times. ... - bringing the thermal flow sensor 2 into contact with the fluid flow 3; - Measuring the flow rate. - placing a thermal property sensor 4 in a measurement cavity 5 in fluid communication with, and preferably adjacent to, the fluid flow 3; - Receiving a portion 6 of fluid into the measurement cavity 5 of the thermal property sensor 4, such that the portion 6 of fluid is substantially stationary within the measurement cavity 5. - heating the heating wire 8 of the thermal property sensor 4 with a direct current (DC) or a very low frequency alternating current (AC) current to heat the portion 6 of the fluid, measuring the voltage of said heating wire 8 during heating using a voltage measuring means 9 connected to the heating wire 8, and relating the measured voltage to the thermal conductivity. and / or - heating a heating wire 8 of a thermal property sensor 4 with a high frequency alternating current (AC) current to heat a portion 6 of the fluid, measuring the phase and amplitude of the third harmonic of the AC voltage of said heating wire 8 during heating using a voltage measuring means 9 connected to the heating wire 8, and relating the phase and amplitude of the measured third harmonic to the heat capacity.

[0066] As shown in Figure 6, the present invention also relates to a method for fabricating the aforementioned thermal property sensor 4. First, a support layer 22, preferably 1 μm of SiRN, is deposited on a Si wafer 25, for example, by LPCVD (1). Next, 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 wires and metal traces (2, 3). While a combination of Cr and Pt at these thicknesses provides excellent results, other thicknesses and metal combinations are also possible. The IBE etching step is performed twice using two different masks: the first step is to transfer the metal pattern, and the second step is to narrow the beam width and define the pattern of the SiRN support layer 22. In (4), the SiRN is etched by plasma etching to open windows for etching the Si. All of these steps are repeated on the back side of the wafer 25, forming heating wires 8 and 14 on both sides (5-7). Finally, the Si is etched with KOH (KOH:pure water=1:3) to realize cavities 5, 24 inside the wafer 25 between / around the heating wires 8, 14.

[0067] 7 shows an exemplary embodiment of a flow path 10 equipped with a thermal flow sensor 2 and a thermal property sensor 4. The flow sensing element 14 may consist of a probe. A pressure sensor 7 and an additional sensor 26 may also be provided. The probe-shaped flow sensing element 14 may be arranged at spaced positions in the flow 3. A measuring cavity 5 equipped with a heating wire 8 is also shown. [Explanation of symbols]

[0068] 1.Fluid, 2. Thermal flow sensor, 3. Fluid flow, 4. Thermal property sensors, 5. Measuring cavity, 6. Stationary part of the fluid, 7. Pressure sensor, 8. Heating wire, 9. Voltage measuring means, 10. Flow path, 11. Sensor body, 12. Flow path cross section, 13. Flow sensor arrangement; 14. Flow detection element, 15. Main body, 16. First base, 17. Second base, 18. Flow detection wire, 19. Chip, 20. Printed circuit boards, 21. Wheatstone Bridge, 22.SiRN layer, 23.Cr / Pt layer, 24. Flow sensing element cavity, 25.Si wafer, 26. Additional sensors, 27. Bonding pad.

Claims

1. A thermal flow sensor (2) for determining the flow rate of a fluid (1), comprising: a sensor body (11) having a flow cross section (12) through which a fluid flows in a flow direction (3) in use; a flow sensor arrangement (13) comprising a plurality of flow sensing elements (14) arranged at a plurality of positions within the cross section of the flow path in order to measure the flow velocity at different positions within the cross section of the flow path; Equipped with The plurality of flow rate detection elements (14) are arranged parallel to each other in a plane parallel to the cross section of the flow path, A thermal flow sensor characterized in that a single flow rate detection element (14) among the plurality of flow rate detection elements (14) is readable.

2. The sensor body (11) a body part (15), a first base portion (16) extending from said body portion; a second base portion (17) extending from said body portion; Equipped with 2. The thermal flow sensor according to claim 1, wherein the flow passage cross section (12) is formed between the main body, the first base and the second base.

3. 3. The thermal flow sensor according to claim 2, wherein the first base portion (16) and the second base portion (17) are parallel to each other.

4. The thermal flow sensor (2) according to claim 1 or 2, wherein the plurality of flow detection elements (14) extend between the first base (16) and the second base (17).

5. 4. The thermal flow sensor (2) according to claim 1, wherein the plurality of flow rate detection elements (14) are arranged at equal intervals within the flow path cross section (12).

6. A thermal flow sensor (2) according to any one of claims 1 to 5, characterized in that each of the flow sensing elements (14) includes a pair of flow sensing wires (18) or a combination of three flow sensing wires (18).

7. The thermal flow sensor (2) according to any one of claims 1 to 6, characterized in that the sensor body is a microelectromechanical device (MEMS).

8. The thermal flow sensor (2) according to claim 6 or 7, characterized in that the flow rate detection wires (18) are arranged parallel to each other.

9. 9. A thermal flow sensor (2) according to any one of claims 6 to 8, wherein each of the flow sensing wires (18) forms a Wheatstone bridge (21) or a part of a Wheatstone bridge (21).

10. a measurement cavity (5) for receiving a portion of a fluid such that the portion of the fluid is substantially stationary; a thermal property sensor (4) including a heating wire (8); Furthermore, The heating wire (8) is heated by a direct current (DC) or a very low frequency alternating current (AC) current to heat a portion of the fluid, and the voltage of the heating wire during heating is measured by a voltage measuring means (9) connected to the heating wire (8), and the measured voltage is related to the thermal conductivity. and / or The heating wire (8) is heated by a high frequency alternating current (AC) current to heat a portion of the fluid, and the phase and amplitude of the third harmonic of the AC voltage of the heating wire during heating are measured by a voltage measuring means (9) connected to the heating wire (8), and the measured phase and amplitude of the third harmonic are related to the heat capacity. A thermal flow sensor (2) according to any one of claims 1 to 9.

11. the measuring cavity (5) has a U-shaped or V-shaped cross section, 11. The thermal flow sensor (2) according to claim 10, characterized in that the heating wire (8) is suspended within the measuring cavity (5).

12. A thermal flow sensor (2) according to any one of the preceding claims, characterized in that, in use, it is releasably inserted into a flow path (10) in which a fluid flow (3) is present.

13. 1. A method for determining a flow rate of a fluid independent of the thermal properties of the fluid, comprising: - placing a thermal flow sensor (2) according to any one of claims 1 to 12 in a fluid flow (3); - placing a thermal property sensor (4) in a measurement cavity (5) in fluid communication with, and preferably adjacent to, said fluid flow; - receiving a portion of said fluid in said measuring cavity (5) so that said portion of fluid is substantially stationary; - at least one thermal property of said fluid (κ, ρ, c ｐ ) measuring the - correcting the measured flow rate for the measured thermal properties; A method comprising:

14. The thermal conductivity (κ) and / or heat capacity (c) of the fluid (1) whose flow rate is to be measured ｐ ) a method for measuring - placing a thermal flow sensor (2) according to any one of claims 1 to 12 in a fluid flow (3) and measuring the flow rate; - placing a thermal property sensor (4) in a measurement cavity (5) in fluid communication with, and preferably adjacent to, said fluid flow; - receiving a portion of said fluid in said measuring cavity (5) so that said portion of fluid is substantially stationary; - heating a heating wire (8) of a thermal property sensor (4) with a direct current (DC) or a very low frequency alternating current (AC) current to heat a portion of the fluid, measuring the voltage of said heating wire (8) during heating using a voltage measuring means (9) connected to said heating wire (8) and relating the measured voltage to the thermal conductivity; and / or - heating a heating wire (8) of a thermal property sensor (4) with a high frequency alternating current (AC) current to heat a portion of a fluid, measuring the phase and amplitude of the third harmonic of the alternating current (AC) voltage of said heating wire (8) during heating using a voltage measuring means (9) connected to said heating wire (8), and relating the phase and amplitude of the measured third harmonic to the heat capacity; A method comprising:

15. A thermal flow meter or control device comprising a thermal flow sensor (2) according to any one of claims 1 to 12.

16. A medical device comprising the heat flow meter or control device according to claim 15.

17. A respiratory device comprising a heat flow meter or control device according to claim 15.

18. A method for manufacturing the thermal flow sensor (2) according to any one of claims 1 to 12, comprising the steps of: (1) depositing a support layer (22) on both sides of a wafer (25); (2) depositing a metal layer (23) on one side of the wafer (25); (3) patterning the metal layer; (4) patterning the support layer to open windows for etching Si underneath; (5) repeating steps 2 through 4 on the other side of the wafer (25); (6) Etching a Si wafer to create a U-groove or V-groove (5) and a flow sensing element cavity (24) inside said wafer (25); A method comprising:

19. 20. The method of claim 18, wherein step (4) includes etching the support layer to open windows for etching the Si wafer underneath.

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