Method for determining fluid flow rate independent of the thermal properties of the fluid

Abstract

The present invention relates to a method for determining the flow rate of a fluid independently of the thermal properties of the fluid, the method comprising: - bringing a thermal flow sensor (2) into thermal contact with a fluid flow (3); - measuring the flow rate; - placing a thermal property sensor (4) in a measurement cavity (5) in fluid contact with the fluid flow (3); - receiving a portion of fluid (6) into a measurement cavity of the thermal property sensor, such that the portion of fluid (6) is substantially stationary within the measurement cavity; - at least one thermal property of the fluid (κ, ρ, c p , ρc p ) measuring the - correcting the measured flow rate for at least one measured thermal property; Includes.

Description

[Technical Field]

[0001] The present invention generally relates to a method for determining the flow rate of a fluid without relying on the thermal properties of the fluid, the thermal conductivity (κ) and heat capacity (c) of the fluid whose flow rate is to be determined. p The present invention relates to a method for determining the flow rate of a fluid, a thermal property sensor for use with a thermal flow sensor through which, in use, a fluid flows whose flow rate is to be determined, a thermal flow sensor for measuring fluid flow, and a method for manufacturing a thermal property sensor. [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. Nowadays, these sensors can also be microdevices.

[0003] The three types of thermal flow sensors generally include a heater and a temperature sensor, and their operating principle is to supply power to the heater to raise its temperature, and measure the change in temperature distribution on the sensor structure as an indicator of flow rate. Many variations are possible, such as keeping the power constant or having the heater also function as a temperature sensor.

[0004] Thermal flow sensors have a simple operating principle and are relatively inexpensive to manufacture. However, they depend on the thermal properties of the medium being flowed, specifically gas or liquid. This means that accurate flow measurement requires knowledge of the fluid and its properties.

[0005] In "Modeling and simulation of a thermal flow sensor for determining the flow speed and thermal properties of binary gas mixtures" by Christoph J. Hepp et al. (EUROSENSORS 2016, pp. 1028-1031), a thermal flow sensor including a heater and a downstream temperature sensor is used to measure the thermal conductivity of the gas. The temperature of the downstream temperature sensor in a specific flow region depends only on κ.

[0006] "Multi-parameter monitoring of binary gas mixtures: concentration and flow rate by DC excitation of thermal sensor arrays" by Christoph J. Hepp et al. (Sensors and Actuators A, pp. 32-39, 2017) describes a thermal sensor that can simultaneously determine the gas concentration and flow rate of a binary gas mixture under flow conditions. The thermal capacity is determined in a flow range where the flow sensor is not sensitive to flow rate, which limits the applicability of the sensor (i.e., the sensor must first be brought to a relatively "high flow range").

[0007] "Measurement and simulation of the frequency response of a thermal flow sensor at different flow speeds" by D.F. Reyes takes advantage of the fact that increasing the frequency reduces the thermal boundary layer and can be lowered closer to the wall. Therefore, due to the non-slip condition of the wall, the heat exchange between the heater and the sensor is independent of the flow velocity, and the heat transfer is affected only by the physical properties of the gas or fluid, not by the flow.

[0008] US Pat. No. 4,685,331 B1 discloses a thermal mass flow meter and controller with a series of flow channels and blocked flow channels. These channels are microetched from a silicon substrate, leaving an integral membrane floating across the channels. The thermal mass flow meter's sensor includes a stagnant gas flow channel running parallel to the etched channels, also known as a "blocked flow channel." The heat transfer equation described in US Pat. No. 4,685,331 B1 is independent of the sensor's flow rate because measurements are taken in the blocked, stagnant flow channel. Therefore, if the fluid composition in the flow channel changes, it takes a significant amount of time to diffuse into the blocked flow channel. Furthermore, different fluids may diffuse at different rates into blocked flow channels. In this field, in-line sensors are strongly preferred because of the requirement for immediate response to changes in gas (air-mixture) composition.

[0009] Therefore, measuring fluid-independent flow rates, especially gas-independent flow rates, remains a major challenge. Summary of the Invention [Problem to be solved by the invention]

[0010] It is therefore an object of the present invention to provide a method that allows for fluid-independent, in particular gas-independent, flow rate measurements. [Means for solving the problem]

[0011] One aspect of the present invention is A method for determining the flow rate of a fluid independent of the thermal properties of the fluid is provided, the method comprising: - placing a thermal flow sensor in thermal contact with the fluid flow; - measuring the flow rate; - placing a thermal property sensor in a measurement cavity in fluid contact with the fluid flow; - receiving a portion of a fluid into a measurement cavity of the thermal property sensor such that the portion of the fluid is substantially stationary within the measurement cavity; - at least one thermal property of the fluid (κ, ρ, cp , ρc p ) or the thermal properties of the fluid (κ, ρ, c p , ρc p ) combinations; - correcting the measured flow rate for at least one measured thermal property; Includes.

[0012] The above method basically involves determining at least one thermal property of a fluid (κ, ρ, cp, ρcp) or a thermal property of a fluid (κ, ρ, c p , ρc p This requires the implementation of a thermal property sensor in a "dead volume" that allows the measurement of a combination of the thermal properties of the fluid. The measurement cavity is, by design, completely open to the flow. Knowing these thermal properties from the measurement allows for a fluid-independent flow measurement to be established.

[0013] The above-described technology is particularly suited to in-channel thermal flow sensors, especially microelectromechanical systems (MEMS)-based thermal flow sensors in microfabricated channels, as well as for "fluidic chip" or "lab-on-a-chip" applications. However, this technology can also be used for heater and / or sensor elements that are fixed to the outer wall of the channel, establishing "indirect" thermal contact with the fluid. Depending on the sensor type, thermal flow sensors can be in direct or indirect thermal contact with the fluid flow. For example, hot wire sensors are in direct contact, while the wire windings of capillary tube sensors are in indirect contact.

[0014] In this patent application, "substantially stationary" means that the fluid is essentially "stationary," but allows for relatively small diffusion and movement of fluid particles within the measurement cavity. Such small diffusion and movement may even be necessary to reflect the changing fluid composition. The present invention has found that this is possible without interfering with the accuracy of the measurement. Grooves, particularly V-grooves and U-grooves, are particularly suitable for this purpose. These grooves can be used most advantageously when facing the flow or when facing away from the flow.

[0015] An embodiment further includes the following steps: - additionally measuring at least one thermal property in the fluid flow using a thermal flow sensor.

[0016] In one embodiment, the thermal property measured using the thermal flow sensor is heat capacity (c p ), density (ρ) or volumetric heat capacity (ρc p )

[0017] In one embodiment, the thermal property measured in a substantially stationary fluid is thermal conductivity (κ).

[0018] A further embodiment relates to the aforementioned method, wherein a combination of thermal properties is measured simultaneously in the flow of the fluid and on the substantially stationary fluid. The thermal properties measured in the flow include heat capacity (c p ) or density (ρ) or volumetric heat capacity (ρc p ), and the thermal property measured on a substantially stationary fluid is thermal conductivity (κ). p ) can be measured even in a substantially stationary fluid. p ) can also be measured.

[0019] One embodiment of the method further includes measuring pressure and / or pressure differential using a pressure sensor to derive viscosity from the measured flow rate and the measured pressure and / or pressure differential. Those skilled in the art may refer, for example, to Chapter 2 of "Distributed thermal microsensors for fluid flow" by John van Baar, 2002, ISBN 90-36518288, and / or "Micro-machined structures for thermal measurements of fluid and flow parameters" by JJ van Baar et al., Journal of Micromechanics and Microengineering, Vol. 11, No. 4 (July), pp. 311-318, 2001. The latter paper, like U.S. Pat. No. 4,685,331 B1, discloses sensors in a "dead volume" that necessarily contains stagnant fluid. Even in this setup, changes in the fluid flow path take a significant amount of time to diffuse into the blocked flow path. Furthermore, different fluids may diffuse into the blocked flow path at different rates. Van Baar (2001) also discloses a relatively large thermal conductivity sensor, 1 mm long, 25 μm wide, and 1 μm thick, and a TCR ("thermal coefficient of resistance") sensor, which is a thick silicon beam, 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 response of the sensor.

[0020] One embodiment relates to the aforementioned method, wherein the thermal property sensor includes a pair of heating wires, probes, or windings. Such heating wires are relatively thin compared to the volume of the measurement cavity and therefore do not significantly affect the thermal properties of the substantially stationary fluid. Optionally, the thermal property sensor can include two or more heating wires, which act as backups or drift checks for each other. A particular advantage of wires is their fast response time (typically less than 1 ms), which the present invention utilizes to improve measurement speed. Other elements with low thermal mass may be used instead of heating wires.

[0021] In another embodiment, the thermal property sensor includes a probe having a platinum sensor element.

[0022] An embodiment of the method described above includes the following steps: - Heating a portion of the fluid with a heating wire using a direct current (DC) or very low frequency alternating current (AC) current to measure the thermal conductivity (κ). - measuring the voltage of the heating wire during heating of the portion of the fluid using a voltage measuring means connected to the heating wire, and relating the measured voltage to the thermal conductivity.

[0023] In one embodiment, the method further comprises: p ) is measured by the following steps: - energizing the heating wire 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 during heating of the portion of the fluid using a voltage measuring means connected to the heating wire, and relating the phase and amplitude of the measured third harmonic of the AC voltage to the heat capacity.

[0024] Thus, AC excitation can be advantageously used to measure the voltage of a thermal property sensor. This exploits the fact that increasing the frequency reduces the thickness of the thermal boundary layer, allowing it to be moved closer to the wall. Therefore, due to the non-slip condition at the wall, the heat exchange between the heater and the sensor is independent of the flow velocity. That is, the heat transfer is affected only by the physical properties of the gas or fluid, not by the flow. The thermal conductivity κ and the volumetric heat capacity ρc p These two physical parameters are derived from the phase and amplitude of the third harmonic of the measured AC voltage. The appropriate frequency can be selected by taking into account the type of sensor and other environmental factors. The flow rate itself is determined by κ and ρc. pIt can be measured with DC excitation, which is dependent on the frequency of the fluid. It is clear that very low frequency AC excitation can be used as an alternative to DC excitation if the frequency is so low that the measurement can be completed in a few cycles, such as half a cycle. Of course, a person skilled in the art will take into account the type of fluid and any additional measures for correction. Therefore, by knowing these characteristics from AC measurements, it is possible to measure the flow rate independently of the fluid / gas.

[0025] Another aspect of the present invention is to determine the thermal conductivity (κ) and / or heat capacity (c) of the fluid whose flow rate is to be determined. p ) includes the following steps: - bringing the thermal flow sensor into thermal contact with the fluid flow. - Measuring the flow rate. - placing the thermal property sensor in a measurement cavity that is in fluid communication with the fluid flow and is completely open to said fluid flow; - Receiving a portion of fluid into a measurement cavity of said thermal property sensor, such that the portion of fluid is substantially stationary within the measurement cavity 5. - heating a heating wire of the thermal property sensor with a direct current (DC) current or a very low frequency alternating current (AC) current to heat a portion of the fluid, measuring the voltage of the heating wire during heating using a voltage measuring means connected to the 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 (6) 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 connected to said heating wire, and relating the phase and amplitude of the measured third harmonic to the heat capacity.

[0026] In one embodiment, the step of thermally contacting the thermal flow sensor with the fluid flow comprises thermally contacting the thermal flow sensor through an outer wall of the flow path.

[0027] Another aspect of the present invention is a thermal property sensor for use with a thermal flow sensor through which, in use, a fluid flows whose flow rate is to be determined, the thermal property sensor comprising: a heating wire arranged in a measurement cavity for receiving a portion of a fluid so that said portion of the fluid is substantially stationary; The heating wire is - heating a portion of the fluid with a direct current (DC) or a very low frequency alternating current (AC) current, the voltage of the heating wire during heating being measured by a voltage measuring means connected to the heating wire, the measured voltage being related to the thermal conductivity; and / or - a portion of the fluid is heated by a high frequency alternating current (AC) current to heat the portion, 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 connected to the heating wire, and the measured phase and amplitude of the third harmonic are related to the heat capacity.

[0028] An embodiment relates to the thermal property sensor as described above, wherein the measurement cavity is a groove and has a U-shaped or U-shaped or V-shaped cross section, and the heating wire is suspended within the measurement cavity.

[0029] An embodiment relates to the aforementioned thermal property sensor, where the wire has a cross section of less than 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, because of its small volume and therefore small heat capacity, resulting in a fast response.

[0030] One embodiment relates to the aforementioned thermal property sensor, where the wire has a cross section with one flat side, such as a square, triangular, semicircular, or, most preferably, rectangular shape. The flat portion of the cross section preferably has a width of less than 10 μm, e.g., 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 μm 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 approximately doubles the sensor's speed, with other variables remaining constant. In the field of flow sensors, where a response time twice as fast is desirable, even greater speed improvements can be achieved by halving the volume or less.

[0031] In one embodiment, the measurement cavity and / or the heating wire having a U-shaped or V-shaped cross section has a length of 1 mm to 3 mm, for example 1.5 mm to 2.5 mm, and / or the measurement cavity having a U-shaped or V-shaped cross section has a width of 20 μm to 60 μm.

[0032] One embodiment relates to the thermal property sensor described above, where the thermal property sensor is positioned to minimize the distance to the thermal flow sensor, i.e., without removing the thermal property sensor from the measurement cavity.

[0033] Another aspect of the present invention relates to a thermal flow sensor for measuring fluid flow, comprising: a measurement cavity in fluid communication with a fluid flow, the measurement cavity being completely open to said fluid flow; - a thermal property sensor according to any one of claims 11 to 13, wherein a heating wire is arranged in the measurement cavity for receiving a portion of the fluid, the portion of the fluid being substantially stationary within the measurement cavity; Equipped with.

[0034] One embodiment relates to a thermal flow sensor as described above, which, in use, is releasably inserted into a flow path through which fluid flow exists, or, if thermal contact occurs via (indirectly) the outer wall of the flow path, is fixed to the outer wall of the flow path.

[0035] Another aspect of the present invention is a thermal flow meter or control device including the thermal property sensor and / or the thermal flow sensor.

[0036] An embodiment relates to the thermal flow meter or control device described above, wherein at least one of the thermal property sensor or the thermal flow sensor is a microelectromechanical system (MEMS) device.

[0037] An embodiment relates to the thermal flow meter or controller described above, further comprising a viscosity sensor, a humidity sensor, a CO2 sensor, a temperature sensor, a dielectric constant sensor, a relative permittivity sensor, a fluid composition sensor or a multi-parameter sensor.

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

[0039] Another aspect of the present invention relates to a breathing apparatus including a thermal flow meter or control device as described above.

[0040] Another aspect of the present invention is a method for manufacturing the thermal flow sensor described above, comprising the steps of: (1) depositing a support layer on both sides of a 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) repeating steps 2 through 4 on the other side of the wafer; (6) Etching a Si wafer to create a U-groove or V-groove and a measurement cavity within the wafer; Includes.

[0041] In one embodiment, step (4) includes etching the support layer to open a window for etching the Si wafer underneath.

[0042] An embodiment relates to the aforementioned manufacturing method, wherein the metal layer comprises a Cr / Pt layer.

[0043] Certain embodiments relate to the aforementioned manufacturing method, wherein the support layer is a SiRN support layer.

[0044] Certain embodiments relate to the aforementioned method, examples of which are as follows: (1) The support layer has a thickness of 1 μm.

[0045] Certain embodiments relate to the aforementioned method, examples of which are as follows: (2) The Cr / Pt layer has a thickness of 20 nm / 200 nm.

[0046] Certain embodiments relate to the aforementioned method, examples of which are as follows: (3) Patterning of the support layer involves etching the Cr / Pt layer.

[0047] Certain embodiments relate to the aforementioned method, examples of which are as follows: (2) The Cr / Pt layer is deposited by sputtering.

[0048] In one embodiment, step 1 uses low pressure chemical vapor deposition (LPCVD).

[0049] In one embodiment, step 6 uses KOH for etching, preferably 1:3 in distilled water.

[0050] An embodiment relates to the thermal flow sensor described above and comprises: - A sensor body having a flow path cross section through which the fluid flows in the flow direction during use. a flow sensor arrangement including a plurality of flow sensing elements arranged at a plurality of positions within the cross-section of the flow path for measuring flow velocity at different positions within the cross-section of the flow path;

[0051] An embodiment relates to the thermal flow sensor described above and comprises: -Main body. a first base portion extending from said body portion; a second base portion extending from said body portion;

[0052] Here, the flow path cross section is formed between the main body, the first base and the second base.

[0053] An embodiment relates to the thermal flow sensor described above, wherein the measurement cavity is disposed in the body portion.

[0054] An embodiment relates to the thermal flow sensor described above, wherein the measurement cavity extends along a side surface of the flow passage cross section.

[0055] One embodiment relates to the thermal flow sensor described above, wherein the flow sensor structure comprises a plurality of flow sensing elements arranged at a plurality of positions across the flow path.

[0056] An embodiment relates to the thermal flow sensor described above, wherein the flow passage cross section is open at a side of the flow passage cross section that is not bounded by the main body, the first base and / or the second base.

[0057] An embodiment relates to the thermal flow sensor described above, wherein the first base and the second base are parallel to each other.

[0058] An embodiment relates to the thermal flow sensor described above, wherein the flow passage cross section has a square or rectangular shape in a plane transverse to the flow direction.

[0059] One embodiment relates to the thermal flow sensor described above, wherein the plurality of flow sensing elements extend between the first base and the second base.

[0060] One embodiment relates to the thermal flow sensor described above, wherein the plurality of flow detection elements are arranged at equal intervals in the cross section of the flow path.

[0061] An embodiment relates to the thermal flow sensor described above, wherein the plurality of flow sensing elements includes three or more flow sensing elements.

[0062] One 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.

[0063] In one embodiment, the distance between the pair of flow detection wires is 300 μm to 500 μm, preferably 350 μm to 450 μm, more preferably 375 μm to 425 μm, in relation to the thermal flow sensor.

[0064] An embodiment relates to the thermal flow sensor described above, wherein the pair of flow sensing wires extend between the first base and the second base.

[0065] One embodiment relates to the thermal flow sensor described above, wherein the sensor body is formed as a chip.

[0066] One embodiment relates to the thermal flow sensor described above, wherein the sensor body is attached to or disposed on a printed circuit board (PCB).

[0067] One embodiment relates to the thermal flow sensor described above, wherein at least one of the sensors is preferably a flow sensing wire, forming part of a Wheatstone bridge.

[0068] One embodiment relates to the thermal flow sensor described above, where one or more pairs of sensor flow sensing wires each form one half of a Wheatstone bridge.

[0069] One embodiment relates to the thermal flow sensor described above, where a fixed resistor disposed in the sensor body forms the other half of the Wheatstone bridge.

[0070] One embodiment relates to the thermal flow sensor described above and is a microelectromechanical system (MEMS) component. [Brief explanation of the drawings]

[0071] 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] 10A and 10B are diagrams illustrating the heating wire of the thermal property sensor connected to the voltage measurement means of the embodiment; [Figure 4] 10A and 10B are cross-sectional views 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] 10A-10C show steps for creating a V-shaped measurement cavity with a heating wire. [Figure 7] FIG. 10 is a diagram showing a flow path provided with a thermal flow sensor and a thermal property sensor according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0072] FIG. 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 is furthermore completely open to the fluid flow 3. The measurement cavity 5 may be disposed within the fluid flow 3, e.g., immersed therein. The measurement cavity 5 may face the fluid flow 3 (or may be remote from 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 can be releasably inserted into a flow path 10 in which the fluid flow 3 is present. FIG. 1 illustrates a chip embodiment with bonding pads. However, those skilled in the art will recognize that other designs and embodiments are possible.

[0073] The thermal flow sensor 2 may include a sensor body 11 having a flow cross section 12 through which, during use, a fluid 1 whose flow rate is to be determined flows in a flow direction. According to the present invention, a flow sensor assembly 13 may be provided, including a plurality of flow sensing elements 14 arranged at a plurality of positions on the flow cross section 12. The thermal flow sensor 2 may include a body portion 15, a first base portion 16 extending from the body portion 15, and a second base portion 17 extending from the body portion 15. The flow cross section 12 may be formed between the body portion 15, the first base portion 16, and the second base portion 17. The measurement cavity 5 may be disposed in the body portion 15. The measurement cavity 5 may extend along a side of the flow cross section 12. The flow sensor assembly 13 may include a plurality of flow sensing elements 14 arranged at a plurality of positions on the flow cross section 12, for example, at three positions, as shown in FIG. 1 . 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 intersecting the direction of the flow 3. A 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. The 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. Such wires 18 are surprisingly robust and insensitive to small particles such as dust that sometimes contaminate fluids. However, appropriate precautions must be taken against overheating. The sensor body 11 may be formed as a chip 19. The sensor body 11 may be attached to or disposed on a printed circuit board (PCB) 20. Each set of one or more flow sensing wires 18 may form one half (R2, R3) of a Wheatstone bridge 21, as shown in FIG. 5.Those skilled in the art will understand that if a differential flow signal is to be 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 (depending on the flow direction) output voltage signal.

[0074] One or more flow sensing elements 14 may also be disposed on the outer wall (not shown) of the schematic flow channel 10 so as to establish "indirect" thermal contact with the fluid flow 3.

[0075] FIG. 2 is a perspective view showing an exemplary embodiment of a thermal property sensor 4 with a measurement cavity 5 in more detail.

[0076] As shown more clearly in FIG. 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 can be configured to be heated with an 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 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 related to the thermal capacity c. p is associated with.

[0077] As shown in Figure 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 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 flow rate of the fluid 3. 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 between 50 and 60 degrees.

[0078] In another embodiment, a method is provided for determining the flow rate of fluid 1 without relying on the thermal properties of fluid 1. The method comprises the following steps: - bringing the thermal flow sensor 2 into contact with the fluid flow 3 or bringing the thermal flow sensor 2 into thermal contact with the fluid flow 3; - Measuring the flow rate. - placing a thermal property sensor 4 in a measurement cavity 5 in fluid contact with, 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.

[0079] The method may further include: - additionally measuring at least one thermal property in the fluid flow 3 using a thermal flow sensor 2;

[0080] The thermal properties measured in the fluid flow 3 are the heat capacity (c p ) or density (ρ).

[0081] The thermal property measured on an essentially stationary fluid 6 is the thermal conductivity (κ).

[0082] The schematic pressure sensor 7 can additionally measure pressure and / or pressure differential to derive viscosity from thermal properties and the measured pressure and / or pressure differential. Viscosity can also be measured "thermally," as demonstrated by John van Baar.

[0083] 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.

[0084] 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.

[0085] 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 comparing the phase and amplitude of the third harmonic of the measured AC voltage with the heat capacity (preferably the volumetric heat capacity ρc p ) to associate with the

[0086] In another embodiment, the thermal conductivity (κ) and / or heat capacity (c p A method is provided for determining the .times. ... - bringing the thermal flow sensor 2 into thermal contact with the fluid flow 3; - Measuring the flow rate. - placing the thermal property sensor 4 in a measurement cavity 5 in fluid communication with, preferably adjacent to, the fluid flow 3, where the measurement cavity 5 is completely open to the fluid flow, e.g., positioned within the fluid flow. - 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) current or a very low frequency alternating current (AC) current to heat the portion 6 of the fluid, measuring the voltage of the 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 the heating wire 8 of the thermal property sensor 4 with a high frequency alternating current (AC) current to heat the portion 6 of the fluid, measuring the phase and amplitude of the third harmonic of the AC voltage of the heating wire 8 during heating using a voltage measuring means 9 connected to the heating wire 8, and converting the phase and amplitude of the measured third harmonic into a heat capacity (preferably a volumetric heat capacity ρc p ) to associate with the

[0087] 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 the wires and metal traces (2, 3). While these thicknesses of Cr and Pt combined produce 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 create measurement cavities 5, 24 inside the wafer 25 between / around the heating wires 8, 14.

[0088] 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 be provided to measure the differential pressure. 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]

[0089] 1.Fluid, 2. Thermal flow sensor, 3. Fluid flow, 4. Thermal property sensors, 5. Measuring cavity, 6. Stationary part of the fluid, 7. Pressure sensors, 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. Support layer (of SiRN); 23.Cr / Pt layer, 24. Measuring cavity, 25.Si wafer, 26. Additional sensors.

Claims

1. 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) in thermal contact with a fluid flow (3); - measuring the flow rate; - placing a thermal property sensor (4) in a measurement cavity (5) in fluid contact with the fluid flow (3); - receiving a portion of fluid (6) in a measurement cavity of said thermal property sensor, such that said portion of fluid (6) is substantially stationary in said measurement cavity; - at least one thermal property of the fluid (κ, ρ, c ｐ , ρ ｐ ) or the thermal properties of the fluid (κ, ρ, c ｐ , ρ ｐ ) combinations; - correcting the measured flow rate for at least one measured thermal property; A method comprising:

2. 2. The method of claim 1, further comprising the step of measuring at least one additional thermal property in the fluid flow (3) using a thermal flow sensor (2).

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

4. 4. A method according to any one of claims 1 to 3, characterized in that the measured thermal property of the substantially stationary fluid (6) is the thermal conductivity (κ).

5. 5. The method according to any one of claims 1 to 4, further comprising measuring the pressure and / or pressure difference with a pressure sensor (7) in order to derive the viscosity from the measured flow rate and the measured pressure and / or pressure difference.

6. 6. The method according to any one of claims 1 to 5, wherein the thermal property sensor (4) or the thermal flow sensor comprises a heated wire (8).

7. To measure the thermal conductivity (κ), - heating said 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 said heating wire during heating of the portion of fluid (6) using voltage measuring means (9) connected to said heating wire and relating the measured voltage to the thermal conductivity; 7. The method according to claim 4 or 6, comprising:

8. Heat capacity (c ｐ ) to measure - powering said heating wire with an alternating current (AC) current; - measuring the phase and amplitude of the third harmonic of the alternating current (AC) voltage of said heating wire during heating of the portion of fluid (6) using voltage measurement means (9) connected to said heating wire, and relating the phase and amplitude of the measured third harmonic of the AC voltage to the heat capacity; 7. The method according to claim 3 or 6, comprising:

9. The thermal conductivity (κ) and / or heat capacity (c) of the fluid 1 whose flow rate is to be determined. ｐ ) to determine - placing a thermal flow sensor (2) in thermal contact with a fluid flow (3); - measuring the flow rate; - placing a thermal property sensor (4) in a measurement cavity (5) in fluid communication with and completely open to the fluid flow (3); - receiving a portion (6) of a fluid in a measurement cavity (5) of said thermal property sensor (4) so ​​that said portion (6) of the fluid is substantially stationary in said measurement cavity (5); - heating a heating wire (8) of said thermal property sensor (4) with a direct current (DC) or a very low frequency alternating current (AC) current to heat a portion (6) of a 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 said thermal property sensor (4) with a high frequency alternating current (AC) current to heat a portion (6) 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 measured phase and amplitude of the third harmonic to the heat capacity; A method comprising:

10. 10. The method according to claim 1, wherein the step of bringing the thermal flow sensor (2) into thermal contact with the fluid flow (3) comprises bringing the thermal flow sensor (2) into thermal contact with the fluid flow (3) through an outer wall of a flow path (10).

11. A thermal property sensor (4) for use with a thermal flow sensor (2) through which, in use, a fluid (1) flows whose flow rate is to be determined, comprising: a heating wire (8) arranged in a measuring cavity (5) for receiving a portion of a fluid (6) so that said portion of the fluid (6) is substantially stationary, The heating wire (8) - a direct current (DC) or a very low frequency alternating current (AC) current is used to heat a portion of said fluid, the voltage of said heating wire during heating being measured by a voltage measuring means (9) connected to said heating wire (8), said measured voltage being related to the thermal conductivity; and / or - A thermal property sensor (4) characterized in that it is heated by a high frequency alternating current (AC) current to heat a portion of said fluid, and the phase and amplitude of the third harmonic of the alternating current (AC) voltage of said heating wire during heating are measured by a voltage measuring means (9) connected to said heating wire (8), and the phase and amplitude of the measured third harmonic are related to the heat capacity.

12. said measuring cavity (5) having a U-shaped or U-shaped or V-shaped cross section, 12. The thermal property sensor (4) according to claim 11, characterized in that the heating wire (8) is suspended within the measuring cavity (5).

13. A thermal property sensor (4) as described in claim 12, characterized in that the measuring cavity (5) and / or the heating wire (8) have a length of 1 mm to 3 mm, for example 1.5 mm to 2.5 mm, and / or the measuring cavity (5) has a width of 20 μm to 60 μm.

14. A thermal flow sensor (2) for measuring a flow rate, a measuring cavity (5) in fluid communication with a fluid flow, the measuring cavity (5) being completely open to said fluid flow; A thermal property sensor (4) according to any one of claims 11 to 13, in which a heating wire (8) is arranged in the measurement cavity for receiving a portion of a fluid (6), the portion of the fluid being substantially stationary in the measurement cavity; A thermal flow sensor (2) comprising:

15. A thermal flow sensor (2) as claimed in claim 14, characterized in that, in use, it is releasably inserted into a flow path (10) in which a fluid flow (3) is present, or, if dependent on claim 10, is fixed to the outer wall of the flow path (10).

16. A thermal flow meter or control device comprising a thermal property sensor (4) according to any one of claims 11 to 13 and / or a thermal flow sensor (2) according to any one of claims 14 to 15.

17. 17. The thermal flow meter or control device of claim 16, wherein at least one of the thermal property sensor (4) or the thermal flow sensor (2) is a microelectromechanical system (MEMS) device.

18. Viscosity sensor, humidity sensor, CO ２ 18. A thermal flow meter or control device according to claim 16 or 17, further comprising a sensor, a temperature sensor, a dielectric constant sensor, a relative permittivity sensor, a fluid composition sensor or a multi-parameter sensor.

19. A medical device comprising a heat flow meter or a control device according to any one of claims 16 to 18.

20. 20. The medical device of claim 19, which is a respiratory device.

21. A method for manufacturing the thermal flow sensor (2) according to any one of claims 14 to 15, 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 U-grooves or V-grooves (5) and measurement cavities (24) within said wafer (25); A method comprising:

22. 22. The method of claim 21, wherein step (4) includes etching the support layer to open windows for etching the Si wafer underneath.

23. 23. The method of claim 21 or 22, wherein the metal layer comprises a Cr / Pt layer.

24. 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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