THERMAL FLOW SENSOR AND METHOD FOR OPERATING THE SAME

Description

[0001] The invention relates to a method for operating a thermal flow sensor, wherein the thermal flow sensor comprises at least one sensor element and an electronic unit. Furthermore, the invention relates to a thermal flow sensor configured to be operated by the method according to the invention.

[0002] Thermal flow sensors are known for determining the flow rate or flow velocity of a measuring medium or fluid, such as a gas, gas mixture, or liquid. These utilize the fact that a (flowing) measuring medium dissipates heat from a heated surface. Thermal flow sensors typically consist of several functional elements, usually at least one low-resistance heating element and one high-resistance resistance element that serves as a temperature sensor. Alternatively, thermal flow sensors are constructed with several low-resistance heating elements as a heater and temperature sensor.

[0003] Calorimetric thermal flow sensors determine the flow rate of the fluid in a channel by measuring a temperature difference between two temperature sensors located downstream and upstream of a heating element. This takes advantage of the fact that the temperature difference is linear with the flow rate up to a certain point. This method is extensively described in the relevant literature.

[0004] Anemometric thermal flow sensors consist of at least one heating element, which is heated during flow measurement. As the measured medium flows around the heating element, heat is transferred into the measured medium, which changes with the flow velocity. By measuring the electrical parameters of the heating element, the flow velocity of the measured medium can be determined. Such an anemometric thermal flow sensor is typically operated in one of the following two control modes: In the "Constant-Current Anemometry" (CCA) control mode, the heating element is subjected to a constant current. As the measured medium flows around it, the resistance of the heating element changes, and thus the voltage drop across the heating element, which represents the measurement signal. The "Constant-Voltage Anemometry" (CVA) control mode works in a similar way, in which the heating element is subjected to a constant voltage.

[0005] With the "Constant-Temperature Anemometry (CTA)" control method, the heating element is kept at an average constant temperature. Relatively high flow velocities can be measured using this control method. Depending on the flow velocity, more or less heat is carried away by the flowing medium, and accordingly, more or less electrical power must be supplied to keep the temperature constant. This supplied electrical power is a measure of the flow velocity of the medium.

[0006] However, the heat dissipated by anemometric or calorimetric thermal flow sensors depends on the thermal parameters of the measuring medium—primarily the thermal conductivity and heat capacity of the measuring medium. Therefore, the thermal flow sensor must be calibrated in advance in a special calibration device for a specific measuring medium. If the measuring medium changes, the thermal flow sensor must be recalibrated in the calibration device before it can again output correct values.

[0007] There are already measurement principle approaches for membrane flow sensors for gases that measure thermal conductivity and heat capacity in a first step (using constant power or AC drive) and use this information in the flow velocity measurement to compensate for the fluid dependence in a second step. However, certain information about the gas properties is often required a priori for this self-calibration to work.

[0008] Furthermore, the thermal conductivity and heat capacity of the measuring medium often have to be measured separately, or rather, they have to be known. This separate determination often requires different measurement conditions or measurement sequences, which makes the process more complex. This means that multiple steps, different sensors, and electronic components may be required. Such a complex implementation can often lead to higher production costs and thus higher prices.

[0009] EP 3 106 851 A1 discloses a temperature sensor that can be used to determine the thermal properties of a fluid, in particular its heat capacity and thermal conductivity. For this purpose, the temperature sensor measures the temperature amplitude and phase shift curves and feeds them into a reduced-order parametric model, which provides the heat capacity and / or its thermal conductivity and the heat capacity as a result.

[0010] Based on this problem, the invention is based on the object of presenting a method which allows a calibration of a sensor to a specific measuring medium without the properties of the measuring medium being known a priori.

[0011] The object is achieved by a method for operating a thermal flow sensor, wherein the thermal flow sensor has at least one sensor element and an electronic unit, comprising: Bringing a measuring medium into thermal contact with the sensor element and periodically heating the measuring medium by means of an alternating voltage introduced into the sensor element, wherein a maximum amplitude of a temperature profile and / or a phase shift between the alternating voltage profile and the temperature profile is simultaneously recorded; adjusting the recorded maximum amplitude and / or the recorded phase shift using calibration data; determining an isoline using the adjusted maximum amplitude and / or the adjusted phase shift on the basis of a mathematical or physical model of the thermal flow sensor, wherein the isoline has a plurality of value pairs of thermal parameters, in particular a thermal conductivity and a heat capacity, of the measuring medium, which value pairs have the same maximum amplitude orthe same phase shift; deriving at least one piece of measuring fluid information from the isoline; and performing a flow measurement using the thermal flow sensor, wherein the sensor element supplies signal values, and converting the signal values ​​into measured values ​​of the effective flow velocity of the measuring medium using the measuring fluid information.

[0012] The advantage of the method according to the invention is that, compared to conventional operating methods of thermal flow sensors, the dependence of the measured flow velocity on the measuring medium can be directly compensated. For this purpose, a so-called "3-omega method" is used, by means of which a maximum amplitude and / or a phase shift is determined. These values ​​are compensated using calibration values, which are determined in particular by the manufacturer. Subsequently, a so-called "isoline" of the heat capacity and thermal conductivity is determined. An isoline represents those pairs of heat capacity and thermal conductivity values ​​for which the measured and compensated maximum amplitude or phase shift are the same. This can apply to several different measuring media, so that the measuring medium itself does not need to be determined, but only its underlying behavior relative to the isoline.

[0013] The model used to determine the isoline is based on the heat conduction equation. This isoline can be fed into a second model, which is used to convert the signal values ​​into measured flow velocity values. A more detailed explanation follows in the description of the figures. Since the measuring medium itself does not need to be determined—and its heat capacity and thermal conductivity are required for this—the method according to the invention is advantageous in terms of effort and cost compared to known methods.

[0014] The term "flow velocity" also includes the volume flow of a measuring medium.

[0015] According to an advantageous embodiment of the method according to the invention, the calibration data are determined in advance using a known measuring medium with a defined pair of thermal parameters, in particular water. Determining the calibration data includes detecting a reference maximum amplitude of a temperature profile and / or a reference phase shift between the alternating voltage profile and the temperature profile. A so-called "3-omega method" is also used for this purpose, by means of which the reference values ​​are determined.

[0016] According to an advantageous embodiment of the method according to the invention, a flow velocity of the measuring medium of zero is required for detecting the maximum amplitude and / or the phase shift and for determining the calibration data. Conditions may also exist in which the flow velocity of the measuring medium is not equal to zero. However, determining the isoline is then significantly more complex and laborious.

[0017] Determining the calibration data can also involve a two-point calibration. For example, one measurement is performed at the specified flow velocity of zero and one measurement is performed at a finite flow value.

[0018] According to an advantageous embodiment of the method according to the invention, the calibration data is stored in the electronics unit. As described above, the calibration data is preferably determined by the manufacturer. In order for the customer or user to be able to perform the subsequent steps of the method, the calibration data must be made available to them. Therefore, this calibration data is stored directly in the electronics unit of the thermal flow sensor, whereupon it can be read out by the customer or user.

[0019] According to an advantageous embodiment of the method according to the invention, the flow measurement is carried out using a constant-temperature anemometry method. As described above, a heating element of the thermal flow sensor is subjected to a constant current. The flow of the measuring medium around it changes the resistance of the heating element and thus the voltage drop across the heating element, which represents the measurement signal.

[0020] According to an advantageous embodiment of the method according to the invention, it is provided that the steps of determining the isoline and deriving the measuring fluid information from the isoline are repeated in the event that one or more of the following cases occur: A change or modification of the measuring medium; checking the sensor element for contamination; changes in the composition of the measuring medium, particularly due to decomposition of the measuring medium.

[0021] These are situations that occur during operation and for which the thermal flow sensor does not need to be returned to the manufacturer. In other words, the calibration data remains valid, so only the behavior between the thermal flow sensor and the measuring medium needs to be re-evaluated by determining a new isoline.

[0022] According to an advantageous embodiment of the method according to the invention, the steps of determining the calibration data, determining the isoline, and deriving the measuring fluid information from the isoline are repeated in the event of a defect in the thermal flow sensor. If the thermal flow sensor is still functional despite the defect, it may have a modified sensor element characteristic. In this case, the thermal flow sensor must be returned to the manufacturer so that the manufacturer can repeat the step of determining the calibration data.

[0023] According to an advantageous embodiment of the method according to the invention, the thermal flow sensor is connected, in particular temporarily, to a calibration device for determining the calibration data. The calibration device comprises a pipeline or channel and a pump element, and the pump element establishes a flow through the pipeline or channel with the known measuring medium at the defined flow velocity value. By means of this calibration device, which is provided in particular by the manufacturer, defined states or values ​​of the flow velocity of the known measuring medium can be generated, so that the calibration data of the thermal flow sensor can be determined with high precision.

[0024] According to an advantageous embodiment of the method according to the invention, the thermal flow sensor for determining the isoline and for performing the flow measurement is mounted at a measuring point in a pipe or channel through which the measuring medium flows. The isoline can thus advantageously be determined directly at the installation location intended for the operation of the thermal flow sensor and does not require any additional, special measuring equipment. Removal of the sensor for a possible change of the measuring medium or for checking the sensor and / or the measuring medium is thus completely eliminated.

[0025] According to an advantageous embodiment of the method according to the invention, the step of detecting the maximum amplitude or phase shift is repeated periodically, and the electronics unit generates a warning if the respectively detected maximum amplitudes or phase shifts deviate from one another by more than a predetermined factor. The thermal flow sensor thus has a self-checking function and enables predictive maintenance, as deviations from the regular or known measured values ​​are detected. This also allows the user to be informed when—as described above—there is a change in the behavior between the thermal flow sensor and the measuring medium and that the isoline should be redefined.

[0026] Furthermore, the object is achieved by a thermal flow sensor designed to be operated using the method according to the invention. It is essential for the thermal flow sensor to be highly sensitive and have a short response time—for example, through a simplified layer structure, which also simplifies the mathematical model of the thermal flow sensor and reduces the associated uncertainties.

[0027] The invention is explained in more detail with reference to the following figures. Fig. 1: an overview of the method according to the invention; Fig. 2: a graph showing isolines for various values ​​of measurement signals; and Fig. 3: an embodiment of a thermal flow sensor according to the invention.

[0028] Fig. 1 shows a flowchart of an exemplary embodiment of the method according to the invention. The method is used to calibrate a thermal flow sensor 100 to a second measuring medium 2 for an application of a user A.

[0029] The first method step a is carried out by the manufacturer H or by a calibration service. The thermal flow sensor 100 is connected to a calibration device. The calibration device has a pipeline, or a channel, and a pump element. The pump element is designed to establish a flow through the pipeline, or channel, with a known measuring medium at a defined flow velocity. The composition of the first measuring medium and its thermal parameters are known—for example, it is water. Particularly relevant thermal parameters for the method according to the invention are, for example, the thermal conductivity and the heat capacity of a measuring medium.

[0030] To acquire calibration data, an alternating current test of the thermal flow sensor 100 is performed. An alternating current signal is applied to the sensor element 101, 102, 103, and the voltage drop across the sensor element 101, 102, 103 is simultaneously recorded. A reference maximum amplitude of a temperature profile and / or a reference phase shift between the alternating voltage profile and the temperature profile is recorded.

[0031] This alternating current investigation can be carried out, for example, using a lock-in method.

[0032] These calibration data are stored in an electronics unit of the thermal flow sensor 100. The thermal flow sensor 100 is then removed from the calibration device and handed over to user A. User A then installs the thermal flow sensor 100 in a measuring point, where the thermal flow sensor 100 serves the purpose of determining the flow velocity of a measuring medium 2 flowing through a pipe or channel at the measuring point. In particular, the thermal parameters of the measuring medium 2 are unknown.

[0033] In a second method step b, the second measuring medium 2 is characterized. In this example, the second measuring medium 2 is an oil of unknown composition. The so-called "3-Omega method" is used for this, whereby the flow velocity of the second measuring medium is zero during this method step b. A sensor element 101, 102, 103 of the thermal flow sensor 100 is periodically excited with an alternating voltage or an alternating current and acts as a heating element. This periodically induces a heat pulse into the second measuring medium 2. At the same time, the temperature of the sensor element is determined. By comparing the excitation signal with the temperature signal, in particular its third harmonic, a maximum amplitude of a temperature curve and / or a phase shift between the alternating voltage curve and the temperature curve can be determined.This measured maximum amplitude or the measured phase shift is then adjusted using the calibration data.

[0034] Anschließend An isoline ILx is determined based on the adjusted maximum amplitude or the adjusted phase shift. In Fig. 2 A graph is shown which represents a plurality of such isolines IL1, IL2, ..., ILx, ..., ILn for liquids. For gases, for which the method according to the invention can also be carried out, the curves may appear different. The x-axis represents the heat capacity Cp, while the y-axis represents the thermal conductivity k. An isoline IL1, IL2, ..., ILx, ..., ILn is now a plurality of value pairs of the heat capacity Cp and the thermal conductivity k for which the adjusted maximum amplitude or the adjusted phase shift assumes the same value. The isolines IL1, IL2, ..., ILx, ..., ILn are based on a mathematical model f ( k, c p ) where k where C p is the thermal conductivity and C p is the heat capacity. The purpose of this model is to describe the physical behavior of temperature during periodic heating. This model is based on the heat conduction equation: dT dt − k ρc p Δ T = 0 , where ρ denotes the density.

[0035] Mathematically speaking, an isoline (of the first model) is defined as ∀ k, c p | f ( k, c p ) = const.

[0036] For the present example, it is assumed that the isoline ILx is determined based on the adjusted measured maximum amplitude or the measured phase shift.

[0037] In process step c, the isoline IIx is transferred to a constant temperature anemometry flow measurement. For this purpose, measured medium information from the isoline IIx is used. The flow measurement is then performed using the thermal flow sensor 100. The sensor elements 101, 102, 103 provide signal values, which are converted into measured values ​​of the effective flow velocity of the measuring medium 2 using the measured fluid information.

[0038] Specifically, a second mathematical model (for constant temperature anemometers) g ( k, c p ). The purpose of this model is to describe the energy / power required for constant-temperature anemometry. This model is based in particular on the conservation of energy during forced convection, which is defined as follows: dE dt = m ˙ c p T medium , out − T medium , in = h k c p A T heater − T medium , in , where E is the energy used, ṁ is the mass flow, T medium,in is the temperature of the medium when reaching the heater, T medium,out is the temperature of the medium after the heater and T heater is the temperature of the heater, h is the heat transfer coefficient and A is the cross-sectional area of ​​the channel / pipe.

[0039] The second model also includes an isoline, which is called ∀ k, c p | g ( k , c p ) = const and is referred to as "further isoline".

[0040] The crucial observation that was established experimentally is the following: ∀ k , c p f k c p = const → g k c p ≅ const

[0041] The ≅ symbol symbolizes the "similarity" of the two models f and g. Based on equation 1, there is no difference in the parameters ( k, c p ) between the first isoline ILx and the further isoline.

[0042] Process step d allows the thermal flow sensor to be readjusted to a changed measuring medium 2 or, for example, in the event of a deposit on one of the sensor elements 101, 102, 103. For this purpose, process steps b and c are repeated, thereby determining a new isoline ILx. It can be provided to redetermine the maximum amplitude or the phase shift at regular intervals and examine it for deviations. In the event of a deviation, this can be displayed to the user, prompting them to initiate the next steps. Alternatively, process steps b and c can be performed automatically.

[0043] Process step e allows the thermal flow sensor 100 to be completely re-characterized and re-calibrated in the event of a defect that alters the measurement properties of the thermal flow sensor 100 but does not impair its functionality. For this purpose, process steps a to c are repeated, with the thermal flow sensor 100 being handed over, at least temporarily, to the manufacturer H or to a calibration service.

[0044] It is essential for the thermal flow sensor used that it is very sensitive and has a short response time. Fig. 2shows an embodiment of such a thermal flow sensor 100 according to the invention. Fig. 2a shows an isometric view of the thermal flow sensor 100. Fig. 2b shows a cross section orthogonal to a flow direction F of the measuring medium 2 through the carrier element 110 of the thermal flow sensor 100.

[0045] A tube is used as the carrier element 110, through which a measuring medium 2 can flow in a flow direction F. The carrier element 110 is made of a metallic material, in particular stainless steel, and has, for example, a wall thickness of 150 µm.

[0046] An electrical insulation layer 140 is applied to a first surface 111 of the carrier element, facing away from the measuring medium, using a thin-film process. In this example, the electrical insulation layer 140 is a ceramic material, for example, Al 2 O 3 , with a layer thickness of approximately 2 µm.

[0047] A functional layer 120 made of platinum is then applied to the electrical insulation layer 140. The functional layer is applied using a thin-film process, for example, sputtering or vapor deposition, and has a layer thickness of approximately 800 nm. The electrical insulation layer serves to electrically insulate the functional layer 120 and the carrier element. The electrical insulation layer 140 and the functional layer 120 have essentially the same convex curvature as the first surface 111.

[0048] In a subsequent process step, the functional layer is structured using a laser ablation process. Here, a laser successively removes the material of the functional layer, creating the structures of sensor elements 101, 102, 103. These structures are, for example, resistance structures. The resistance structure serves to determine the temperature of the measuring medium. For this purpose, the resistance value of the resistance structure can be measured. The measured resistance value is a directly proportional measure of the temperature at the resistance structure. By applying an electrical voltage or an electrical current to such a resistance structure, it is heated, so that a sensor element 101, 102, 103 can also serve as a heating element.

[0049] Another method for structuring the functional layer is a 3D lithography process. However, the advantage of using the laser ablation process over the 3D lithography process is lower effort, resulting in lower costs, and increased environmental friendliness.

[0050] Subsequently, a passivation layer 130, in particular consisting of a ceramic material, is applied to the functional layer 120 and the electrical insulation layer 140. This layer has a layer thickness of approximately 30 µm and serves to protect the functional layer 120 and the electrical insulation layer 140 from environmental influences, such as corrosion.

[0051] An alternative embodiment of the sensor element 100 consists in using a non-conductive material for the carrier element 110. In this case, the electrically insulating layer 140 can be omitted, so that it is provided that the functional layer 10 is applied directly to the carrier element 110.

[0052] The thermal flow sensor has at least one sensor element 101. In such a case, the sensor element 101 serves to alternately temporarily heat the measuring medium 2 and to determine the temperature of the measuring medium 2.

[0053] It can be provided to use two or more sensor elements 101, 102, 103, in particular arranged in the flow direction, wherein one of the sensor elements 101 is designed as a heating element and arranged centrally and wherein one of the sensor elements 102, 103 is designed as a temperature sensor and is arranged upstream in the flow direction F and downstream in the flow direction F from the heating element.

[0054] Such a thermal flow sensor 100 can be operated in the conventional operating modes "calorimetric flow measurement", "anemometric flow measurement" and "time-of-flight flow measurement" and, in addition to the sensor elements 100, has an electronic unit comprising a control unit and an evaluation unit, a voltage / current source and wiring.

[0055] Calorimetric thermal flow sensors 100 determine the flow or flow rate of the measuring medium 2 in a channel via a temperature difference between two sensor elements 102, 103 in the form of temperature sensors arranged downstream and upstream of a sensor element 101 designed as a heating element. This takes advantage of the fact that the temperature difference is linear to the flow or flow rate up to a certain point. This method is extensively described in the relevant literature.

[0056] Anemometric thermal flow sensors consist of at least one sensor element 101 in the form of a heating element, which is heated during the flow measurement. As the measuring medium 2 flows around the heating element, heat is transferred into the measuring medium, which changes with the flow velocity. By measuring the electrical parameters of the heating element, the flow velocity of the measuring medium can be determined.

[0057] Flow sensors based on the so-called "time-of-flight" measurement principle have at least one sensor element 101 in the form of a heating element and one sensor element 102, 103 in the form of a temperature sensor. The heating element emits a brief heat pulse into the measuring medium 2, which causes local heating of the measuring medium 2. The flowing measuring medium 2 causes a movement of the local heating corresponding to the existing flow. If the local heating approaches the temperature sensor, it is detected by the temperature sensor. An evaluation unit determines the time difference between the induction of the heat pulse and the detection of the local heating by the temperature sensor. The time difference represents a measure of the flow velocity of the measuring medium. The lower the time difference, the higher the existing flow velocity of the measuring medium 2, and vice versa.

[0058] Instead of designing a sensor element 101, 102, 103 as a heating element, it can also be designed as a cooling element, for example, a Peltier element. The operating modes described above can also be implemented using a cooling element, in which case a cooling pulse is induced in the measuring medium.

[0059] In addition to this specific embodiment of a thermal flow sensor 100, it will be understood by those skilled in the art that any other embodiments and types of thermal flow sensors can be used in conjunction with the method according to the invention, as long as they have a sufficiently high sensitivity. For example, a sensor element 101, 102, 103, which is applied to a substrate, can be soldered to the substrate on the carrier element 110, which also enables high sensitivity. List of reference symbols

[0060] 100 thermal flow sensor 101, 102, 103 sensor elements 110 support element 111 first surface 112 second surface 120 functional layer 130 passivation layer 140 electrical insulation layer 2 measuring medium a, b, c, d, e process steps A user C p , k thermal parameters H manufacturer IL1, IL2, ..., ILx, ..., ILn isolines

Claims

1. A method for operating a thermal flow sensor (100), wherein the thermal flow sensor (100) has at least one sensor element (101, 102, 103) and an electronic unit, comprising: - bringing a measuring medium (2) into thermal contact with the sensor element (101, 102, 103) and periodically heating the measuring medium (2) by means of an alternating voltage applied to the sensor element (101 , 102, 103) and simultaneously detecting a maximum amplitude of a temperature profile and / or a phase shift between the profile of the alternating voltage and the profile of the temperature; - adjusting the detected maximum amplitude and / or the detected phase shift with the aid of calibration data; - determining an isoline (ILx) using the adjusted maximum amplitude and / or the adjusted phase shift on the basis of a mathematical or physical model of the thermal flow sensor (100), the isoline (ILx) ) of a multiplicity of value pairs of thermal parameters (Cp, k), in particular a thermal conductivity and a heat capacity, of the measurement medium (2), which value pairs are associated with the same maximum amplitude or the same phase shift; - deriving at least one measurement fluid datum from the isoline (ILx); and carrying out a flow measurement by means of the thermal flow sensor (100), the sensor element (101, 102, 103) supplying signal values, and converting the signal values into measured values of the effective flow velocity of the measuring medium (2) by means of the measuring fluid information.

2. Method according to claim 1, wherein the calibration data are determined in advance with a known measuring medium with a defined pair of values of thermal parameters (Cp, k), in particular water, wherein the determination of the calibration data comprises detecting a reference maximum amplitude of a temperature profile and / or a reference phase shift between the alternating voltage profile and the temperature profile.

3. Method according to at least one of the previous claims, wherein a flow velocity of the measuring medium (2), or of the known measuring medium, is zero for detecting the maximum amplitude and / or the phase shift and for determining the calibration data.

4. Method according to at least one of the previous claims, wherein the calibration data are stored in the electronic unit.

5. method according to at least one of the previous claims, wherein the flow measurement is carried out by means of constant-temperature anemometry.

6. Method according to at least one of the previous claims, wherein the steps of determining the isoline (ILx) and deriving the measurement fluid information from the isoline (ILx) are repeated in the event that one or more of the following cases occurs: - a change or modification of the measuring medium (2); - checking the sensor element (101, 102, 103) for contamination; - change in the composition of the measuring medium (2), in particular due to decomposition of the measuring medium (2).

7. Method according to at least one of the previous claims, wherein the steps of determining the calibration data, determining the isoline (ILx) and deriving the measurement fluid information from the isoline (ILx) are repeated in the event that a defect occurs in the thermal flow sensor (100).

8. Method according to at least one of claims 2 to 7, wherein the thermal flow sensor (100) is connected to a calibration device, in particular temporarily, for determining the calibration data, wherein the calibration device device has a pipe or a channel and a pumping element, and the pumping element establishes a flow through the pipe or the channel with the known measuring medium at a defined flow rate.

9. Method according to at least one of the previous claims, wherein the thermal flow sensor (100) for determining the isoline (ILx) and for performing the flow measurement is attached to a pipe or duct through which the measuring medium (2) flows, at a measuring point.

10. Method according to at least one of the previous claims, wherein the step of detecting the maximum amplitude or the phase shift is repeated periodically and wherein a warning is generated by the electronic unit if the respective detected maximum amplitudes or phase shifts differ from one another by more than a predetermined factor.

11. Thermal flow sensor (100) which is designed to be operated by means of the method according to at least one of claims 1 to 10.