Thermal measuring device for determining fluid properties by varying the surrounding reference properties

Abstract

The invention relates to a measuring method for determining at least one property of a measuring fluid by means of a thermal sensor. The thermal sensor comprises a heating element which is actuated in an oscillating manner with at least one frequency. The at least one property of the measuring fluid is determined based on a measurement of a frequency-dependent temperature response of the thermal sensor. The measuring fluid is guided on a top side above the heating element, while a reference substance is located on a bottom side below the heating element. The reference substance differs in at least one thermal property from the measuring fluid in order to influence the frequency-dependent temperature response of the thermal sensor. The invention is also directed to a thermal sensor which is used for the method according to the invention.

Description

[0001] THERMAL MEASURING DEVICE FOR DETERMINING FLUID PROPERTIES BY VARIING THE AMBIENT REFERENCE PROPERTIES

[0002] DESCRIPTION

[0003] The invention relates to a measuring method for determining at least one property of a measuring fluid using a thermal sensor. The thermal sensor comprises a heating element that is controlled to oscillate at at least one frequency. The at least one property of the measuring fluid is determined based on a measurement of a frequency-dependent temperature response of the thermal sensor. The measuring fluid is guided on an upper side above the heating element, while a reference substance is located on a lower side below the heating element. The reference substance differs from the measuring fluid in at least one thermal property in order to influence the frequency-dependent temperature response of the thermal sensor.

[0004] Furthermore, the invention is directed to a thermal sensor which is used for the method according to the invention.

[0005] Background and state of the art

[0006] Microsystems technology is now used in many applications to manufacture compact, mechanical-electronic devices. The resulting microsystems (microelectromechanical systems, or MEMS) are extremely compact (in the micrometer range) while simultaneously offering excellent functionality and ever-decreasing manufacturing costs.

[0007] MEMS technologies offer a wide variety of mechanisms for actuation and sensing. MEMS designs and components can also be used for temperature measurements or the resulting determination of the thermal properties of a wide variety of materials. In these cases, they are usually referred to as thermal MEMS sensors, or simply as thermal sensors when the MEMS context is known. A wide variety of applications for thermal sensors are known in the state of the art.

[0008] Thermal sensors can be used in particular to measure the properties of a fluid. One such thermal sensor is disclosed, for example, in Ernst, Jachimowicz & Urban (2001). The thermal sensor comprises a heating element which surrounds a thermistor as a temperature sensor. For reference measurements, two further thermistors are located laterally spaced from the heating element as temperature sensors. The measuring process consists of two phases. On the one hand, the heating element is heated in cycles by applying an electrical voltage. On the other hand, the local transition temperature of the fluid is measured by a thermal sensor. The measurements are carried out on stationary fluids, i.e. fluids that are not flowing. The reference measurement is carried out in air or vacuum. The transition temperature represents a temperature response of the thermal sensor, which enables the characterization of thermal properties of the fluid to be measured.DE 10 2017 215 527 A1 discloses a thermal sensor for measuring the concentration of a gas to be analyzed, the analysis gas. For this purpose, a heating element, the analysis heating element, is located on a first membrane. A second heating element, the reference heating element, is located on a second membrane. This heating element serves to heat a reference gas. The thermal sensor operates using a resistance measurement principle. An electronic circuit enables the detection of a change in the resistance of the analysis heating element caused by the analysis gas relative to an electrical resistance of the reference heating element. The first membrane and the second membrane are arranged adjacent to each other in a sensor substrate.A base substrate arranged on one side of the sensor substrate between the first membrane and the base substrate provides a measurement volume, and a reference volume is provided between the second membrane and the base substrate. The reference volume contains the reference gas, while the analysis gas can flow through the measurement volume.

[0009] A thermal reference principle can also be applied in biological contexts, as demonstrated, for example, in Wang, Wang & Lin (2008). A MEMS-based differential scanning calorimetry measurement is used to characterize the thermal properties of biomolecules. The sensor comprises a pair of microchambers and microchannels for fluid delivery. The chamber is located on a freestanding membrane that extends along a substrate. Heating elements and temperature sensors are integrated within the microchambers. The first chamber is filled with a biomolecular solution, while the second chamber is filled with a reference material. The applied temperatures by the heating element are continuously varied over a certain range, allowing the temperature response to be transmitted via a thermopile.

[0010] It is also known in the prior art to use oscillating excitation of a heating element to determine the thermal properties of a measuring fluid based on the resulting temperature response. One known method is the 3-omega method, which can be used in particular to determine the thermal conductivity of a material. This method uses a heating element that is excited by an alternating current, thereby creating an oscillating thermal field. There is a certain delay between the heating of the measuring fluid and the temperature response, which depends on the thermal properties of the thermal sensor and / or the measuring fluid. This temperature response is measured by recording the amplitude and phase delay over a frequency range between the activation of the heating element and the temperature response.Although the 3-omega method is an established method for determining the thermal conductivity of a material, it also has limitations regarding the range of applications for the materials to be examined and accuracy.

[0011] DE 102016200270 A1 discloses a device and a method for detecting a fluid, preferably a gas. The device comprises a substrate having a measuring cavity and a reference cavity adjacent to one another. The measuring cavity and reference cavity are each bounded on one side by a measuring membrane and reference membrane, respectively, beneath which a first and second heating device are located. A temperature sensing device encloses the heating devices and has a first and a second temperature measuring region. The fluid to be detected is guided to a measuring membrane in the measuring cavity. This can be done via an opening in the measuring cavity from a second outer side of the substrate. Ambient air or a reference fluid, such as water, hydrogen, or nitrogen, is guided into the reference cavity. Ambient air or a reference fluid can also be present on the first outer side.

[0012] DE 102021107080 A1 relates to a method for operating a thermal sensor and to a thermal sensor itself. In a measuring method, the thermal sensor can be inserted into a wall of a container. The container can, for example, be a pipe through which a fluid flows as the measuring medium. The thermal sensor has a substrate on which sensor elements are mounted. The sensor elements can be formed by a centrally arranged heating element and two temperature sensors. Preferably, a 3-omega measuring method is used to determine a measured variable of the measuring fluid. This method exploits the fact that an amplitude and / or phase difference between the alternating voltage introduced into the sensor element and the trailing temperature of the sensor element depends on the thermal properties of the measuring fluid.The penetration depth of the temperature emitted by the alternating voltage can be influenced by the frequency of the alternating voltage. The frequency should preferably be selected such that the penetration depth reaches into the measuring medium, but lies in a range where the flow velocity of the measuring medium is almost zero.

[0013] US 2017 / 0307553 A1 describes a microdevice comprising two microchambers. A measurement sample and a reference sample for calorimetric measurements can be contained therein. The microchambers are located above a substrate, which can comprise multiple layers. In one layer, the substrate preferably comprises a thermopile. Below this layer, a microheater and a temperature sensor are preferably located for each of the respective microchambers. Cavities provided below are intended to support thermal insulation. The temperature sensors can monitor the temperature within the respective microchamber, while the microheaters heat the respective microchambers, so that a constant differential power can preferably be supplied for calorimetric calibration.Using the microdevice, for example, thermal properties of an analyte can be determined by detecting a temperature difference between a measurement sample (with analyte) in the first microchamber and a reference sample (without analyte) in the second microchamber. In preferred embodiments, the microchamber can also be heated using an oscillating signal. This is particularly advantageous for measuring the relaxation times of biomolecules or biochemical reactions.

[0014] In general, thermal sensors that operate based on an oscillating heating element have limitations regarding the possible frequencies at which a frequency-dependent temperature response can provide sufficient amplitudes to infer a property of the measuring fluid. Current state-of-the-art measurement methods limit the frequency band, which in turn also results in limited operation for the heating element control. Therefore, there is a need to provide improved or alternative methods for measuring material properties using a thermal sensor based on an oscillating heating mechanism.

[0015] Object of the invention

[0016] The object of the invention is to eliminate the disadvantages of the prior art. In particular, the object of the invention is to provide a measuring method and a measuring sensor that enables the properties of a measuring fluid to be determined with high precision and sensitivity using a thermal sensor with oscillating excitation.

[0017] Summary of the invention

[0018] The object of the invention is achieved by the independent claims. Advantageous embodiments of the invention are described in the dependent claims.

[0019] In a first aspect, the invention relates to a measuring method for determining at least one property of a measuring fluid by means of a thermal sensor comprising a heating element, wherein an oscillating control of the heating element takes place at least one frequency and the at least one property of the measuring fluid is determined based on a measurement of a frequency-dependent temperature response of the thermal sensor, characterized in that the measuring fluid is guided on an upper side above the heating element and a reference substance is located on a lower side below the heating element, wherein the reference substance differs from the measuring fluid in at least one thermal property in order to influence the frequency-dependent temperature response of the thermal sensor.

[0020] By providing a reference substance that differs from the measuring fluid in at least one thermal property, it is advantageously possible to optimize the temperature behavior, in particular the temperature response, of the thermal sensor. For example, depending on the choice of reference substance, the amplitude of the sensor's temperature response can be increased. Furthermore, a reference substance can be selected and / or adjusted in such a way that a desired phase shift of the temperature response can be achieved. In this way, an increase in the cutoff frequency of the temperature response can be achieved, allowing a thermal sensor to be operated at a higher frequency with the same output signal.By providing a reference substance below the heating element, which differs from the measuring fluid in at least one thermal property, the amplitude and / or phase of a temperature response can be advantageously optimized in order to align the measuring method with high sensitivity to a property of the measuring fluid to be determined.

[0021] The temperature response preferably refers to a signal detected by the thermal sensor as a result of the heating of the measuring fluid by the oscillating control of the heating element. The frequency-dependent temperature response thus preferably represents a reaction of the thermal sensor to an oscillating control of the heating element, whereby a dependence on the influence of the measuring fluid enables statements to be made about the thermal properties of the latter. The temperature response can, for example, be detected by a temperature sensor, which is preferably present as a component of the thermal sensor. The temperature response can also be a signal from the heating element that is influenced by the measuring fluid. For example, the temperature response can result from a resistive measuring principle if the heating element changes its resistance at an increased temperature.In other words, the temperature response comprises an effect of the measuring fluid on the thermal sensor with a corresponding oscillating excitation of the heating element. The temperature response preferably corresponds to an oscillating measurement signal, the frequency of which is preferably predetermined by the oscillating control of the heating element, wherein the temperature response can have a different phase and / or amplitude. The temperature response can vary depending on the frequency of the excitation and can therefore also preferably be referred to as a frequency-dependent temperature response. The temperature response can preferably be an oscillating electrical signal, the amplitude of which can be described by an electrical voltage and / or an electrical current. The temperature response can also preferably relate to a thermal variable that can be read out by the thermal sensor, such as, for example,the temperature, the thermal conductivity, the volumetric heat capacity, etc.

[0022] The measuring principle of a thermal sensor is based on a (measurable) dependence of the temperature response of the thermal sensor on the (to be determined) properties of the measuring fluid. The inventors also recognized that the frequency-dependent temperature response of a thermal sensor with respect to a measuring fluid can be specifically influenced by providing a reference substance. Instead, in the prior art, it is usually common practice to allow a measuring fluid to flow around a heating element on both the top and bottom sides. According to the invention, a reference substance is now provided on a side facing away from the measuring fluid, which influences the temperature response in the desired manner.

[0023] For example, it may be preferable to select a reference substance that differs from the measurement fluid in terms of lower thermal conductivity and lower volumetric heat capacity. Such a reference substance, on the one hand, increases the signal amplitudes of the temperature response and, on the other hand, shifts the phase to higher cutoff frequencies. The increase in amplitude or shift in phase to higher cutoff frequencies should be understood in comparison to a temperature response that would occur if a measurement fluid were present on both sides of the heating element that did not differ in at least one thermal property.

[0024] The potential increase in the amplitude of the temperature response advantageously extends the measurement range, achieving greater sensitivity. Thus, even small changes in the properties of the measuring fluid can be measured with high precision.

[0025] Furthermore, the provision of a reference substance according to the invention makes it possible to induce a desired phase shift in the temperature response. The phase shift preferably refers to a change in the phase relationship between the oscillating control signal for the heating element and the resulting frequency-dependent temperature response. The at least one property of the measuring fluid can be determined in particular via the temperature response of the thermal sensor. The temperature response is preferably determined by a harmonic measurement signal, which varies depending on the composition and / or flow parameters and / or thermodynamic parameters of the measuring fluid. The temperature response of the thermal sensor can preferably be characterized by an amplitude and phase.Here, the phase of the temperature response refers specifically to the phase difference between the temperature response and a control signal for oscillating excitation of the heating element. The amplitude and phase advantageously enable a precise determination of at least one property of the measuring fluid. For example, determining the phase between the control signal and the temperature response can be used, in particular, to determine thermal conductivity. The amplitude of the temperature response also depends, in particular, on the thermal conductivity and volumetric heat capacity. Thus, the phase and amplitude of the temperature response can be used to measure, for example, thermal conductivity and volumetric heat capacity with high sensitivity.Based on thermal parameters such as thermal conductivity or volumetric heat capacity, further properties of the measuring fluid can also be derived, such as the composition of a mixed gas.

[0026] Preferably, the determination of the at least one property of the measuring fluid occurs independently of the flow of the measuring fluid. In other words, it is preferred that the measuring fluid flows over or along the heating element and the determination of the at least one property remains substantially unaffected by the flow of the measuring fluid. For this purpose, it may be preferred that a preferred control signal for the heating element is several times higher than the flow velocity of the measuring fluid. This can preferably be approximately described by the quotient of the average velocity and the radius and / or diameter of the flow around the measuring fluid and the frequency of a control signal for the heating element, which can be less than 2.5 (see Hong & Kim (2012)).Advantageously, noise caused by the flow of the measuring fluid is reduced or eliminated by the measuring method according to the invention, so that the at least one property of the measuring fluid to be determined has an improved resolution.

[0027] In particular, it is possible to effect a phase shift by providing a reference substance such that a cut-off frequency of the temperature response is shifted either towards a higher or lower frequency (compared to a temperature response without the reference substance according to the invention, ie when a measuring fluid flows around the heating element on both sides).

[0028] The cutoff frequency preferably corresponds to a characteristic frequency at which an amplitude of the frequency-dependent temperature response drops, in particular to a previously defined value depending on the context of the measurement. The cutoff frequency can preferably be the frequency above which the temperature response has dropped to approximately 1 / 2 times the maximum temperature response. The latter preferred option for defining the cutoff frequency preferably corresponds to a drop of approximately 70% of a maximum value of the temperature response. Above the cutoff frequency, a measurement with only reduced sensitivity is possible. In metrological terms, the cutoff frequency therefore refers in particular to a maximum frequency at which the heating element can (still) be controlled and a sufficient amplitude of a temperature response is measurable, which allows at least one property of the measuring fluid to be detected.

[0029] If the heating element is controlled with an oscillating signal with a frequency significantly above the cutoff frequency, a meaningful measurement of at least one property of the measuring fluid is no longer possible because the temperature response has too low an amplitude. At frequencies above the cutoff frequency, the heating element is instead excited at a higher frequency than any heat dissipation from the measuring fluid could influence the temperature response. In other words, the frequency-dependent temperature response becomes insensitive to the measuring fluid at frequencies above the cutoff frequency, so that a measurement of at least one of its properties is not possible or only possible with reduced significance.

[0030] The cutoff frequency therefore preferably refers to a frequency at which at least one property of the measuring fluid can still be measured with the desired resolution using an oscillating excitation. Increasing the cutoff frequency can therefore expand the frequency range at which the heating element can be controlled. This allows for higher sensitivity for the thermal sensor at higher control frequencies, which allows for a wider measurement dynamic range. In particular, the measurement method also allows for greater selectivity with regard to changes in the properties of the measuring fluid, which become visible particularly through high-frequency measurements.

[0031] The measuring method according to the invention serves in particular to determine at least one property of the measuring fluid. The at least one property can preferably relate to one or more of the parameters selected from a group comprising a thermal conductivity, a density, a pressure, a specific heat capacity, a temperature, a volume, a thermal diffusivity, a concentration, a composition, an amount of substance, a mass, a molar mass and / or a flow of the measuring fluid. The at least one property of the measuring fluid can be determined in particular by an amplitude of the temperature response and / or a phase difference between the temperature response and the control signal for an oscillating control of the heating element. In this case, the temperature response is preferably dependent on the frequency. Depending on the frequency, for example, the amplitude of the temperature response can vary.Advantageously, the attachment of the reference substance enables optimization of the temperature response, thereby achieving an improvement in the measurement of at least one property of the measuring fluid. The reference substance preferably differs from the measuring fluid in at least one thermal property. Accordingly, it is preferred that the reference substance behaves differently from the measuring fluid in at least one property as a result of a temperature change caused by the heating element. For example, it may be preferred that the reference substance is present as a solid below the heating element and the measuring fluid flows above the heating element as a gas. When the temperature increases, the density of the gas changes, whereas the density of the solid as the reference substance changes only insignificantly.Furthermore, it may be preferred that, in addition to a difference in a thermal property, one or more thermal properties between the measurement fluid and the reference substance are similar or essentially the same. For example, it may be preferred that the measurement fluid and the reference substance be a noble gas, such as helium and argon. The volumetric heat capacity (product of density and specific heat capacity) for helium and argon is approximately the same (0.852 kJ / (m³). 3 *K) for helium and 0.855 kJ / (m 3 *K) at 20°C and 1 bar), but their thermal conductivities differ (153 mW / m*K for helium and 17.5 mW / m*K for argon). Thus, by varying at least one material property above and below the heating element, a change in the temperature response can be achieved, leading to a more efficient measurement method for determining at least one property of the measuring fluid.

[0032] The measuring fluid preferably refers to a fluid whose at least one property is to be determined by the measurement method. With respect to the thermal sensor used, the measuring fluid is guided on a top surface above the heating element.

[0033] In the context of the invention, the information regarding the top and bottom sides as well as the terms “above” and “below” preferably relate to the spatial introduction of the measuring fluid and the reference substance. The term “above” or “top side” preferably refers to a spatial area in which the measuring fluid is located. A “bottom side” or the statement “below”, on the other hand, means a spatial area in which a reference substance is present. The terms “above” and “below” are therefore not defined by a direction of gravity. The top and bottom sides of the thermal sensor are preferably separated by a partition wall or membrane, with the heating element located on or within the partition wall or membrane.

[0034] The heating element creates a heat field that spreads outward from the heating element. The ratio of the heat field spread between the top and bottom is essentially determined by the thermal properties of the measuring fluid and the reference material. If the two materials differ significantly in their thermal conductivity and heat capacity, the heat field spreads differently between the top and bottom. Oscillating heating element control is referred to as the penetration depth x. n a damped oscillation in which the amplitude has decreased to the nth part of the excitation. In the context of the invention, the underside of the heating element preferably refers to a side of the heating element facing away from the measuring fluid.

[0035] The heating element is controlled in an oscillating manner, so that the provision of the heat field emanating from the heating element is also oscillating. The term "oscillating" preferably means that a periodic heating of the measuring fluid is effected. Thus, there are alternating phases in which the measuring fluid is heated and cooled. The corresponding phases are preferably caused by a periodic control signal for the heating element. Preferred control signals for an oscillating control of the heating element can preferably be described by a function which is selected, for example, from a group comprising a harmonic function (such as a sine and / or cosine function), a rectangular function, an impulse function, a triangular function and / or a sawtooth function, without being limited thereto.Since an oscillating control is particularly dependent on a frequency, the control of the heating element is also correspondingly frequency-dependent. The heating element can preferably be controlled with one frequency or with a plurality of frequencies across a frequency range. In the latter case, in particular, it is possible to determine a spectrum of a frequency-dependent temperature response using a frequency scan. A frequency scan preferably refers to the control of the heating element and the reading of a temperature response across multiple frequencies within a frequency range, for example, from 10 mHz to 10 kHz, preferably 1 Hz to 1 kHz. The oscillating control can preferably have a constant amplitude, with only the frequency being increased.By means of a corresponding frequency scan, a frequency-dependent temperature response, in particular its amplitude and / or phase relationship to the exciting signal, can be advantageously determined over a wide frequency range, so that a more meaningful analysis of the measuring fluid is possible.

[0036] The expansion of the heat field emanating from the heating element on the upper side and the corresponding guidance of the measuring fluid on the upper side above the heating element also clarify the meaning of the term "beneath" or "beneath the heating element." Accordingly, the underside preferably refers to the side of the heating element along which the heat field should preferably spread less. In this respect, the temperature response is primarily determined by the influence of the heating or cooling of the measuring fluid. As explained, the temperature response can be optimized by providing a reference substance.

[0037] Preferably, there is a spatial separation between the top and bottom of the thermal sensor. The separation between the top and bottom is achieved in particular by the presence of a heating element, preferably by a further structural component, for example, a membrane, on which the heating element can preferably be positioned. Preferably, the separation between the top and bottom can be such that no fluid communication between the top and bottom is possible.

[0038] In the context of the invention, it is provided that the reference substance is located on the underside below the heating element. Accordingly, the reference substance is preferably located on a side of the thermal sensor facing away from the heating element and the measuring fluid. The reference substance refers to a material that is arranged as described and differs from the measuring fluid in at least one thermal property, including, for example, thermal conductivity, density, pressure, specific heat capacity, and / or thermal diffusivity.

[0039] The reference substance can also preferably exhibit a property change of a different magnitude than the measuring fluid when ambient conditions vary, with the variation in ambient conditions occurring through the control of the heating element. By guiding the measuring fluid on an upper side and the presence of the reference substance on an underside of the thermal sensor, both the measuring fluid and the reference substance are in close proximity to the heating element. As a result, the thermal field emanating from the heating element affects the measuring fluid and the reference substance. However, the consequence of this effect, as a change in a property, is different because the reference substance and the measuring fluid differ in at least one thermal property.In particular, the temperature response of the thermal sensor exhibits a different amplitude and / or phase relative to the control signal for the heating element than would be the case without the provision of a reference substance, i.e., with the measuring fluid flowing around the heating element on both sides. According to the invention, it was recognized that by varying the material properties in the vicinity of the heating element, an advantageous optimization of the temperature response with regard to the desired measurement of properties of the measuring fluid can be achieved.

[0040] The physical state of the reference substance can preferably correspond to the physical state of the measuring fluid, i.e., liquid or gaseous. Likewise, the measuring fluid and the reference substance can preferably have different physical states, so that the reference substance can also be present, for example, as a solid. The thermal sensor is preferably designed such that the reference substance is encapsulated beneath the heating element. This can be achieved, for example, by enclosing the reference substance within a cavity of a substrate or carrier to which the heating element is attached. In further preferred embodiments, a flow of the reference substance beneath the heating element may be preferred.

[0041] The measuring fluid is guided on the upper side above the heating element in order to measure the measuring fluid through the oscillating control of the heating element and based on the resulting temperature response. The guiding of the measuring fluid can preferably refer to a flow of the measuring fluid above the heating element. Preferably, a spatial limitation for the guiding of the measuring fluid can be provided, for example, by a housing-like component of the thermal sensor, for example, a package.

[0042] The inventors recognized that attaching the reference substance on the underside below the heating element influences the frequency-dependent temperature response of the thermal sensor. This is particularly the case when the reference substance and the measuring fluid differ in at least one thermal property. It is preferred that there is at least a relative or absolute deviation in at least one thermal property between the measuring fluid and the reference substance. The invention is therefore preferably characterized by the possibility of a targeted influence on the frequency-dependent temperature response by the reference substance. In this respect, the invention preferably goes beyond the placement of, for example, (arbitrarily) different fluids above and below a measuring device, but rather relates to a dedicated use of a reference substance to influence a temperature response of the thermal sensor.The invention is therefore based on the new finding that parameters for characterizing the temperature response could be advantageously influenced by the choice or design of the reference substance, such as an amplitude and / or cutoff frequency of the temperature response.

[0043] In a further preferred embodiment, the measuring method is characterized in that two or more measuring fluids are measured. Preferably, several methods can also be carried out for different measuring fluids. In this case, it may be preferable to provide one and the same reference substance or different reference substances on the underside below the heating element. The latter preferred embodiments are particularly suitable if the heating element is to be controlled at different frequencies. In particular, the frequency behavior of the temperature response changes for a constant reference substance depending on the selection of the measuring fluid. If there is a different thermal conductivity compared to the reference substance, for example, a change in the amplitude of the temperature response at low frequencies can preferably be brought about.If the thermal conductivity differs from that of the reference material, the behavior of the temperature response changes preferentially at higher frequencies.

[0044] In a preferred embodiment, the measuring method is characterized in that the at least one thermal property in which the measuring fluid and the reference substance differ from each other is selected from a group comprising a thermal conductivity, a specific heat capacity, a thermal diffusivity, a density, a temperature and / or a pressure.

[0045] The use of a reference substance that differs from the measuring fluid in at least one of the aforementioned thermal properties has proven particularly advantageous in order to optimize the frequency-dependent temperature response of the thermal sensor for the measuring process.

[0046] A thermal property preferably refers to a parameter that can be used to describe the energy behavior of a material as a result of temperature changes. In particular, the effect of temperature and / or heat can influence a thermal property. Parameters such as thermal conductivity, heat capacity, specific heat capacity, thermal diffusivity, density, and / or pressure fall under the term "thermal property" in the context of the invention.

[0047] Preferably, there is a distinction in at least one thermal property between the measuring fluid and the reference substance. A distinction can preferably be described by an absolute or relative difference. In particular, a relative distinction in at least one thermal property can preferably be characterized by a distinction factor, wherein the distinction factor preferably comprises a dimensionless number by which a conversion of the property to be distinguished between the measuring fluid and the reference substance can take place. The distinction factor can preferably be 2, 3, 4, 5, 6, 7, 8, 9, 10 or more. For example, the thermal property of the measuring fluid can be higher than the thermal property of the reference substance by a distinction factor of 2, 3, 4, 5, 6, 7, 8, 9, 10 or more, or vice versa.It may also be preferred for the reference substance and the measuring fluid to differ in several thermal properties. Furthermore, it may be preferred that, in addition to the different thermal properties, there are also thermal properties in which the reference substance and the measuring fluid are essentially the same or similar. For example, it may be preferred for a noble gas to be used both as the measuring fluid and as the reference substance, such as neon and helium, which have essentially the same or similar volumetric heat capacity but different thermal conductivities. Thermal conductivity preferably refers to a property by which the heat flow through the material is determined due to thermal conduction. Thermal conductivity indicates how well the material conducts heat or how well it is suitable for thermal insulation.The lower the thermal conductivity value, the better the thermal insulation. The higher the thermal conductivity value, the better the material can function as a heat conductor. In the SI system, thermal conductivity is expressed as watts per meter and Kelvin. It can be specified, for example, according to DIN 4108-4.

[0048] Thermal conductivity is linked to thermal diffusivity. Thermal conductivity is determined from the quotient of thermal conductivity and the product of the specific heat capacity and density of the fluid. Thermal diffusivity describes the unsteady behavior of a temperature field, i.e., the temporal propagation of temperature due to heat conduction in the fluid. Thermal diffusivity has the SI unit m 2 / s.

[0049] The specific heat capacity primarily provides information about a material's ability to store heat and thus thermal energy and is material-specific. It describes the energy required to heat 1 kilogram of a specific substance by 1 degree Celsius. It can be calculated using the heat added to or removed from a substance, the mass of the substance, and the associated temperature change. The SI unit of specific heat capacity is J / kg*K. Another way to describe heat capacity is by volumetric heat capacity. The volumetric heat capacity of a material is the heat capacity of a material divided by its volume. It describes the amount of energy that must be added in the form of heat to a unit volume of the material to cause a temperature increase of 1 degree Celsius. The SI unit of volumetric heat capacity is J / m 3*K. Depending on the material behavior and / or the heating element control, particularly with regard to isochoric behavior (constant volume) or isobaric behavior (constant pressure), an isochoric or isobaric heat capacity may be preferentially considered. A volumetric heat capacity can also be specified for an isochoric or isobaric heat capacity by dividing it by the volume.

[0050] By selecting a reference substance with lower thermal conductivity and / or lower volumetric heat capacity compared to the measurement fluid, for example, the amplitude of the temperature response can be increased and a cutoff frequency can be shifted to higher frequencies. For example, it may be preferable to use xenon as a reference substance for determining the properties of hydrogen, which would represent the measurement fluid, as xenon has a lower thermal conductivity and lower volumetric heat capacity than hydrogen. By selecting a reference substance in this way with respect to a measurement fluid, the measurement dynamics can be advantageously increased.

[0051] However, the measurement behavior or temperature response can also be optimized by using a different density and / or pressure of the reference substance.

[0052] Density is the quotient of mass and volume. Density can be expressed, for example (without restriction), in grams per cubic centimeter, kilograms per cubic meter, or kilograms per liter. Similarly, volume describes the amount of space a substance can occupy. Since density and volume are inversely proportional to each other, density increases when volume decreases, and conversely, density decreases when volume increases.

[0053] For gases, the relationship also arises via a (gas) pressure, whereby a higher density exists for the same volume and a higher pressure. The pressure, especially in a gas, comprises the sum of all forces acting on a surface. This applies in particular to the forces transferred to a surface by collisions between gas molecules and / or gas atoms. In a gas, the pressure can be described in a first approximation by the ideal gas equation. It follows directly that the pressure depends on the amount of substance, the temperature and the volume. In particular, pressure and volume are inversely proportional to one another. In a liquid, the pressure can be described in a first approximation by a dynamic pressure or back pressure, which is proportional to the square of the flow velocity and the density.

[0054] A person skilled in the art will understand that the aforementioned quantities relating to thermal properties are also interdependent. For example, material properties of a measuring fluid and / or reference substance, such as thermal conductivity, specific heat capacity, thermal diffusivity, or density, depend on environmental parameters such as temperature or pressure.

[0055] By selecting a different pressure and / or density for the reference substance, the temperature response and thus the measurement behavior can be further optimized. Preferably, the reference substance differs from the measurement fluid in addition to a different chemical or material composition, thus also in physical properties such as pressure and / or density. In some embodiments, it may also be preferred for the reference substance and the measurement fluid to have essentially the same chemical composition, but to differ in a physical property, such as pressure and / or density, to optimize the temperature behavior.

[0056] In a further preferred embodiment, the measuring method is characterized in that the at least one thermal property in which the measuring fluid and the reference substance differ from each other is selected in order to shift, preferably to increase, an amplitude and / or cutoff frequency of the frequency-dependent temperature response.

[0057] As explained above, the temperature response essentially refers to a reaction of the thermal sensor to an oscillating control of the heating element, whereby, due to a dependence on the influence of the measuring fluid, statements about the thermal properties of the same are made possible.

[0058] The temperature response thus preferably also corresponds to an oscillating signal with an amplitude and phase relationship to the exciting signal of the heating element. The influence on the temperature response, which results in particular from the attachment of the reference substance on an underside beneath the heating element, comprises, for example, a shift in the amplitude and / or the cutoff frequency of the temperature response (compared to a measurement situation without a reference substance or when a measuring fluid flows around the thermal sensor on both sides). This can in particular concern an increase or decrease in the amplitude and / or cutoff frequency. The degree of the shift in the amplitude and / or cutoff frequency of the temperature response can be determined in particular by selecting one or more of the differing thermal properties.

[0059] For example, the reference substance can be selected such that it has a lower thermal conductivity and a lower volumetric heat capacity compared to the measurement fluid. A reference substance with a lower thermal conductivity and volumetric heat capacity relative to the measurement fluid can achieve both a positive shift in the cutoff frequency and an increase in the heat capacity.

[0060] Therefore, in a further preferred embodiment, the measuring method is characterized in that the reference substance has a lower thermal conductivity and / or volumetric heat capacity compared to the measuring fluid in order to increase an amplitude and / or cutoff frequency of the frequency-dependent temperature response.

[0061] By increasing the cutoff frequency, the frequency at which the heating element can be controlled can be increased accordingly. This significantly improves the operational capability of the thermal sensor. Thus, when applying control signals, a sufficient amplitude can be achieved even for higher frequencies. This approach thus achieves higher sensitivities at higher excitation frequencies, expanding the measurement dynamic range and enabling a more precise determination of at least one property of the measuring fluid.

[0062] In a further preferred embodiment, the reference substance can comprise a composition of several materials, for example a composition of several gases. In this case, it is particularly advantageous that, with a composition of several gases, one or more thermal properties, such as thermal conductivity and / or heat capacity, can be adjusted as such by the reference substance with a high degree of flexibility. For example, two gases with two different thermal conductivities can be combined with one another, so that, depending on the selection of the proportion of a gas, a thermal conductivity for the reference substance can be adjusted which corresponds to a thermal conductivity between the two values. For example, neon has a thermal conductivity of approximately 49.77 W / (m*K) at a temperature of 300 K and a pressure of 1 bar. Helium, in turn, has a thermal conductivity of 156.0 W / (m*K) at 300 K and 1 bar. The volumetric heat capacity for neon and helium at 300 K and 1 bar is approximately 833 J / (m 3*K) are essentially identical. By selecting the concentration accordingly and / or selecting neon and helium, a thermal conductivity of the reference substance of approximately 49.77 - 156.0 W / (m*K) can be set for the specified parameters for temperature and pressure. Advantageously, parameters of the reference substance can be adjusted flexibly and with high precision by selecting a composition of several materials and / or their proportions. Accordingly, the influence of the reference substance on the temperature response can be better optimized. Terms such as essentially, approximately, about, approx. etc. preferably describe a tolerance range of less than ± 40%, preferably less than ± 20%, particularly preferably less than ± 10%, even more preferably less than ± 5% and in particular less than ± 1% and always include the exact value. Similar preferably describes quantities that are approximately the same.Partially preferably describes at least ± 5%, particularly preferably at least ± 10%, and especially at least ± 20%, in some cases at least ± 40%.

[0063] In a further preferred embodiment, the measuring method is characterized in that the measuring method is carried out by means of at least two thermal sensors or at least one thermal sensor with at least two heating elements, wherein a different reference substance is located on an underside below the respective heating elements.

[0064] The two or more different reference substances preferably differ from each other in at least one thermal property, both relative to the measurement fluid and relative to each other. Therefore, it is preferred that each reference substance differs from each other in at least one thermal property and additionally differs from the measurement fluid in at least one thermal property. Furthermore, it may be preferred that the reference substances are essentially the same or similar to each other in another thermal property.

[0065] By introducing multiple reference substances, a combined signal can advantageously be generated. Therefore, in a further preferred embodiment, the measuring method is characterized in that the at least one property of a measuring fluid is determined based on the respective temperature responses of the at least two heating elements, wherein a combined signal is preferably generated from the respective temperature responses of the at least two heating elements.

[0066] A combination signal preferably comprises a measurement signal that depends on the respective temperature responses of the at least two heating elements. A combination signal can preferably be formed by one or more of the arithmetic operations of the at least two temperature responses, wherein the arithmetic operations can be selected, for example, from a group comprising subtraction, addition, multiplication, and / or division. A differential signal is preferably a signal that is formed by calculating a difference between at least two temperature responses.

[0067] By generating a combination signal, a particularly comprehensive and precise characterization of at least one property of the measuring fluid can be achieved. This allows for different temperature responses to be taken into account for the same measuring fluid, resulting from the provision of different reference substances. By selecting the reference substances and the combination signal, the measurement can also be advantageously optimized for high sensitivity to a specific property of a measuring fluid.

[0068] Furthermore, by using two or more reference substances, it is possible to obtain a characteristic thermal fingerprint of a measurement fluid, which can be highly specific to the respective measurement fluid (e.g., its composition) or its ambient conditions (e.g., pressure). For this purpose, it may be preferable to perform further mathematical operations on the combined signal to infer properties of the measurement fluid, for example, a fast Fourier transform.

[0069] Furthermore, measuring at least one property using a combination signal allows for error minimization by providing multiple reference substances. The choice of the combination signal also allows for the elimination or compensation of any interference factors, such as temperature fluctuations in the thermal sensor. This enables a high-resolution measurement that is also characterized by minimal influence from any interference.

[0070] In preferred embodiments, the measuring method can be carried out by means of at least three, four, five or more thermal sensors or at least one thermal sensor with at least three, four, five or more heating elements, wherein a different reference substance is located on a bottom side below the respective heating elements.

[0071] By providing three, four, five or more reference substances, more complex combination signals can be generated, which are particularly meaningful or sensitive with regard to determining specific properties of the measuring fluid.

[0072] When using multiple reference substances through multiple thermal sensors or multiple heating elements on a carrier, it is preferable that the reference substances differ in at least one thermal property. For example, it may be preferable that the multiple reference substances differ in at least one thermal property by at least a factor of 2. Furthermore, reference substances are preferably used that exert a differently pronounced influence on the temperature response. For example, a first reference substance can preferably have a particularly strong dominant influence on the temperature response. The first reference substance therefore determines the temperature response comparatively strongly. A further, second reference substance can preferably have a less strong influence on the temperature response, so that the temperature response of the second reference substance is determined more strongly by the measuring fluid.Furthermore, it may be preferred that, when using multiple reference substances, they behave essentially the same or similarly in at least one thermal property. For example, at least one thermal property of the first and second reference substances may change in an essentially identical or similar manner when the ambient conditions, in particular pressure and / or temperature, vary.

[0073] The use of two or more reference substances to enable the measurement of combination signals as described above can preferably be carried out by means of two variants.

[0074] On the one hand, it may be preferable, for example, to provide two thermal sensors, each having a heating element, with a different reference substance located beneath each heating element. In this case, it may be preferable for each thermal sensor to comprise a heating element mounted on a membrane, which is preferably located on a carrier. Thus, there are at least two carriers, each having a membrane with a heating element mounted thereon. A reference substance is mounted beneath each respective membrane, which may form a separation between the top and bottom. Each reference substance preferably differs from the measuring fluid in at least one thermal property. Furthermore, it is preferable for the reference substances to differ from one another in at least one thermal property.Furthermore, it may be preferable to provide two, three, four, or more thermal sensors, by which a combined signal can be determined. Preferably, the plurality of thermal sensors can be mounted within a single package.

[0075] It may also be preferable to provide a thermal sensor with at least two heating elements, wherein the thermal sensor comprises a carrier and at least two heating elements are located on the carrier. The carrier itself can preferably be designed such that one or more reference substances are introduced, preferably in one or more cavities. It may also be preferable for the at least two heating elements to be located on support structures, wherein the support structure is attached to the carrier. Preferably, a support structure can have a cavity in which the reference substance can be introduced. Furthermore, it may be preferable for the support structure itself to be used as the reference substance.

[0076] In a further preferred embodiment, the measurement method is characterized in that the reference substance is present as a solid. Solids with low thermal conductivity and low specific heat capacity are preferred. This advantageously allows an increase in the amplitude and / or cutoff frequency of the frequency-dependent temperature response.

[0077] Particularly preferably, the reference substance comprises a material selected from a group comprising porous silicon, aerogels, silicon oxide, polyimide, silicon nitride and / or a photoresist.

[0078] Applying the reference substance as a solid is particularly advantageous in that it ensures mechanical stability under changes in temperature and / or pressure. Furthermore, the reference substance can be applied as a solid underneath the heating element using simple, known means from semiconductor and / or microsystem technology. There are advantageously several options for providing the reference substance as a solid. For example, this can be selected through the material and / or layer thicknesses and / or a coating of a material can be used as a backfill for a membrane on which the heating element is arranged. Likewise, a reference substance can preferably be formed as a solid using a carrier that has different etching depths for the design of the reference substance as a solid.

[0079] Porous silicon has proven to be a particularly advantageous reference material. Firstly, porous silicon exhibits high resistance to potential heat transfer and can therefore ensure good thermal insulation. This is due in particular to the low thermal conductivity of porous silicon. Furthermore, comparatively thick layers of porous silicon can be achieved, further improving stability and robustness. Providing porous silicon as a reference material can be carried out using known, rapid, and cost-effective technologies. The porous silicon preferably comprises micropores (pore sizes smaller than <2 nm) or mesopores (pore sizes between 2 and 50 nm), the size distribution of which can be adjusted within the porous silicon. For example, it may be preferable to provide porous silicon with layers of different pore sizes.The thermal properties of porous silicon can be tuned, in particular, by the design of the pores. As explained in Lysenko, V., et al. (2002), the volumetric heat capacity of porous silicon can be reduced proportionally with the number of atoms removed to create the pore structure. The volumetric heat capacity, which can also be described as the product of density and specific heat capacity, can also be determined for porous silicon using the relationship p*c, according to Lysenko, V., et al. (2002). por ous silicon « (1 - P)*p*csiiizium, where P denotes the porosity, ie the ratio of void volume to total volume and p*csiiizium = 1 ,66*10 6 J / (m 3*kg) represents the volumetric heat capacity of silicon. Thus, by using porous silicon, a reference material with desired, different thermal properties from the measuring fluid can be precisely adjusted.

[0080] Furthermore, porous silicon can be applied as a reference material using simple and known state-of-the-art means, for example, as a backfill for a cavity on a carrier. Porous silicon is therefore particularly advantageous for the thermal sensor from both a process engineering and functional perspective.

[0081] In a further preferred embodiment, the measuring method is characterized in that the reference substance is present as a liquid or a gas, wherein the reference substance is preferably selected from a group comprising hydrogen, carbon dioxide, helium, neon, argon, krypton, xenon, nitrogen, sulfur hexafluoride and / or air.

[0082] Applying the reference substance as a liquid or gas is particularly advantageous due to its particularly good thermal insulation properties. This means that no or only a reduced amount of heat is transferred through the reference substance itself, thus avoiding heat loss and ensuring efficient measurement of the measuring fluid.

[0083] The preferred fluids mentioned as reference substances are suitable for a wide variety of applications in which the measurement method according to the invention can be implemented. For example, noble gases such as argon, neon, and helium have different thermal conductivities but almost similar volumetric heat capacities. In particular, by combining several gases to provide a mixture of different gases and using them as a reference substance, more versatile optimizations of the temperature response can be achieved, thus obtaining a broader frequency band for the measurement.

[0084] In a further aspect, the invention relates to a thermal sensor for use in a measuring method according to the previously described, comprising a heating element, wherein the heating element is designed to be excited in an oscillating manner at least at one frequency and the thermal sensor is configured to determine at least one property of a measuring fluid based on a measurement of a frequency-dependent temperature response, characterized in that the measuring fluid is guided on an upper side above the heating element and a reference substance is located or can be introduced on an underside below the heating element, wherein the reference substance differs from the measuring fluid in at least one thermal property.

[0085] The average person skilled in the art will recognize that technical features, definitions and advantages of embodiments disclosed for the measuring method described above equally apply to the thermal sensor, and vice versa.

[0086] In particular, it may be preferred for the thermal sensor to have (at least) one reference substance, wherein the reference substance is preferably positioned below the heating element. By attaching a reference substance on an underside below the heating element, which reference substance differs from the measuring fluid in at least one thermal property, it is advantageously possible to optimize the temperature behavior, in particular the temperature response of the thermal sensor. For example, depending on the choice of reference substance, the amplitude of the temperature response of the thermal sensor can be increased. Furthermore, a reference substance can be selected and / or adjusted such that a desired phase shift of the temperature response is achieved. In this way, an increase in the cutoff frequency of the temperature response can be achieved, so that a thermal sensor can be operated at a higher frequency with the same output signal.By providing a reference substance below the heating element, which differs from the measuring fluid in at least one thermal property, the amplitude or phase of a temperature response can be advantageously optimized in order to align the measuring method with a higher sensitivity to a property of the measuring fluid to be determined.

[0087] The measuring fluid is periodically heated by an oscillating control of the heating element so that an oscillating temperature response can be recorded by the thermal sensor. The temperature response differs in amplitude and / or phase from the control signal for the heating element. The heating element can be excited via one or more frequencies. Accordingly, the temperature response can also be dependent on one or more frequencies. The reference substance optimizes the temperature response, in particular with regard to an increase in amplitude and / or a shift in phase in a positive direction, so that the cutoff frequency can also be increased. The difference in amplitude and / or phase between a control signal for the heating element and the temperature response can be used to determine at least one property of the measuring fluid.

[0088] The temperature response preferably relates to a measurement signal from the thermal sensor that changes as a result of the heating of the measurement fluid. In other words, the temperature response relates to the ability of the thermal sensor to detect a temperature change in the measurement fluid. The temperature response can preferably be recorded by a temperature sensor. The temperature sensor can, for example, be a thermocouple or a thermopile. In particular, the heating element and the thermocouple or thermopile can be optimized independently of one another as a temperature sensor. Preferably, a low resistance and / or a low temperature coefficient of resistance (TCR) of the heating element is selected, so that the heating element can advantageously be controlled in such a way that it is essentially independent of the ambient temperature.In other words, a low TCR of the heating element allows for virtually temperature-independent power supply or control of the heating element. At the same time, it is preferable to achieve high sensitivity for the temperature sensor, for example, a thermocouple. This allows for individual optimization of the heating element and the temperature sensor. The thermocouple can, for example, comprise a material composition comprising n-polysilicon / aluminum, p-polysilicon / aluminum, or n-polysilicon / p-polysilicon (the suffixes n- and p- denote the corresponding n- and p-doping, respectively).

[0089] The selection of the material for a temperature sensor, especially a thermocouple, depends in particular on the figure of merit. The figure of merit can be described by (Van Herwaarden & Sarro (1986)) z = a 2 / (K*p), where a denotes the Seebeck coefficient, K the thermal conductivity, and p the electrical resistance. It is preferable to use materials that exhibit a high figure of merit in the temperature range to be measured. This, in turn, requires adjustment of the parameters of the Seebeck coefficient, thermal conductivity, and electrical resistance. For metals, the figure of merit is maximum when the Seebeck coefficient is at its highest. For semiconductors, the absolute Seebeck coefficient can be up to one or more millivolts per Kelvin of temperature difference.

[0090] The temperature response can preferably also be measured via the heating element itself. This can be achieved, for example, by a heating resistor that provides the heating element. With a heating resistor, the resistance changes when activated to heat the measuring fluid. The heat absorption by the measuring fluid itself depends on its thermal properties. The change in resistance of the heating resistor for heating the measuring fluid can act as the temperature response. The change in resistance can be measured, for example, using a Wheatstone bridge circuit. In particular, two terminals of the heating element of the Wheatstone bridge circuit can be electrically connected so that the resistance value can be read off as a function of the flow of the measuring fluid through the heating element.

[0091] It is preferred that temperature measurement be performed using a temperature sensor, in particular a thermocouple or thermopile. This also offers several advantages over the known 3-omega method. The 3-omega method determines a thermal property, such as thermal conductivity. A heating element itself is used as a temperature sensor and is fed with an oscillating control signal. The applied power causes the temperature of the heating element to change at the same frequency as the control signal. The amplitude and phase shift relative to the applied power depend on the thermal conductivity and the frequency.The temperature oscillations lead to resistance oscillations in the heating element, which can be described, among other things, by a resistance amplitude, which in turn depends on a temperature coefficient of the electrical resistance of the heating element. From this, the voltage across the heating element can be calculated by applying Ohm's law. This voltage contains a component with three times the frequency (3 μJ) and a corresponding amplitude proportional to the temperature amplitude of the heating element.

[0092] To determine thermal conductivity, for example, a relationship between the temperature oscillation and the thermal conductivity can be used, which can depend, among other things, on the amplitude of the temperature oscillations, the geometric design, and the resistance of the heating element. A disadvantage of the 3-omega method, for example, is that, with regard to thermal conductivity, it is difficult to distinguish between, for example, the thermal conductivity of the material being tested and the material at the interface of the heating element. Known disadvantages of the 3-omega method are eliminated by the thermal sensor according to the invention, since the application of the reference substance enables a precise measurement of at least one property of the measuring fluid.

[0093] In a further preferred embodiment, the thermal sensor is characterized in that the thermal sensor has an electronic circuit, wherein the electronic circuit is configured to excite the heating element in an oscillating manner with at least one frequency and / or to evaluate a frequency-dependent temperature response of the thermal sensor in order to determine a property of the fluid.

[0094] An electronic circuit therefore preferably functions as a computing unit, which preferably acts as a data processing unit, processing data recorded by the heating element or a preferred temperature sensor. Likewise, the electronic circuit can preferably assume the functions of a control unit, exciting the heating element to oscillate at at least one frequency.

[0095] Preferred electronic circuits include, without limitation, an integrated circuit (IC), an application-specific integrated circuit (ASIC), a programmable logic circuit (PLC), a field-programmable gate array (FPGA), a microprocessor, a microcomputer, a programmable logic controller, and / or another electronic circuit that is preferably programmable. The electronic circuit may preferably also include a memory unit. A memory unit allows the backup and / or intermediate storage of data. Non-limiting examples of memories, preferably semiconductor memories, are volatile memories, (RAM) memories, or non-volatile memories, such as ROM memories, EPROM memories, EEPROM memories, or flash memories, and / or other memory technologies.

[0096] In a further preferred embodiment, the thermal sensor is characterized in that the heating element is attached to a membrane, wherein the membrane is held by a carrier, the measuring fluid can be guided on an upper side of the membrane and the reference substance is located or can be introduced on an underside of the membrane. The membrane represents a component which serves in particular to position the heating element. The membrane preferably extends along a carrier so that the heating element can be attached to the membrane. The membrane is preferably flat, i.e. it preferably has a length and / or width which is many times greater than its thickness. The thickness here refers to an extension along an axis which is essentially orthogonal to a length and / or width of the membrane. Thus, the length and / or width can be greater than the thickness by a factor of 1.5, 2.5, 10, 100, 1,000, 10,000 or more.The thickness of the membrane is preferably between 0.1 - 1000 pm, preferably between 0.5 - 500 pm, particularly preferably between 1 - 300 pm, most particularly preferably between 5 - 200 pm.

[0097] The membrane is preferably configured to be sufficiently stable to prevent vibrations that could occur due to a flowing measuring fluid. This prevents distortions and ensures good measurement results even under strong flow conditions.

[0098] In a further preferred embodiment, the thermal sensor is characterized in that the heating element is mounted on a membrane, wherein the membrane comprises a material selected from a group comprising silicon nitride, silicon dioxide and / or polyimide.

[0099] The preferred materials for the membrane have proven advantageous in that, in addition to their excellent design options as a membrane, they also offer thermal insulation, thus reducing or eliminating heat conduction through the membrane. Consequently, one or more of the preferred materials mentioned efficiently utilize the heat field emanating from the heating element to cause oscillating heating of the measuring fluid. The resulting heat conduction through the membrane is advantageously reduced, thus reducing heat loss, thus ensuring optimal use of the heat field.

[0100] In a further preferred embodiment, the heating element is applied to the membrane, while a passivation layer is present on the heating element. The passivation layer can preferably be applied to the heating element in a surface-conforming or tight-fitting manner, so that the heating element is preferably embedded between the membrane and the passivation layer. In this respect, the heating element is preferably located on the membrane or integrated within the membrane or the membrane layer system (membrane and passivation layer). The passivation layer can advantageously provide electrical insulation and / or a protective function for the heating element. The passivation layer can preferably comprise a material selected from a group comprising silicon nitride and / or silicon dioxide.

[0101] The membrane is preferably located on a carrier. The carrier is preferably a frame structure which is essentially formed by a continuous outer border in the form of side walls of a remaining flat area. The frame structure is preferably stable and rigid. In the case of a square frame shape, the individual side regions which preferably essentially form the frame structure are in particular referred to as side walls. The membrane is preferably held by at least two side regions of the carrier. The carrier can preferably be formed from a wafer or substrate using structuring processes proven in the prior art, for example for providing a cavity on the carrier over which a preferred membrane can extend.

[0102] The cavity preferably refers to a recess in the carrier, which defines a hollow space into which the reference substance can be introduced. The membrane is preferably attached to the carrier in such a way that it extends over the preferred cavity and defines an upper boundary of the cavity. The cavity is thus preferably located on the underside of the thermal sensor. The cavity provides a spatial section through which the reference substance, preferably a liquid or a gas, can flow or in which the reference substance is enclosed.

[0103] In preferred embodiments, the cavity is open. An open cavity refers to a cavity that preferably has at least one, preferably at least two, openings so that the reference substance can flow into or through the cavity. An open cavity advantageously allows flexibility in providing a reference substance that can flow into or through the cavity. Thus, the reference substance can be easily provided depending on the desired orientation of the measurement method. The system is characterized by a high degree of adaptability, allowing the measurement method to be optimized for the specific application.

[0104] In other embodiments, the cavity is closed, meaning there are no openings for fluid communication with a measurement environment. In a closed cavity, the reference substance is instead encapsulated, preventing the reference substance from being introduced or removed. Preferred embodiments provide a hermetic seal. The introduction of a gas or liquid into a closed cavity, on the other hand, allows for the provision of a reference substance with the desired thermal properties with particularly high repeatability. Furthermore, such thermal sensors are ready for immediate use, without the need for connection to a reservoir for the reference substance.

[0105] Due to their high flexibility, designs of the thermal sensor with an open cavity are particularly suitable for laboratory-scale applications and for test purposes.

[0106] However, versions of the thermal sensor with a closed cavity can be used particularly on an industrial scale for specifically defined applications due to their high reproducibility and ease of handling.

[0107] In a further preferred embodiment, the thermal sensor is characterized in that the reference substance is present as a solid body, wherein the heating element is mounted on the reference substance and wherein the reference substance is located on a carrier or wherein the heating element is mounted on a support structure, wherein the support structure has a cavity and is arranged on a carrier, wherein the reference substance is located as a solid body within the cavity of the support structure.

[0108] A preferred design for the thermal sensor comprises a carrier with a cavity, onto which a membrane is attached, extending over the cavity. A heating element is preferably attached to the membrane, which is controlled in an oscillating manner, for example by an electrical signal, in order to provide a correspondingly oscillating thermal field and thereby thermally excite the measuring fluid. The measuring fluid can be passed through on an upper side above the heating element. The reference substance is attached to the underside below the heating element, in particular within a cavity of the carrier. The carrier is preferably located within a package, so that the measuring fluid can be passed through within the package. The reference substance can preferably be encapsulated, in particular within the cavity of the carrier. In further preferred embodiments, multiple reference substances can be attached.Furthermore, it may be preferred that several thermal sensors, in particular within a package, are used for a preferred measuring method.

[0109] In a further preferred embodiment, the thermal sensor is characterized in that the thermal sensor comprises at least two heating elements, wherein a different reference substance is located or can be introduced on a bottom side below the at least two heating elements.

[0110] By using two (or more) reference substances, a characteristic thermal fingerprint of the measurement fluid can advantageously be obtained, which can be highly specific to the respective measurement fluid (e.g., its composition) or its ambient conditions (e.g., pressure). For this purpose, it may be preferable to perform further mathematical operations on the combined signal to infer properties of the measurement fluid, for example, a fast Fourier transform.

[0111] The thermal sensor preferably comprises a carrier on which the at least two heating elements are preferably mounted. At least two reference substances are preferably used, i.e. in particular one reference substance is located underneath each heating element. It is preferred that the thermal sensor comprises at least two reference substances, with one reference substance being located on the underside of each heating element. These are preferably different reference substances. The difference between the two reference substances preferably lies in at least one thermal property, in particular in the fluid composition. The carrier preferably has a corresponding design so that different reference substances can be located underneath the at least two heating elements.It may also be preferred for the at least two heating elements to be located on at least two corresponding support structures, wherein the at least two support structures are attached to the carrier. A reference substance can preferably be introduced into a preferred cavity of the support structure, so that introduction of the reference substances on the underside below the heating elements is still ensured. For example, a first heating element can be attached to a first support structure and a second heating element can be attached to a second support structure, wherein different reference substances are located in the cavity of the first and second support structures. As a result, different reference substances are present on the underside below the first heating element and the second heating element.

[0112] It may also be preferred for the thermal sensor to comprise a carrier, a first heating element, and a second heating element. Preferably, the first heating element is located on the first reference material, and the second heating element is located on a second reference material, with the first reference material and the second reference material preferably being positioned on the carrier. The latter preferred embodiment is particularly suitable when the different reference materials are present as solids.

[0113] The embodiments in which at least two reference substances are used are, as described above, particularly well suited for obtaining a combined signal, which advantageously allows for a particularly comprehensive and precise characterization of at least one property of the measuring fluid. Different temperature responses resulting from the provision of different reference substances can be taken into account for the same measuring fluid. Furthermore, by selecting the reference substances and the combined signal, the measurement can be advantageously optimized for high sensitivity to a specific property of the measuring fluid.

[0114] The preferred embodiments, in which the thermal sensor comprises at least two heating elements, wherein a different reference substance is located or can be introduced on a respective underside below the at least two heating elements, are further advantageous in that the individual heating elements and / or their positioning can be processed and combined independently of one another. This provides a high degree of flexibility with regard to the design of the thermal sensor. Furthermore, attaching multiple heating elements to a carrier achieves a high degree of compactness, thus significantly simplifying the installation of the thermal sensor and enabling a wide range of application options.

[0115] In a further preferred embodiment, the thermal sensor is characterized in that the thermal sensor comprises a first heating element, on the underside of which a first reference substance is located or can be introduced, and a second heating element, on the underside of which a second reference substance is located or can be introduced, wherein the thermal sensor has an electronic circuit which is configured to form a combination signal based on a temperature response of the first and second heating elements, so that a measured variable of a measuring fluid can be determined based on the combination signal.

[0116] As already described, a combination signal comprises a measurement signal formed by one or more of the arithmetic operations of multiple temperature responses, including subtraction, addition, multiplication, and / or division. With several different reference substances, there are correspondingly several different temperature responses that can be linked together using one or more of the aforementioned arithmetic operations.

[0117] Therefore, in a further preferred embodiment, the thermal sensor is characterized in that the combination signal is selected from a group comprising a sum, a difference, a product and / or a quotient of a first and second measurement signal of the temperature response of the first and second heating element.

[0118] In a further preferred embodiment, the thermal sensor is characterized in that the first and second heating elements are incorporated within a package, wherein preferably the first and second heating elements are each held by different carriers or by one and the same carrier.

[0119] The package preferably refers to a component of the thermal sensor, which can be optionally present and fulfills a protective function, for example against foreign particles and / or substances such as dust, moisture and / or liquids. At the same time, the package maintains functional properties of the thermal sensor, such as the transfer of heat emanating from the heating element to the measuring fluid. The package can preferably surround the thermal sensor, so that it forms, in particular, a cover for the heating element. Furthermore, it is preferred that the package surrounds the heating element in such a way that it provides a base for a preferred carrier, on which the heating element can preferably be attached. Thus, it is preferred that the package fulfills a housing function, which enables encapsulation of a preferred carrier, a preferred membrane and / or, in particular, the heating element.A measuring fluid is preferably carried within the package.

[0120] Preferably, several heating elements, in particular a first heating element and a second heating element, are present within the package, wherein the first heating element and the second heating element are preferably positioned on different supports, i.e., a first support and a second support. This advantageously offers greater design freedom for arranging the heating elements for heating the measuring fluid.

[0121] It may also be preferable to mount multiple heating elements, in particular a first heating element and a second heating element, on a single carrier. This advantageously enables a compact design within the package. Furthermore, the influence on a possible flow of the measuring fluid on the top side is reduced by reducing the number of components, allowing for more precise measurement. Accordingly, the susceptibility to errors is also advantageously reduced.

[0122] Any potential susceptibility to errors can also be further reduced by including multiple reference substances, especially multiple temperature responses. In particular, several reference substances can exhibit different aggregate states.

[0123] In a further preferred embodiment, the thermal sensor is characterized in that the reference substance is present as a liquid or a gas, the heating element is mounted on a membrane, wherein the membrane is held by a carrier and the carrier has a cavity below the membrane, wherein preferably the cavity is closed and the reference substance is introduced within the cavity or the cavity is open and the reference substance can flow in.

[0124] The use of a solid as a reference substance is advantageous in that it ensures high mechanical robustness. This allows for reliable measurements even when the measurement fluid undergoes significant temperature and / or pressure changes. Using a solid as a reference substance also offers advantages in processing. In particular, there are advantageously several design options for providing the reference substance as a solid. In a preferred embodiment, the solid is introduced into a cavity of a support structure. In a further preferred embodiment, the support structure as a solid itself can form the reference substance. A support structure preferably refers to a component which is attached to a preferred carrier and onto which at least one heating element can be positioned. A cavity is preferably located on the support structure.This provides sufficient space for attaching the reference substance. Thus, a reference substance can be attached to a preferred support below the heating element. The support structure preferably has a smaller length, height, and / or width than the support. In particular, it is preferred that the support structure be positioned as an intermediate component between the support and the heating element. A preferred cavity on the support structure is particularly well-suited for a solid body as a reference substance, in order to introduce it into the cavity of the support structure below the heating element.

[0125] It may also be preferable to apply the solid as a material layer to the carrier in order to position the heating element on the solid. Thus, several options are available for the design of the thermal sensor when a solid is used as the reference substance, which, depending on the application, enable an efficient measurement process for determining at least one property of a measurement fluid. A reference substance can also preferably be formed by a carrier having different etching depths for the design of the reference substance as a solid. In other words, the carrier can preferably have at least two sections that have a different height between a cavity and an interface, on which one or more heating elements are attached. Different etching depths or heights between the cavities on the carrier and an interface enable different thermal insulation of the heating element.This allows different amplitude and phase signals to be achieved.

[0126] In a further preferred embodiment of the thermal sensor, the thermal sensor has at least two heating elements comprising a first heating element and a second heating element, wherein the at least two heating elements are preferably mounted on a membrane which is located on a carrier. The membrane preferably has at least one opening, preferably two or more, at the location where the first heating element is located. Furthermore, it is preferred that the membrane is designed to be continuous at the location where the second heating element is located. A grid is preferably mounted between the first and second heating elements so that the measuring fluid is divided into a stagnant and a flowing region. The measuring fluid diffuses through the openings below the first heating element so that the measuring fluid itself acts as a reference substance therein.Below the second heating element there is a reference substance that differs from the measuring fluid in at least one thermal property.

[0127] The aspects of the invention will be described below by way of example with reference to figures, without being limited to these figures.

[0128] FIGURES

[0129] Short description of the characters

[0130] TI Fig. 1 Schematic representation of a preferred embodiment of a thermal

[0131] Sensors

[0132] Fig. 2 Schematic representation of measurement results regarding a temperature response

[0133] Fig. 3 Schematic representation of measurement results to illustrate an optimization of the amplitude and phase shift of the temperature response

[0134] Fig. 4 Schematic representation of measurement results with a variation of measuring fluids

[0135] Fig. 5 Schematic representation of a heating element comprising two heating elements, each with a heating element on a carrier

[0136] Fig. 6 Schematic representation of a measuring principle for the formation of a combination signal

[0137] Fig. 7 Schematic representation of measurement results of a thermal sensor comprising three heating elements, each heating element being mounted on a carrier

[0138] Fig. 8 Schematic representation of a thermal sensor comprising two heating elements on a carrier

[0139] Fig. 9-10 Schematic representation of embodiments of the thermal sensor comprising a solid body as a reference substance

[0140] Fig. 11 Schematic representation of a thermal sensor comprising two heating elements on a carrier

[0141] Detailed description of the characters

[0142] Fig. 1 shows a preferred embodiment of a thermal sensor 1 for use in a measuring method according to the invention. The thermal sensor 1 comprises a heating element 5, which is designed to be excited in an oscillating manner at least at one frequency. Furthermore, the thermal sensor 1 is configured to determine at least one property of a measuring fluid 3 based on a measurement of a frequency-dependent temperature response. A measuring fluid 3 is guided on an upper side O above the heating element 5. A reference substance 7 is located on a lower side U below the heating element 5, wherein the reference substance 7 differs from the measuring fluid 3 in at least one thermal property.

[0143] In the illustrated embodiment according to Fig. 1, the heating element 5 is mounted on a membrane 9. The membrane 9 is held by a carrier 1 such that the measuring fluid 3 is guided on an upper side O of the membrane 9. Furthermore, the carrier 11 and the membrane 9 are incorporated within a package 13. The package 13 fulfills a protective function, for example against foreign particles and / or substances, such as dust, moisture and / or liquids. The package 13 surrounds the heating element 5 such that it provides a base for the carrier 11. The package 13 can advantageously offer protection against external influences for components on an upper side O and / or the lower side U.

[0144] By providing a reference substance 7 that differs from the measuring fluid 3 in at least one thermal property, it is advantageously possible to optimize the temperature behavior, in particular the temperature response, of the thermal sensor 1. For example, depending on the choice of reference substance, the amplitude of the sensor's temperature response can be increased. Furthermore, a reference substance 7 can be selected and / or adjusted such that a desired phase shift of the temperature response can be achieved. In this way, an increase in the cutoff frequency of the temperature response can be achieved, so that a thermal sensor 1 can be operated at a higher frequency with the same output signal.By providing a reference substance 7 below the heating element 5, which differs from the measuring fluid 3 in at least one thermal property, an optimization of the amplitude or phase of a temperature response can thus be advantageously carried out in order to align the measuring method with high sensitivity to a property of the measuring fluid 3 to be determined.

[0145] The temperature response refers to a signal detected by the thermal sensor 1 as a result of the heating of the measuring fluid 3 by the oscillating activation of the heating element 5. The frequency-dependent temperature response thus represents a reaction of the thermal sensor 1 to an oscillating activation of the heating element 5, whereby, due to a dependence on the influence of the measuring fluid 3, statements about the thermal properties of the latter are made possible.

[0146] Fig. 2 shows measurement results obtained by applying the measurement method according to the invention. According to the invention, at least one property of the measurement fluid is determined by measuring a frequency-dependent temperature response using a thermal sensor comprising a heating element, wherein the heating element is oscillated at at least one frequency. The measurement fluid is guided on an upper side above the heating element. A reference substance is located on a lower side below the heating element, wherein the reference substance differs from the measurement fluid in at least one thermal property in order to influence the frequency-dependent temperature response of the thermal sensor.

[0147] The solid measurement curve (model 1) represents measurement results using an arrangement according to the invention. On the top side above the heating element, hydrogen is conducted as the measurement fluid. On the bottom side below the heating element, xenon, which has low thermal conductivity and low volumetric heat capacity, is located. The dashed measurement curve (model 2) represents measurement results using a measurement method known from the prior art, in which hydrogen is located on a top and bottom side. It can be seen that the temperature response of the thermal sensor is optimized by the measurement method according to the invention. The amplitude of the temperature response is advantageously increased. Furthermore, a desired phase shift to higher cutoff frequencies can advantageously be brought about.This approach thus achieves higher sensitivities at higher excitation frequencies, which expands the measurement dynamics and enables a more precise determination of at least one property of the measurement fluid. Fig. 3 shows further measurement results using a preferred embodiment of the measurement method according to the invention. On the top side above the heating element, hydrogen is conducted as the measurement fluid at atmospheric conditions. On the bottom side below the heating element, xenon is located. The dashed measurement curve (model 2) represents measurement results using a measurement method known in the prior art. The solid line (model 1) illustrates the measurement results using a measurement method preferred according to the invention. In Fig. 3A it can be seen that the amplitude of the temperature response is increased for low frequencies, in particular in the range of less than 10 Hz. In Fig.Figure 3B shows an advantageous phase shift for increasing the cutoff frequency, which can be achieved by the measurement method according to the invention. By increasing the cutoff frequency, the frequency range at which the heating element can be controlled can be expanded. Thus, higher sensitivities for the thermal sensor can be achieved at higher control frequencies, which allows for a higher measurement dynamic range.

[0148] Fig. 4 schematically shows measurement results in an embodiment of the invention in which a variation of measurement fluids occurs. A frequency scan of the measurement method is carried out for several different measurement fluids. The reference substance in the measurement results is air, while air, hydrogen (H2) and carbon dioxide (CO2) are used as measurement fluids. Of the related gases, carbon dioxide has the lowest thermal conductivity compared to air and hydrogen. The thermal conductivity of hydrogen, on the other hand, is many times (a factor of 5 - 10) higher than that of air and carbon dioxide. It can be observed that the frequency behavior of the temperature response changes depending on the selection of the measurement fluid for a constant reference substance. If the thermal conductivity differs from the reference substance, a change in the amplitude of the temperature response at low frequencies can be caused.If the thermal diffusivity differs from the reference material, the behavior of the temperature response, especially the phase, changes at higher frequencies.

[0149] Fig. 5 shows a schematic representation of another preferred embodiment of the thermal sensor 1. The thermal sensor 1 has a first heating element 5 and a second heating element 5. The first heating element 5 and the second heating element 5 are located within a package 13. The first heating element 5 and the second heating element 5 are held by membranes 9 on different supports 11. On the underside U below the respective heating elements 5 there is a reference substance 7 which differs from the measuring fluid 3 in at least one thermal property. At least one property of the measuring fluid 3 is determined based on the respective temperature responses of the two heating elements 5. A combination signal can be formed from the respective temperature responses of the two heating elements 5, whereby a particularly comprehensive and precise determination of at least one property of the measuring fluid 3 can be carried out.

[0150] Fig. 6 illustrates an inventive principle for generating a combination signal for a thermal sensor comprising two heating elements. The heating elements are mounted on a carrier. However, the measuring principle for generating a combination signal is largely independent of whether multiple heating elements are mounted on different carriers or just a single carrier. A combination signal takes into account different temperature responses resulting from different reference substances. This allows characterization of the measuring fluid based on multiple reference substances, so that the different temperature responses enable a reliable measurement. Furthermore, the measurement of at least one property using a combination signal allows error minimization by providing reference substances.By selecting the combination signal, the influence of any interference factors, such as temperature fluctuations in the thermal sensor, can be eliminated or compensated. This enables a high-resolution measurement that is also characterized by minimal influence from any interference.

[0151] Fig. 7A shows a further schematic representation of a preferred embodiment of the thermal sensor 1. The thermal sensor 1 has three heating elements 5, which are incorporated within the package 13. The three heating elements 5 are held by membranes 9 on different supports 7. On the underside U below the respective heating elements 5 is a different reference substance 7. Sulfur hexafluoride SFe, nitrogen N2, and hydrogen H2 are used as reference substances 7.

[0152] Figures 7B and 7C show the temperature responses that can be generated by different combination signals from thermal sensor 1. Figure 7B shows a combination signal generated by the temperature responses of the heating elements, below which sulfur hexafluoride and nitrogen are arranged. Figure 7B shows the combination signal generated by the difference between all heating elements and the reference substance. Several measurement fluids are examined, in particular hydrogen (H2), nitrogen (N2), sulfur hexafluoride (SFe), carbon dioxide (CO2), and helium (He). The measurement curves exhibit clearly distinguishable profiles, particularly with regard to the frequency of peaks and minima. This makes it easier to identify specific properties of the respective measurement fluids.Thus, by using the three reference substances mentioned, a characteristic thermal fingerprint of measuring fluids can be advantageously obtained, which is extremely specific for the respective measuring fluid.

[0153] Fig. 8 shows a preferred embodiment of the thermal sensor 1. The thermal sensor 1 comprises a first heating element 5 and a second heating element 5, which are held by the same carrier 11. Below the heating elements 5 are the reference substances 7, which are different from one another. The reference substance 7 is designed as a solid body that is positioned on the carrier 11. The heating elements 5 are applied directly to the reference substances 7, so that the reference substances 7 are located below the heating elements 5. The attachment of the reference substance 7 as a solid body is advantageous, among other things, in that mechanical stability is ensured during changes in temperature and / or pressure. Furthermore, the reference substance 7 can be attached as a solid body below the heating element 5 using simple, known means from semiconductor and / or microsystem technology. Fig.9 shows a thermal sensor 1 in which the reference substance is present as a solid. The thermal sensor 1 in Fig. 9A comprises a carrier 11 on which support structures 15 are arranged. The heating elements 5 are attached to the support structures 15. The support structures 15 have a cavity. The reference substance 7 is located in the cavity of the left support structure 15 as a solid. The solid can be provided by a sacrificial layer. In the cavity of the right support structure 15 there is no solid as the reference substance 7. Fig. 9B shows a further embodiment in which the reference substance 7 can be formed by a solid. In this case, cavities on the carrier 11 are etched over different depths, so that different heights hi and fi2 lie between the cavities on the carrier 11 and an interface of the carrier 11. Different etching depths orHeights between the cavities on the carrier 11 and an interface allow for varying thermal insulation of the heating element. This allows for different amplitude and phase signals to be achieved.

[0154] Fig. 10 schematically illustrates another preferred embodiment of the thermal sensor 1 according to the invention. The thermal sensor 1 has a first heating element 5 and a second heating element 5, each mounted on a support structure 15. The support structures 15 are located on a carrier 11. Below the first heating element 5 and the second heating element 5, reference substances 7 are located in a cavity of the support structures 15. The reference substances 7 are in the form of solids. By interconnecting at least two individual sensors with different thermal insulation, the thermal sensors 1 can be made less sensitive to fluid flows. The thermal sensors 1 also have different sensitivities to the fluid properties of the measuring fluid 3.As explained above, by analyzing the respective signal based on a characteristic fingerprint, properties of the measuring fluid, such as its composition, can be determined. The system also becomes less sensitive to temperature changes. To infer the properties of the measuring fluid, further mathematical operations can be performed, such as fast Fourier transform.

[0155] Fig. 11 schematically shows another preferred embodiment of the thermal sensor 1. In Fig. 11A, the thermal sensor 1 has a first heating element 5 and a second heating element 5. The first heating element 5 and the second heating element 5 are mounted on a membrane 9. The membrane comprises two openings at the location where the first heating element 5 is positioned. The membrane 9 is continuous at the location where the second heating element 5 is mounted. A grid is mounted between the first and second heating elements 5 so that the measuring fluid 3 is divided into a stagnant and a flowing region. The measuring fluid 3 diffuses through the openings below the first heating element 5. Below the second heating element 9 is a reference substance 7, which differs from the measuring fluid 3 in at least one thermal property. Fig. 11B shows another preferred embodiment of the thermal sensor 1.The thermal sensor also has a first heating element 5 and a second heating element 5. A measuring fluid 3 is located above the heating element 5. Different reference substances 7 are located below the first heating element 5 and the second heating element 5. LIST OF REFERENCE SYMBOLS.

[0156] 1 Thermal sensor

[0157] 3 Measuring fluid

[0158] 5 Heating element

[0159] 7 Reference substance

[0160] 9 Membran

[0161] 11 carriers

[0162] 13 Package

[0163] 15 Support structure

[0164] LIST OF SOURCES

[0165] Ernst, Herbert, Artur Jachimowicz, and Gerald Urban. "Dynamic thermal sensor principles in MEMS for fluid characterization." IEEE sensors journal 1.4 (2001): 361-367.

[0166] Wang, Li, Bin Wang, and Qiao Lin. "Demonstration of MEMS-based differential scanning calorimetry for determining thermodynamic properties of biomolecules." Sensors and Actuators B: Chemical 134.2 (2008): 953-958.

[0167] J. Hong and D. Kim. Measuring the Thermal Conductivity of Flowing Liquid Samples Using the Three Omega Method. Journal of Heat Transfer, 134(9), p. 094502, 2012. doi: 10.1115 / 1.400638.

[0168] Lysenko, V., et al. "Thermal isolation in microsystems with porous silicon." Sensors and Actuators A: Physical 99.1-2 (2002): 13-24.

[0169] Van Herwaarden, A. W., and P. M. Sarro. "Thermal sensors based on the Seebeck effect." Sensors and Actuators 10.3-4 (1986): 321-346.

Claims

PATENT CLAIMS 1. A measuring method for determining at least one property of a measuring fluid (3) by means of a thermal sensor (1) comprising a heating element (5), wherein an oscillating control of the heating element (5) takes place at at least one frequency and the at least one property of the measuring fluid (3) is determined based on a measurement of a frequency-dependent temperature response of the thermal sensor (1), characterized in that the measuring fluid (3) is guided on an upper side (O) above the heating element (5) and a reference substance (7) is located on a lower side (U) below the heating element (5), wherein the reference substance (7) differs from the measuring fluid (3) in at least one thermal property in order to influence the frequency-dependent temperature response of the thermal sensor (1).

2. Measuring method according to the preceding claim, characterized in that the at least one thermal property in which the measuring fluid (3) and the reference substance (7) differ from one another is selected from a group comprising a thermal conductivity, a density, a pressure, a temperature, a specific heat capacity and / or a thermal diffusivity.

3. Measuring method according to one or more of the preceding claims, characterized in that the at least one thermal property in which the measuring fluid (3) and the reference substance (7) differ from one another is selected in order to shift, preferably to increase, an amplitude and / or cut-off frequency of the frequency-dependent temperature response.

4. Measuring method according to one or more of the preceding claims, characterized in that the reference substance (7) has a lower thermal conductivity and / or volumetric heat capacity compared to the measuring fluid (3) in order to increase an amplitude and / or cut-off frequency of the frequency-dependent temperature response.

5. Measuring method according to one or more of the preceding claims, characterized in that the measuring method is carried out by means of at least two thermal sensors (1) or at least one thermal sensor (1) with at least two heating elements (5), wherein a different reference substance (7) is located on an underside (U) below the respective heating elements (5).

6. Measuring method according to the previous claim characterized in that a determination of the property of a measuring fluid (3) is carried out on the basis of the respective temperature responses of the at least two heating elements (5), wherein a combination signal is preferably formed from the respective temperature responses of the at least two heating elements (5).

7. Measuring method according to one or more of the preceding claims, characterized in that the reference substance (7) is present as a solid, wherein the reference substance (7) preferably comprises a material selected from a group comprising porous silicon, aerogels, silicon oxide, polyimide, silicon nitride and / or a photoresist.

8. Measuring method according to one or more of the preceding claims, characterized in that the reference substance (7) is present as a liquid or a gas, wherein the reference substance (7) is preferably selected from a group comprising hydrogen, carbon dioxide, helium, neon, argon, krypton, xenon, nitrogen, sulfur hexafluoride and / or air.

9. Thermal sensor (1) for use in a measuring method according to one or more of the preceding claims, comprising a heating element (5), wherein the heating element is designed to be excited in an oscillating manner at least at one frequency and the thermal sensor (1) is configured to determine at least one property of a measuring fluid (3) based on a measurement of a frequency-dependent temperature response, characterized in that the measuring fluid (3) is guided on an upper side (O) above the heating element (5) and a reference substance (7) is located or can be introduced on a lower side (U) below the heating element (5), wherein the reference substance (7) differs from the measuring fluid (3) in at least one thermal property.

10. Thermal sensor (1) according to the preceding claim, characterized in that the thermal sensor (1) has an electronic circuit, wherein the computing unit is configured to excite the heating element in an oscillating manner with at least one frequency and / or to evaluate a frequency-dependent temperature response of the thermal sensor (1) in order to determine a property of the fluid.

11. Thermal sensor (1) according to one or more of the preceding claims 9-10, characterized in that it comprises at least two heating elements (5), wherein a different reference substance (7) is located or can be introduced on a bottom side (U) below the at least two heating elements (5).

12. Thermal sensor (1) according to one or more of the preceding claims 9-11, characterized in that the thermal sensor (1) comprises a first heating element (5), on the underside of which a first reference substance (7) is located or can be introduced, and a second heating element (5), on the underside (U) of which a second reference substance (7) is located or can be introduced, wherein the thermal sensor (1) has a computing unit which is configured to form a combination signal on the basis of a temperature response of the first and second heating elements (5), so that a measured variable of a measuring fluid (3) can be determined on the basis of the combination signal.

13. Thermal sensor (1) according to one or more of the preceding claims 9-12, characterized in that the first and second heating elements (5) are incorporated within a package (13), wherein preferably the first and second heating elements (5) are each held by different carriers (11) or by one and the same carrier (11).

14. Thermal sensor (1) according to one or more of the preceding claims 9-13, characterized in that the reference substance (7) is present as a liquid or a gas, the heating element (5) is mounted on a membrane (9), wherein the membrane (9) is held by a carrier (11) and the carrier (11) has a cavity below the membrane (9), wherein preferably the cavity is closed and the reference substance (7) is introduced within the cavity or the cavity is open and the reference substance (7) can flow in.

15. Thermal sensor (1) according to one or more of the preceding claims 9-14, characterized in that the reference substance (7) is present as a solid body, wherein the heating element (5) is attached to the reference substance (7) and wherein the reference substance (7) is located on a carrier (11) or wherein the heating element (5) is attached to a support structure (15), wherein the support structure (15) has a cavity and is arranged on a carrier (11), wherein the reference substance (7) is located as a solid body within the cavity of the support structure.

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