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EP-4740003-A1 - THERMAL MEASURING DEVICE FOR DETERMINING FLUID PROPERTIES BY VARYING THE SURROUNDING REFERENCE PROPERTIES

EP4740003A1EP 4740003 A1EP4740003 A1EP 4740003A1EP-4740003-A1

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.

Inventors

  • Raimann, Philipp
  • HEDRICH, FRANK
  • BILLAT, SOPHIE
  • Dehé, Alfons

Assignees

  • Hahn-Schickard-Gesellschaft für angewandte Forschung e. V.

Dates

Publication Date
20260513
Application Date
20240705

Claims (15)

  1. 1. 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. 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. 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 increase, an amplitude and/or cutoff frequency of the frequency-dependent temperature response.
  4. 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 cutoff frequency of the frequency-dependent temperature response.
  5. 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 a lower side (U) below the respective heating elements (5).
  6. 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. 7. Measuring method according to one or more of the preceding claims, characterized in that the reference substance (7) is present as a solid body, 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. 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. 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. 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. 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. 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 based on 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. 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. 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. 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.

Description

THERMAL MEASURING DEVICE FOR DETERMINING FLUID PROPERTIES BY VARIING THE AMBIENT REFERENCE PROPERTIES DESCRIPTION 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 which is controlled in an oscillating manner with at least one frequency. The at least one property of the measuring fluid is determined by measuring 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. Furthermore, the invention is directed to a thermal sensor which is used for the method according to the invention. Background and State of the Art Microsystems technology is now used in many areas of application to produce compact, mechanical-electronic devices. The microsystems (microelectromechanical systems, MEMS) produced in this way are very compact (in the micrometer range) while at the same time offering excellent functionality and ever lower production costs. MEMS technologies offer a variety of mechanisms for actuation or sensor technology. Designs and components of MEMS technologies can also be used for temperature measurements or for determining the thermal properties of a wide variety of materials based on these. In this case, one usually speaks of thermal MEMS sensors or simply thermal sensors when the MEMS context is known. In the state of the art, a wide variety of areas of application for thermal sensors are known. 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 as temperature sensors are located laterally spaced from the heating element. The measuring process comprises 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 thermal properties of the fluid to be measured to be characterized. DE 10 2017 215 527 A1 discloses a thermal sensor that can measure the concentration of a gas to be analyzed, the analysis gas. For this purpose, a heating element is located on a first membrane, the analysis heating element, which is intended to heat the analysis gas. On a second membrane there is another heating element, the reference heating element, which serves to heat a reference gas. The thermal sensor works with a resistance measurement principle. An electronic circuit enables the detection of a change in 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 one another 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 measuring volume and a reference volume between the second membrane and the base substrate. The reference gas is located in the reference volume, while the analysis gas can flow in the measuring volume. 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 for the characterization of thermal properties of biomolecules. The sensor comprises a pair of microchambers and microchannels for fluid delivery. The chamber is located on a free-standing membrane that extends along a substrate. Heating elements and temperature sensors are integrated in 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 so that the temperature response can be transmitted via a thermopile. In the prior art, it is also known to use an oscillating excitation of a heating element to determine the thermal properties of a measuring fluid based on the resulting temperature response. For example, the 3-omega method is known in the prior art, which can be used in particular to determine the thermal conductivity of a material. A heating element is used which is excited with an