Thermal Flow Sensor Self-Calibration Using Periodic Heating
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Solution Overview
Problem
Thermal flow sensors require calibration for specific measuring media, which is complex and costly, and often necessitates prior knowledge of the medium's thermal properties, making them inefficient for use with unknown media.
Innovation Solution
A method involving periodic heating with alternating voltage to detect temperature curve amplitudes and phase shifts, using calibration data to determine an isoline of thermal parameters, allowing for flow measurement without prior medium property knowledge, utilizing the 3-omega method and mathematical models based on heat conduction equations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If thermal flow sensors are calibrated for specific measuring media using conventional methods, then measurement precision is improved, but device complexity and production costs increase due to requiring prior knowledge of thermal properties and separate determination of thermal conductivity and heat capacity
Solution Approach 1:
The patent combines the determination of thermal conductivity and heat capacity into a single simultaneous measurement process using one sensor element. Instead of requiring separate measurements or multiple sensors, the method measures both thermal parameters together by analyzing the transient thermal response to periodic heating, thereby reducing device complexity and calibration requirements
Solution Approach 2:
The sensor performs self-calibration by automatically determining the thermal parameters of the measuring medium through its own periodic heating and temperature measurement functions. The evaluation unit calculates thermal conductivity and heat capacity from the measured transient thermal response, eliminating the need for external calibration devices or prior knowledge of medium properties
2Measurement precision
If separate determination of thermal conductivity and heat capacity is performed using different measurement conditions, then measurement precision is improved, but loss of time increases due to multiple measurement steps and sequences
Solution Approach 1:
The patent implements continuous simultaneous determination of thermal conductivity and heat capacity through uninterrupted periodic heating and continuous temperature measurement. The sensor element is continuously heated with alternating current while the temperature is continuously monitored, allowing both thermal parameters to be determined in one continuous process rather than through separate sequential measurements
Solution Approach 2:
The method performs preliminary characterization of the measuring medium by determining its thermal parameters during the initial measurement phase. The transient thermal response to periodic heating provides immediate information about both thermal conductivity and heat capacity, eliminating the need for separate preliminary calibration steps before flow measurement
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables self-calibration of thermal flow sensors for various media, reducing production costs and complexity by compensating for medium dependence, with a self-checking function for predictive maintenance and simplified implementation.
Implementation Method 1
periodically heating the measuring medium by means of an alternating voltage introduced into the sensor element
Implementation Method 2
a (flowing) measuring medium dissipates heat from a heated surface
Implementation Method 3
As the measured medium flows around the heating element, heat is transferred into the measured medium
Data Source
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AI summary
The invention relates to a method for operating a thermal flow sensor (100), said thermal flow sensor (100) having at least one sensor element (101, 102, 103) and an electronic unit. The method has the steps of: - bringing a measuring medium (2) into thermal contact with the sensor element (101, 102, 103) and periodically heating the measuring medium (2) using an AC voltage introduced into the sensor element (101, 102, 103), wherein simultaneously the maximum amplitude of the curve of the temperature and/or the phase offset between the curve of the AC voltage and the curve of the temperature is detected; - adapting the detected maximum amplitude and/or the detected phase offset using calibration data; - determining an isoline (ILx) using the adapted maximum amplitude and/or the adapted phase offset on the basis of a mathematical, or physical, model of the thermal flow sensor (100), wherein the isoline (ILx) has a plurality of value pairs of thermal parameters (Cp, k), in particular thermal conductivity and thermal capacitance, of the measuring medium (2), said value pairs being associated with the same maximum amplitude or the same phase offset; - deriving at least one piece of measurement fluid information from the isoline (ILx); and - carrying out a flow measurement using the thermal flow sensor (100), said sensor element (101, 102, 103) providing signal values, and converting the signal values into measurement values of the effective flow speed of the measuring medium (2) using the measuring fluid information. The invention also relates to a thermal flow sensor.