Thermal Flow Meter Circuit with Periodic Heating and Measurement
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Solution Overview
Problem
Conventional thermal flow meters require multiple resistors aligned within the measuring tube, leading to complex designs and potential inaccuracies due to non-constant heating currents and self-heating effects, which affect the precision of flow measurements.
Innovation Solution
A thermal flow meter circuit with a first and second resistor, where a switch controls the connection to either a first current source for heating or a second current source for measurement, utilizing a shunt resistor to measure heating current and an offset resistor to improve measurement accuracy, allowing for precise calculation of flow based on heating and measurement currents and temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple resistors are permanently heated with constant current sources, then flow measurement can be performed, but device complexity increases and self-heating effects cause measurement inaccuracies
Solution Approach 1:
The patent implements periodic heating phases alternating with measurement phases. During heating phases, current flows through the resistor to heat it; during measurement phases, the current is interrupted and temperature is measured. This periodic action eliminates the need for permanent heating, reducing self-heating effects and improving measurement precision while simplifying the circuit design.
Solution Approach 2:
The patent applies preliminary heating before measurement by conducting heating phases immediately before measurement phases in each measurement cycle. This ensures the resistor reaches the desired temperature state before temperature measurement begins, allowing accurate flow measurement while maintaining a simplified periodic circuit structure.
2Device complexity
If voltage sources are used to supply heating current, then circuit simplification is achieved, but heating current becomes non-constant leading to measurement inaccuracies
Solution Approach 1:
The patent uses periodic switching between heating and measurement phases with constant current sources. During heating phases, constant current ensures stable heating; during measurement phases, the same constant current source is switched to measurement mode. This periodic switching maintains measurement precision while achieving circuit simplification through shared current sources.
Solution Approach 2:
The patent dynamically switches the function of constant current sources between heating and measurement modes using switches. The current sources are not permanently dedicated to heating but are dynamically reassigned to measurement functions during measurement phases. This dynamic switching achieves circuit simplification without sacrificing measurement precision.
3Measurement precision
If measurement current flows continuously through resistors, then temperature can be continuously measured, but self-heating effects occur causing measurement inaccuracies
Solution Approach 1:
The patent implements periodic measurement phases where measurement current flows only during designated measurement periods, not continuously. Between measurement phases, heating phases occur without measurement current flow. This periodic measurement approach eliminates continuous self-heating effects while maintaining adequate temperature measurement precision through regular sampling.
Solution Approach 2:
The patent maintains continuous useful action by alternating between heating phases (which prepare the resistor for measurement) and measurement phases. The heating phases ensure the resistor is properly conditioned, and measurement phases capture temperature data. This continuous alternation eliminates idle time while preventing continuous measurement current flow that would cause self-heating.
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
This configuration simplifies the design, reduces measurement inaccuracies by maintaining constant heating currents and accounting for measurement currents, enhancing the precision of flow determination through the measuring tube.
Implementation Method 1
a constant heating current Ih from a first current source flows through a first resistor (1), which first resistor (1) is exposed to the measurement medium
Implementation Method 2
a constant measurement current Im from a second current source flows through the first resistor (1), with a first voltage Ur,1 dropping across the first resistor (1) during the first measurement phase
Implementation Method 3
both resistors are heated with a heating current and that the temperature of the measurement medium is measured with a third resistor
Data Source
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AI summary
The invention relates to a thermal flow measuring device and to a method for operating a thermal flow measuring device for determining the flow of a measurement medium through a measurement tube, wherein a constant heating current lh of a first current source flows through a first resistor during a heating phase of prescribed length th during a first measurement cycle, said first resistor being exposed to the measurement medium, wherein a constant measurement current lm of a second current source flows through the first resistor during a first measurement phase of prescribed length tr,1 during the first measurement cycle, wherein a first voltage Ur,1 drops across the first resistor during the first measurement phase tr,1, wherein the constant measurement current lm of the second current source flows through a second resistor during a second measurement phase of prescribed length tr,2 during the first measurement cycle, said second resistor being exposed to the measurement medium, wherein a second voltage Ur,2 drops across the second resistor during the second measurement phase tr,2, wherein the constant heating current lh of the first current source flows through a shunt resistor during the heating phase th, wherein a fifth voltage Uh,5 drops across the shunt resistor during the heating phase th, wherein at least the first voltage Ur,1, the second voltage Ur,2, and the fifth voltage Ur,5 are incorporated in the calculation of the flow rate, and wherein the constant heating current lh of the first current source flows through a bypass resistor during the first measurement phase tr,1.