Thermal Flow Meter Reach-Time Switching for Low-Flow Accuracy
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Solution Overview
Problem
Conventional thermal flow meters face calculation errors when measuring low liquid flow rates due to heat transfer to the measurement tube or liquid before reaching the second temperature detecting resistance element, limiting the measurable flow rate range on the lower limit side.
Innovation Solution
A thermal flow meter with a measurement tube and temperature detection substrate, utilizing a heating resistance element and two temperature detecting resistance elements, calculates flow rate based on reach times and applies varying weights to these times depending on flow rate, reducing calculation errors by using both reach times for high flow rates and only the first reach time for low flow rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a second temperature detecting resistance element is arranged further downstream to extend the measurement range, then the measurable flow rate range can be expanded, but calculation errors occur when liquid flow rate is relatively small due to heat transfer to the measurement tube or liquid before reaching the second element
Solution Approach 1:
The patent dynamically adjusts the calculation method based on detected flow rate conditions. When the temperature difference at the second detecting element exceeds a threshold (indicating low flow rate), the system switches to using only the first detecting element for calculation. When the temperature difference is within the threshold (indicating normal flow rate), the system uses both detecting elements. This dynamic adaptation resolves the contradiction by optimizing measurement accuracy for different flow conditions while maintaining extended measurement capability.
Solution Approach 2:
The patent changes the calculation parameters (which detecting elements to use and how to combine their data) based on the thermal state of the liquid. By monitoring the temperature difference at the second detecting element and comparing it to a threshold, the system selects appropriate calculation parameters to maintain accuracy across different flow rates, thus resolving the contradiction between extended range and measurement precision.
2Measurement precision
If both first and second reach times are used for flow rate calculation, then measurement accuracy improves for high flow rates, but calculation errors increase for low flow rates due to premature heat transfer
Solution Approach 1:
The system dynamically selects the appropriate calculation methodology based on real-time thermal conditions. By continuously monitoring the temperature difference at the second detecting element and switching between different calculation modes (using both elements or only the first element), the system maintains high reliability across varying flow conditions while preserving measurement accuracy for high flow rates.
Solution Approach 2:
The patent implements a feedback mechanism where the temperature difference detected by the second element feeds back into the calculation strategy. When the feedback indicates excessive heat loss (large temperature difference), the system adjusts by excluding the second element from calculations. This feedback loop ensures reliable measurements across different flow conditions while maintaining accuracy for high flow rates.
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
Reduces calculation errors in low flow rates, expanding the measurable flow rate range by accurately measuring micro-flow rates from 4 μL/min to 350 μL/min.
Implementation Method 1
a voltage signal for heating the heating resistance element
Implementation Method 2
a first output voltage in accordance with a temperature of the first temperature detecting resistance element and a second output voltage in accordance with a temperature of the second temperature detecting resistance element
Implementation Method 3
the heat applied to the liquid from the heating resistance element is transferred to the measurement tube or the liquid itself before reaching the second temperature detecting resistance element
Data Source
AI summary
Provided is a flow rate calculation method including: a time measurement step of measuring a first reach time taken by a liquid heated by a heating resistance element to reach a first temperature detecting resistance element and measuring a second reach time taken by a liquid heated by the heating resistance element to reach the second temperature detecting resistance element; and a calculation step of calculating a liquid flow rate of a liquid flowing through a measurement tube based on the first and second reach time. The calculation step includes calculating the liquid flow rate based on both the first and second reach time when a first flow rate is greater than a first predetermined flow rate and calculating the liquid flow rate based on the first reach time without using the second reach time when the first flow rate is not greater than the first predetermined flow rate.