MEMS Thermal Flow Sensor Fluid Composition Compensation
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Solution Overview
Problem
MEMS thermal flow sensors are sensitive to thermal and mechanical properties of fluids and environmental variables, limiting their ability to operate with different fluids without expensive and time-consuming calibration, and existing thermal flow meters, such as capillary tube sensors, struggle to accurately measure flows of unknown pure gases or mixtures due to dependencies on fluid composition and properties.
Innovation Solution
A MEMS thermal flow sensor that determines a response curve by plotting sensor output voltage against volume flow rate divided by fluid thermal diffusivity, using a conversion factor derived from the ratio of thermal time constants to correct flow rate measurements for unknown fluids, allowing calibration on one fluid to measure a wide variety of fluids and mixtures without additional calibration, and employing in-situ measurements of thermal diffusivity and conductivity to achieve accurate flow rate determination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the MEMS thermal flow sensor is calibrated for one specific fluid, then it provides accurate measurements for that fluid, but it cannot accurately measure other fluids with different thermal and mechanical properties without additional calibration
Solution Approach 1:
The patent measures thermal conductivity and mass density of the fluid as physical parameters, then uses these parameter values to select appropriate calibration data from stored calibration sets. This allows the sensor to adapt to different fluids by changing the selected calibration parameters rather than requiring physical recalibration for each fluid type.
Solution Approach 2:
The patent performs preliminary measurements of thermal conductivity and mass density when a new fluid is introduced, before actual flow measurement begins. These preliminary actions enable the system to identify the fluid type and select the appropriate calibration curve in advance, ensuring accurate measurements from the start without requiring recalibration.
2Measurement precision
If empirical calibration is performed for each fluid to achieve accurate measurements, then measurement precision improves, but time consumption and cost increase significantly
Solution Approach 1:
The patent performs preliminary measurements of thermal conductivity and mass density when a new fluid is introduced, before actual flow measurement begins. These preliminary actions enable the system to identify the fluid type and select the appropriate calibration curve in advance, ensuring accurate measurements from the start without requiring recalibration.
Solution Approach 2:
The system automatically measures fluid properties (thermal conductivity, mass density) and self-identifies the fluid type, then automatically selects the appropriate calibration data. This self-service capability eliminates the need for manual calibration procedures, reducing both time loss and operational complexity.
3Adaptability or versatility
If the sensor operates with different fluid compositions, then versatility improves, but measurement accuracy deteriorates due to sensitivity to thermal and mechanical properties variations
Solution Approach 1:
The patent measures thermal conductivity and mass density of the fluid as physical parameters, then uses these parameter values to select appropriate calibration data from stored calibration sets. This allows the sensor to adapt to different fluids by changing the selected calibration parameters rather than requiring physical recalibration for each fluid type.
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 the MEMS thermal flow sensor to measure flow rates of various fluids and mixtures without separate calibration, providing accurate and proportional mass flow rate measurements, even for fluids with unknown thermal properties, by using a universal response curve and in-situ measurements of thermal diffusivity and conductivity.
Implementation Method 1
employs a central heater flanked by two or more upstream and downstream temperature detectors all placed in direct thermal contact with the flowing fluid
Implementation Method 2
the existence of fluid flow in the upstream or downstream direction causes an imbalance in the temperature detectors indicative of the fluid flow rate
Data Source
AI summary
The present invention provides a MEMS thermal flow sensor or meter for measuring the flow rate of a fluid without need for calibration of the flow sensor for that particular fluid. A response curve is determined by plotting the sensor output voltage against the volume flow rate divided by fluid thermal diffusivity for a calibration fluid of known thermal diffusivity, and storing response curve data in memory. A conversion factor is employed to provide a measure of correct flow rate of an unknown fluid. This conversion factor is derived from the ratio of the thermal time constant of the calibration fluid to the thermal time constant of the measured fluid, the time constants being measured at zero flow. These time constants are stored in memory. This conversion factor in conjunction with the response curve data is utilized by the processor to produce the correct flow rate. The invention also encompasses a method for measuring fluid flow rate of fluids of differing properties without necessity of a separate flow calibration for each fluid.