Airflow Sensor Gas Composition Correction
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Solution Overview
Problem
Conventional thermal based MEMS flow sensors struggle to accurately measure gas flow when the gas composition varies or is unknown, as they rely on thermal diffusivity measurements, which are difficult due to low gas density and require additional information.
Innovation Solution
A cost-effective, low-power thermal conductivity and kinematic viscosity sensor is integrated into or used concurrently with conventional flow sensors, incorporating additional circuitry to measure thermal conductivity and kinematic viscosity, allowing for correction of mass flow values by determining the Prandtl number and using differential pressure to calculate standard volumetric flow rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If thermal based MEMS flow sensors are used to measure gas flow, then flow rate can be detected, but measurement precision deteriorates when gas composition varies or is unknown due to reliance on thermal diffusivity measurements
Solution Approach 1:
The patent changes the measurement parameters from relying solely on thermal diffusivity to measuring both thermal conductivity and kinematic viscosity. By determining the Prandtl number (ratio of thermal diffusivity to kinematic viscosity) and using differential pressure measurements, the system can accurately measure gas flow rates regardless of gas composition, resolving the contradiction between measurement precision and adaptability to unknown gases.
Solution Approach 2:
The patent introduces thermal conductivity and kinematic viscosity measurements as intermediary parameters to bridge the gap between thermal diffusivity measurements and accurate gas flow measurement. These intermediary measurements allow the system to compensate for unknown gas compositions by determining the Prandtl number and using it to correct the flow measurement.
2Measurement precision
If thermal diffusivity measurements are used for flow sensing, then flow rate can be measured, but device complexity increases due to additional circuitry and measurements required
Solution Approach 1:
The patent makes the flow sensor multi-functional by integrating capabilities to measure thermal conductivity, kinematic viscosity, and flow rate into a single device. The sensor uses a heated element that serves multiple purposes: generating the thermal field for flow measurement while also enabling thermal conductivity and viscosity measurements through additional circuitry that analyzes temperature and pressure variations.
Solution Approach 2:
The patent combines multiple measurement functions into a single integrated sensor system. By merging thermal conductivity measurement, kinematic viscosity measurement, and flow rate measurement capabilities into one device with shared heating elements and sensing circuits, the system reduces overall complexity compared to using separate devices for each measurement.
3Device complexity
If conventional flow sensors are used, then simple flow measurement is possible, but measurement precision deteriorates when gas composition is unknown or varies
Solution Approach 1:
The patent changes the physical parameters measured from simple thermal diffusivity to a combination of thermal conductivity, kinematic viscosity, and differential pressure. This parameter change enables the sensor to maintain high measurement precision across varying gas compositions while keeping the overall device structure relatively simple through integrated sensing elements.
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 approach enhances the accuracy of gas flow measurement by compensating for unknown gas compositions, providing precise standard volumetric flow rates through thermal and viscosity property determination, even when the gas identity is unknown.
Implementation Method 1
a cost-effective, low-power thermal conductivity and kinematic viscosity sensor is integrated into or used concurrently with conventional flow sensors, incorporating additional circuitry to measure thermal conductivity and kinematic viscosity
Implementation Method 2
a cost-effective, low-power thermal conductivity and kinematic viscosity sensor is integrated into or used concurrently with conventional flow sensors, incorporating additional circuitry to measure thermal conductivity and kinematic viscosity
Implementation Method 3
using differential pressure to calculate standard volumetric flow rates
Data Source
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AI summary
A sensor for sensing a flow rate of a fluid comprises an upstream resistive element having a first resistance that changes with temperature, a downstream resistive element having a second resistance that changes with temperature, at least one tail resistor configured to determine thermal conductivity of the fluid, at least one pressure sensor configured to determine a differential pressure in the flow direction of the fluid, and circuitry configured to use the differential pressure with the thermal conductivity to determine a kinematic viscosity of the fluid, and compensate an output of the bridge circuit. The downstream resistive element is situated downstream of the upstream resistive element in the flow direction of the fluid, and the upstream resistive element and the downstream resistive element are operatively connected in a bridge circuit.