Thermal Flow Sensor Linear Range Extension via Heater Control
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Solution Overview
Problem
Flow sensors employing differential thermal anemometry have a limited linear range, making them ineffective at both low and high flow rates, as the relationship between temperature difference and flow rate becomes nonlinear at elevated flow rates, leading to decreased sensitivity and accuracy.
Innovation Solution
A method and apparatus that utilize a heating element with temperature sensors to calculate a flow scalar value and a heating element compensation factor, allowing for accurate flow rate determination beyond the maximum linear range by compensating for temperature differences and maintaining a uniform heater temperature, thereby extending the linear range of flow sensing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a thermal flow sensor operates at elevated flow rates, then the flow rate increases, but the relationship between temperature difference and flow rate becomes nonlinear, decreasing measurement precision
Solution Approach 1:
The patent changes the operational parameter by maintaining constant heater temperature instead of constant power input. This parameter change ensures that the temperature difference between upstream and downstream remains proportional to flow rate across the entire operating range, eliminating the nonlinearity that occurs at elevated flow rates when heater temperature drops due to convective cooling.
2Device complexity
If constant power is applied to the heater, then the heater operates simply, but at high flow rates the heater temperature decreases causing the ΔT response to flow rate changes to decrease
Solution Approach 1:
The patent implements a feedback control system where the heater temperature is continuously monitored and adjusted to maintain a constant setpoint. This feedback mechanism compensates for the convective cooling effect at high flow rates, ensuring that the heater temperature remains stable and the temperature difference response remains linear with flow rate across the extended operating range.
3Reliability
If a more massive heater with larger surface area is used, then the sensor can handle large fluid flows without decreasing heater temperature, but the response to flow changes becomes very slow
Solution Approach 1:
The patent employs dynamic temperature control of the heater, adjusting the heating power in real-time based on feedback from the temperature sensors. This dynamic control allows the system to respond quickly to flow changes while maintaining heater temperature stability, avoiding the need for a massive heater that would provide stability but slow down the response time.
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
The solution effectively extends the linear range of flow sensing, enabling accurate measurement of both low and high flow rates by compensating for temperature changes and maintaining heater stability, improving the sensitivity and reliability of flow rate calculations.
Implementation Method 1
A discrete thermal plug introduced into a liquid filled tube/channel will disperse in both the upstream and downstream directions due to thermal conduction or diffusion, respectively
Implementation Method 2
A discrete thermal plug introduced into a liquid filled tube/channel will disperse in both the upstream and downstream directions due to thermal conduction or diffusion, respectively
Implementation Method 3
If the liquid in the tube/channel is permitted to flow, the fluid temperatures at P1 and P2 will now also depend upon the rate of liquid flux and the resulting heat convection. As liquid begins to flow past the heated zone, a temperature profile similar to CB in FIG. 1 will develop since, in addition to the symmetrical diffusion of the heat, asymmetrical convection of the heated fluid will occur in the direction of the fluid flow
Data Source
AI summary
One aspect of the invention provides a flow sensing apparatus including: a fluid channel that allows a fluid to flow in a first direction; a first temperature sensor arranged at a first position along the fluid channel; a second temperature sensor arranged at a second position along the fluid channel and separated from the first sensor by a predetermined distance along the fluid channel; a heating element arranged between the first and second thermoelectric sensors, the heating element being substantially equally spaced from the first and second thermoelectric sensors; a heating element temperature sensor for sensing a temperature of the heating element; and a control device configured to maintain the heating element at a substantially uniform temperature.