Mass Flow Controller With Dual Pressure Transducers for Low Setpoints
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Solution Overview
Problem
Pressure-based mass flow controllers face challenges in maintaining accuracy and repeatability at low setpoints due to significant pressure variations from near zero outlet pressure to atmospheric pressure, leading to degraded flow sensor signal-to-noise ratio and stability.
Innovation Solution
A mass flow controller design incorporating a combination of absolute and differential pressure transducers, with the absolute pressure transducer upstream and differential pressure transducer downstream of the laminar flow element, along with a PID controller to adjust the flow control valve, ensuring accurate mass flow rate control across varying pressures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If absolute pressure sensors with downstream laminar flow elements are used to handle large downstream pressure variations, then the device can operate across a wide pressure range, but the flow sensor signal-to-noise ratio and stability degrade at low setpoints
Solution Approach 1:
The pressure measurement function is segmented into two independent transducers: an absolute pressure transducer upstream of the laminar flow element and a differential pressure transducer downstream. This segmentation allows each transducer to operate in its optimal range, with the upstream absolute pressure providing stable reference measurements independent of downstream pressure variations, thereby maintaining signal-to-noise ratio while achieving wide pressure range adaptability
Solution Approach 2:
The upstream absolute pressure transducer acts as an intermediary that decouples the flow measurement from downstream pressure variations. By measuring absolute pressure upstream before the laminar flow element, the system obtains a stable reference that is not affected by downstream pressure excursions, thus maintaining measurement precision across the full pressure range
2Adaptability or versatility
If the outlet pressure transitions from near zero to atmospheric pressure, then the device can handle varying process conditions, but the pressure drop of the flow restrictor reduces by a factor of 50 or more
Solution Approach 1:
The system dynamically adapts to varying outlet pressures by using two transducers that can independently measure pressure at different operating points. The upstream absolute pressure transducer maintains accurate measurements across the full dynamic range from vacuum to atmospheric pressure, while the downstream differential pressure transducer compensates for local pressure variations, ensuring the system remains adaptable throughout the entire outlet pressure range without losing pressure drop measurement capability
3Device complexity
If a single downstream pressure sensor is used, then the device structure is simpler, but the flow sensor signal is reduced by potentially a factor of 50 or more at low setpoints
Solution Approach 1:
The pressure sensing function is segmented into two separate transducers positioned at different locations: upstream absolute pressure and downstream differential pressure. This segmentation prevents signal degradation by ensuring that the primary flow measurement reference (upstream absolute pressure) is not contaminated by downstream pressure variations, thereby maintaining strong signal levels even at low setpoints while accepting increased device complexity
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 enhances the accuracy and repeatability of mass flow rate measurements at low setpoints by minimizing the impact of pressure variations, providing more precise control and stability across a wide range of pressures.
Implementation Method 1
an absolute pressure transducer having an absolute pressure membrane and exposed to absolute pressure in the third cavity
Implementation Method 2
a differential pressure transducer having a first differential pressure membrane and a second differential pressure membrane and exposed to differential pressure between the third cavity and the second cavity
Implementation Method 3
a laminar flow element adjacent to the first cavity and the second cavity; wherein the first cavity is upstream of the laminar flow element and the second cavity is downstream of the laminar flow element
Data Source
AI summary
Included are mass flow controllers and methods of use. An example mass flow controller comprises a flow pathway through the mass flow controller; the flow pathway comprising a first cavity and a second cavity. The mass flow controller further comprises a laminar flow element. The mass flow controller additionally comprises a combination absolute and differential pressure transducer assembly comprising: a third cavity in fluid communication with the first cavity, an absolute pressure transducer exposed to absolute pressure in the third cavity, and a differential pressure transducer exposed to differential pressure between the third cavity and the second cavity. The mass flow controller also comprises a flow control valve assembly downstream of the laminar flow element and the combination absolute and differential pressure transducer assembly.