Mass Flow Controller Pressure Sensing for Low-Flow Stability
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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 absolute and differential pressure transducer assembly, 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 signal, ensuring accurate mass flow rate control across varying pressures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If pressure-based mass flow meters use a defined flow restriction to create a pressure drop for measuring mass flow rate, then the mass flow rate can be calculated using temperature and pressure measurements, but large downstream pressure variations may reduce the low-flow pressure drop of the flow restrictor by as much as a factor of 50 or more, degrading the flow sensor signal-to-noise ratio and stability
Solution Approach 1:
The patent segments the pressure measurement function into two separate measurement locations: an upstream pressure sensor that measures pressure before the flow restrictor and a downstream pressure sensor that measures pressure after the flow restrictor. This segmentation allows the system to independently measure the pressure drop across the restrictor while compensating for large downstream pressure variations, thereby maintaining signal stability and measurement accuracy simultaneously.
Solution Approach 2:
The patent introduces an intermediary computational process that uses the upstream pressure measurement as a reference to compensate for downstream pressure variations. By calculating the pressure drop as the difference between upstream and downstream pressures, the system effectively uses the upstream pressure as an intermediary reference point, isolating the measurement from the harmful effect of large downstream pressure excursions.
2Adaptability or versatility
If absolute pressure sensors with downstream laminar flow elements are used to handle large downstream pressure variations, then the controller can operate across a wider pressure range, but the upward swing from near zero outlet pressure to atmospheric outlet pressure reduces the low-flow pressure drop by as much as a factor of 50 or more, causing signal-to-noise ratio degradation
Solution Approach 1:
The patent segments the measurement system into upstream and downstream components with dedicated pressure sensors at each location. This segmentation enables the system to maintain adaptability across wide pressure ranges while preserving measurement precision by independently characterizing the pressure conditions at both ends of the flow restrictor.
Solution Approach 2:
Instead of placing the flow sensor downstream where it is vulnerable to pressure variations, the patent inverts the measurement approach by placing the primary pressure measurement upstream of the flow restrictor. This inversion allows the system to reference measurements to a stable upstream pressure point, thereby maintaining signal quality across varying downstream pressures.
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, using differential pressure transducers for improved precision and stability.
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
Pressure-based mass flow meters use a defined flow restriction to create a pressure drop responsive to the flow to be measured
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.