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Title: Thermal Capillary Tube Mass Flow Sensing For Low Flow Metrology Transfer Standards
Author: Thomas O. Maginnis
Source: 2009 International Symposium on Fluid Flow Measurement
Year Published: 2009
Abstract: Flow calibration requires 1) the ability to establish and maintain a precisely constant flow at any desired value 2) the ability to measure any such constant flow with high accuracy, relative to a primary flow standard. Much effort has been devoted to part 2) developing transfer standards with the necessary accuracy, but often the calibration result is limited instead by part 1) the inability to maintain sufficiently constant steady flows, as evidenced by the noise on the flow signal. Thus flow measurement and flow control are synergistic arts, and high proficiency in both is required for superior flow calibration. In the 35 years since their commercialization, thermal mass flow controllers have become the dominant flow measurement technology for gas flow measurement and control in the semiconductor industry, but are little known or understood outside that venue. In flow range, the technology covers a span from 3 SCCM full scale to more than 100 LPM. In the lowest flow ranges, below 100 sccm , no other continuous flow measurement technology is comparable in cost, accuracy, and flow resolution. Despite the clear commercial dominance of this flow sensing technology for the low end of the gas flow range, it remains relatively unexploited for flow metrology applications. Why? Existing semiconductor industry flow controller designs are specialized for small size, rapid response to a control setpoint change, low cost, moderate accuracy (1%), ultra-clean gas delivery and ability to operate with the most corrosive gases. Successful design for this commercial environment involves design tradeoffs that degrade flow turndown, flow resolution, and the repeatability required for a metrology grade transfer standard flow controller that could facilitate high accuracy automated flow calibrations at low flows. Also, the absence of an adequate published physical theory of operation of the thermal sensing technology severely limits the ability of metrology institutes to design or develop such transfer standards for themselves. This paper will review the potential of the thermal capillary tube flow sensing technology for high turndown, high performance gas flow controllers as calibration transfer standards in low to moderate gas flow ranges (100 SLPM).

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