Integrating Analytical and Numerical Approaches to Quantify Flow Work in Rate-of-Rise Gas Flow Standards

Published

February 20, 2026

Author(s)

Max Low, Aaron Johnson, Pragya Natarajan, Jodie Pope

Abstract

Flow work makes it challenging to directly measure the gas temperature in rate-of-rise (RoR) standard collection tanks during filling. Also known as the heat of compression, flow work produces a transient, spatially non-uniform temperature profile that probes have difficulty measuring. Knowing the average gas temperature during filling is necessary for the accuracy of mass flow rate calculation in RoR standards, and so overcoming the flow work issue is vital. To address this challenge, the Fluid Metrology Group at the National Institute of Standards and Technology (NIST) has developed an analytic thermal model to infer the gas temperature in the tank during filling rather than measure it directly. This model shows how to minimize the change in gas temperature due to flow work in a cylindrical tank and was used to guide design for constructing NIST's Semiconductor Low Flow Standard (SLowFlowS). The standard has been experimentally validated to work within the range of 0.01 sccm to 1000 sccm, with expanded uncertainties in flow rates as low as 0.06%. However, the thermal model relies on many assumptions that do not hold for flow rates outside of the range of 0.01 sccm to 1000 sccm, alternative collection tank geometries, or gases with exotic thermophysical properties. It is impractical for NIST to experimentally test all these edge cases; therefore, we have performed computational fluid dynamics (CFD) to both assess the validity of assumptions in the analytical model and to extend the capability of SLowFlowS. These validated CFD models provide a physics-based framework for optimizing future RoR standards and quantifying the influence of flow work across a wide range of operating conditionsHowever, the thermal model relies on many assumptions that do not hold for flow rates outside of the range of 0.01 SCCM to 1000 SCCM, alternative collection tank geometries, or gases with exotic thermophysical properties. It is impractical for NIST to experimentally test all these edge cases; therefore, we have performed computational fluid dynamics (CFD) using COMSOL Multiphysics to both assess the validity of assumptions made and to extend predictive capabilities for SLowFlowS. These validated CFD models provide a physics-based framework for optimizing future RoR standards and quantifying the influence of flow work across a wide range of operating conditions.

Conference Dates

May 17-20, 2026

Conference Location

Nara, JP

Conference Title

Proceedings of the International Symposium on Flow Measurement (FLOMEKO 2026)

Pub Type

Conferences

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Citation

Low, M. , Johnson, A. , Natarajan, P. and Pope, J. (2026), Integrating Analytical and Numerical Approaches to Quantify Flow Work in Rate-of-Rise Gas Flow Standards, Proceedings of the International Symposium on Flow Measurement (FLOMEKO 2026), Nara, JP, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=961638 (Accessed February 21, 2026)