Accurate Volume Determination for Low Gas Flow Standards

Published

March 17, 2026

Author(s)

Jodie Pope, Christopher Crowley, Max Low, John Wright, Aaron Johnson

Abstract

The National Institute of Standards and Technology (NIST) has developed the Semiconductor Low Flow Standard (SLowFlowS), a new rate-of-rise (RoR) gas flow standard capable of measuring flows as low as 0.01 standard cm³/min. SLowFlowS incorporates two independent RoR standards, based on collection tanks of nominal volumes 2.5 liters and 0.3 liters, respectively. These standards have associated relative expanded uncertainties of 0.06 % and 0.12 % in flow, with the higher uncertainty attributed to the smaller volume. Accurately determining small volumes is challenging but essential for efficiently measuring gas flows at ultra-low flowrates. To independently determine the tank volumes, we employed a gas expansion technique using a reference volume determined gravimetrically with low uncertainty. Recognizing the limitations in characterizing very small volumes, we also compared the performance of both RoR standards by calibrating the same laminar flow elements (LFEs) across a wide range of flow rates and gas species. These comparisons revealed a consistent and repeatable calibration offset between the two standards. This persistent offset, confirmed across many LFEs, provides a basis for relating the smaller standard to the larger, more accurate one, allowing the lower uncertainty of the larger volume to be effectively transferred to the smaller standard through statistical correlation. By doing this, the relative expanded uncertainty in flow using the smaller collection tank is lowered from 0.12 % to 0.07 %.

Proceedings Title

Proceedings of FLOMEKO 2026 20th International Flow Measurement Conference

Conference Dates

May 17-20, 2026

Conference Location

Nara, JP

Conference Title

FLOMEKO 2026, The 20th International Flow Measurement Conference

Pub Type

Conferences

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Keywords

gas flow, standard, volume determination

Citation

Pope, J. , Crowley, C. , Low, M. , Wright, J. and Johnson, A. (2026), Accurate Volume Determination for Low Gas Flow Standards, Proceedings of FLOMEKO 2026 20th International Flow Measurement Conference, Nara, JP, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=961628 (Accessed February 21, 2026)