This paper recounts the evolution of a design for a primary mass flow standard (PMFS) targeted at measuring flows down to the 0.0012 grams per minute*1level of gas with a target accuracy of 0.1% of reading. It describes the basic concept of direct mass-flow measurement, identifies external factors that can corrupt the mass measurement and recounts system-design revisions intended to reduce the corrupting influences. A formal uncertainty analysis of the PMFS is not presented as the project is still evolving. However the core task of achieving the needed PMFS stability and noise levels at low-flow levels is discussed for a series of four evolving prototypes, each showing continuing performance improvements.
*1: 0.0012 Grams per minute is equals 1 standard cubic centimeter, 1 sccm, of N2. A sccm is a mass flow term equal to one cubic centimeter per minute of a gas at 0 °C and 1 atm.