Development of a gas micro-flow transfer standard

LNE has ability to calibrate gas micro-flow rates using the dilution method for nitrogen flow rates in the range from 2 µg/s to 200 µg/s or helium ones in the range 0.75–30 µg/s. In addition, a primary constant pressure flowmeter for leak rate measurements from 0.05 µg/s to 35 µg/s is also available. This equipment will be used to validate the dilution method below 30 µg/s. In order to compare these reference facilities, LNE is developing a micro-flow transfer standard (µFTS) in collaboration with ATEQ France, a manufacturer of control equipment for leak testing. The flowmeter consists mainly of an array of three stainless steel capillaries designed to cover the ranges from 0.035 µg/s to 0.35 µg/s, 0.35 µg/s to 3.5 µg/s and 3.5 µg/s to 35 µg/s for nitrogen (0.1–100 ml/h). A dynamic model of the µFTS determines the mass flow rate from the input pressure, the differential pressure of the capillary, the gas temperature, viscosity and density and the length and radius of the capillary. A comparison of both reference methods is carried out with the µFTS from 0.35 µg/s to 35 µg/s.