Some national laboratories of metrology carry out the continuous expansion method to ensure measurements traceability for low absolute pressure. This method implies to generate a reference molar gas flow rate ranging from 4 × 10-12 mol·s-1 to 4 × 10-7 mol·s-1. To improve calibrations uncertainties for pressures below 10-3 Pa, Laboratoire national de métrologie et d’essais (LNE) had to initially design a primary reference for gas flow measurement in the aforementioned range, with an expected expanded uncertainty better than 1%. With the gas flowmeter, it will also be possible to calibrate reference leaks, which is an usual activity of the pressure department of the LNE, for the leaks with reference to vacuum between 4 × 10-12 mol·s-1 and 4 × 10-7mol·s-1 (from 1×10-8 Pa·m3·s-1 to 1×10-3 Pa·m3·s-1) and leaks of the cooling agent R-134a with reference to atmospheric pressure between 3×10-9 mol·s-1 and 2 × 10-8 mol·s-1 (from 10 g per year to 60 g·per year). The calibration results of a capillary leak are compared with the pressure rise calibration method, usually performed for this kind of instrument. An uncertainty budget on the gas flow measurement is established for this calibration and applied to the continuous expansion method.

Key words

constant pressure flowmeter
gas flow rate
continuous expansion method
low absolute pressure
reference leak