Laminar flow meter with straight glass capillary

The Center for Measurement Standards (CMS) developed a laminar flow meter (LFM) as a transfer standard. This LFM consists of a single, straight glass capillary or multiple straight glass capillaries connected in parallel. Two gauges and one thermometer measure the inlet/outlet pressure and inlet temperature, respectively, and the differential pressure was restricted to between 2 kPa and 100 kPa. The glass capillaries were manufactured by laser machining, which resulting in consistent inner diameter and straight flow path. Characteristics of the glass material also prevented the capillary from bending during installation. The LFMs were calibrated against the CMS piston prover primary standard and the calibration data were fitted using a theoretical LFM model and a 2nd order polynomial. The turndown ratio of this approach was higher than 20 and the residuals were within 0.11%. The reproducibility/repeatability within 0.03% indicated that this LFM could be used as a transfer standard. Measurement with dry air demonstrated that these four LFE meters could span flows from 0.8 to 986 µmol/s within the deviation of ± 0.15% (1µmol/s is 1.3 cm3/min at 0°C and 101.325 kPa). Additional measurement with nitrogen demonstrated the feasibility of measurement with multiple gases.