Center for Measurement Standards (CMS) developed a laminar flow element (LFE) type flow meter as a transfer standard. This LFE consists of a single straight glass capillary or multiple straight glass capillaries connected in parallel. Two gauges and one thermometer measured the inlet/outlet pressure and inlet temperature, respectively, and the differential pressure was restricted from 2 kPa to 100 kPa. The glass capillaries were manufactured by laser machining, resulting in consistent inner diameter and straight flow path. Characteristics of glass material also prevented the capillary from bending during installation. By means of regression, the turndown ratio of this LFE flow meter could be higher than 20 and the residual would be still within 0.11 %. The reproducibility within 0.03 % indicated that this LFE flow meter can be used as a transfer standard. Measurement with dry air demonstrated that these four LFE meter could span the flow rate at (0.8 to 986) umol/s within the deviation of ±0.15 %. (1 umol/s could be converted as 1.3 cm3/min at 0 °C and 101.325 kPa) Additional measurement with nitrogen demonstrated the feasibility of measurement with multiple gases.
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