The Fluid Metrology Group (FMG) at NIST is developing a primary, dynamic gravimetric liquid flow standard for use in the range 1 mL/min to 100 nL/min (and eventually lower). An elevated reservoir of water with a pressure head of a few centimeters provides a flow to the meter under test and the discharged water from it flows to a micro-balance. The flow is collected in a beaker which is weighed at intervals while it fills. The time-rate-of-change of the buoyancy-corrected mass of the beakers contents gives the mass flow. The FMGs implementation of the flow standard will allow accurate measurements of non-steady flows and heterogeneous flows (liquids with cells, proteins, and other soluble and non-soluble components). The dynamic-flow feature is vital, given the difficulty of maintaining steady micro flows. Evaporation of the water from the beaker is a significant effect and must be measured or controlled accurately. Intermittent liquid wetting or patchy wettability of the pipette by the water in the beaker is also a significant effect. At present we are exploring two techniques: (1) using an oil film to limit evaporation, and (2) using a porous glass element to control capillary forces and evaporation losses. A preliminary calibration of a commercial flow meter was within 5% of the unofficial calibrations conducted by the Danish and Swiss National Metrology Institutes.
9th International Symposium on Fluid Flow Measurement (ISFFM)
April 14-17, 2015
micro-flow, liquid flow, flow calibration, flow standards