Micro-Flow Calibration Facility at NIST

The Fluid Metrology Group (FMG) at the National Institute of Standards and Technology (NIST) is developing a primary, dynamic gravimetric liquid flow standard for use in the range 100 nL/min to 1 mL/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 beaker’s contents gives the mass flow. The FMG’s 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). 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. We present an uncertainty analysis for the first iteration of the flow standard. A preliminary calibration of a commercial flow meter was within 3 % or better down to 2 L/min of the unofficial calibrations conducted by the Danish and Swiss National Metrology Institutes.