Liquid Flow Standards

The Fluid Metrology Group has four liquid flow standards spanning the flow range 0.002 L/s to 65 L/s with uncertainties less than 0.06 % at a 95 % confidence level. These standards are used to perform customer calibrations, including proficiency tests that check whether commercial calibration laboratories are achieving their claimed uncertainties. These standards are also research tools that NIST uses to improve flow metering techniques and to study the sensitivity of flow meters to liquid properties.  The liquid flow standards (LFSs) have capacities of 0.1 L/s, 2.5 L/s, 15 L/s, and 65 L/s. The two smaller standards are bi-directional piston provers that function much like syringe pumps. Flow is calculated by measuring the time-dependent displacement of a piston sweeping liquid from a cylinder of known diameter. The piston provers are filled with mixtures of propylene glycol and water that have kinematic viscosities ranging from 1 cSt to 50 cSt, depending on the mixing ratio. This wide viscosity range was exploited to validate a physical model for turbine meters.  The model enables NIST-calibrated flow meters to accurately measure flows of diverse liquids with known kinematic viscosities.  Normally, the piston provers are filled with a 1.2 cSt mixture that mimics the properties of jet fuel. The two larger flow standards (15 L/s, and 65 L/s) use the dynamic gravimetric method to measure water flows: the flow is the rate of change of buoyancy-corrected mass of water collected in a tank on a weigh scale. The 65 L/s LFS can also perform the static gravimetric method to calculate flow using a diverter design that minimizes timing errors. The 65 L/s standard was used to validate the simpler dynamic gravimetric approach.