Weighing system with low evaporation error for liquid microflow down to 1 mg/min

Abstract A static weighing system for a liquid microflow calibration rig, whose minimum flow rate is 1 mg/min (equivalent to 1 μL/min for water), was designed and evaluated. The developed weighing system, which can collect over 10 g of water, has two features: (i) an automatic detachable device with a sealing and wiper mechanism in contact with a needle and (ii) a flexible air bag to contain the air from the weighing vessel. Having these two features, the weighing system succeeds in reducing the evaporation effect as well as preventing tension on the weighing scale. However, uncertainty due to the mass difference of adhering liquid on the outside surface of needle before needle insertion and after needle extraction should be considered. Theoretical and experimental analysis were conducted. The mass difference due to the adhering liquid on the needle was estimated to be smaller than 1/3 of the target standard uncertainty of mass measurement set at 0.1 mg, indicating that the detachable procedure was not a dominant uncertainty source for mass measurement. The evaporation rate during collection was experimentally estimated to be less than 0.001 mg/min, which corresponds to less than 0.1% for the lowest flow rate of 1 mg/min.