The buoyancy correction is a correction to determine the true mass of an object on a balance if not in vacuum. A rock weighs less under water, and the same goes for weights in air compared to vacuum. When performing buoyancy correction for nanoflow measurement, the buoyancy correction becomes extra complicated, as effects of offset masses, which normally are negligible, become more important. As a part of the MeDD project, the Danish Technological Institute has developed an equation to determine the true mass of dispensed water when using a gravimetric set-up for flow measurement. You can find more information here.