1. A method, comprising: deriving calibration data representing relationships between heights and densities of fluids within a bell nipple, and flow rates of the fluids through a flow line branched off the bell nipple; measuring pressures of a fluid within the bell nipple with at least two pressure transducers vertically spaced apart along the bell nipple horizontally adjacent to the flow line; calculating height and density of the fluid within the bell nipple based on the measured pressures; and calculating volumetric flow rate and mass flow rate of the fluid through the flow line based on the calibration data and the calculated height and density.