A system for ascertaining absolute fluid flow in a passage way comprises: a force beam that is inserted into a fluid; a carriage to support the beam, the carriage enabling the beam to move vertically in a fixed plane; one or more load cells incorporated with the beam to measure force applied to the beam by the fluid flow; a data recorder and processor for integrating the measurements and for running algorithms that convert the integrated measurements into a value of absolute flow and a controller to operate the system. In some embodiments the force beam may be light and neutrally buoyant in the fluid. Measurements are taken at pre-specified intervals while moving the beam in a vertical plane perpendicular to the direction of fluid flow. One application is the calibration of Winter-Kennedy piezometers in the barrels of large water turbines. A method of use is also provided.