Essentially a hydraulic modeler designed to increase, upstream of the restriction, the fluid head during its outflow. The Palmer-Bowlus upstream fluid head, increases or decreases in function of the fluid quantity flowing over it. The head measured by a level transmitter is then used to calculate the instantaneous flow rate value. The Palmer-Bowlus is mainly used in pipes or ducts accessible through the manholes. The easy installation, and the contained installation cost, are the reason for the applications number increasing of this system to measure the flow rate.