Rotameter is called as a variable area flow meter because in rotameter the area is varied i.e, bottom area is small and it increases towards top, to maintain steady pressure difference. When there is no flow through the Rota meter, the float rests at the bottom of metering tube. When fluid enters the tube, the float moves up and the flow area increases. The float moves up until the lifting force produced by flow and gravitational force acting on the float reach an equilibrium. Thus, the differential pressure and lifting force increase with rise in flow rate. A calibrated scale is printed on the tube. With the help of float position and calibrated scale, we can measure the flow rate.