Aster PP Flange Online Rotameters (BSTD)-The Rotameter's operation is based on the variable area principle: fluid flow raises a float in a tapered tube, increasing the area for passage of the fluid. The greater the flow, the higher the float is raised. The height of the float is directly proportional to the flow rate. The float reaches the stable position in the tube when the upward force entered by the flowing liquid equals to the downwards gravitational force entered by the weight of float.