A rotameter consists of a tapered tube, typically made of glass with a ‘float’. The float moves in a tapered Glass Tube because of the drag force of the flow and buoyancy force of the fluid and it is pulled down because of the gravitational force acting on the float and viscosity of the fluid. The float attends a steady condition when the upwards and downwards forces acting on the float gets balanced. In short, the forces attends the state of equilibrium. Drag force for a given fluid and float cross section is a function of flow speed squared only, see drag equation.

A higher volumetric flow rate through a given area results in increase in flow speed and drag force, so the float will be pushed upwards. However, as the inside of the rotameter is cone shaped (widens), the area around the float through which the medium flows increases, the flow speed and drag force decrease until there is mechanical equilibrium with the float’s weight.

Floats are made in many different shapes, with spheres, ellipsoids and conical being the most common. Involutes shaped cuts on the periphery rotate the float along its vertical axis. This rotation prominently indicates the float position and also confirms that the float is not stuck in the tapered glass tube. Readings are usually taken at the top of the widest part of the float; the center for an ellipsoid, or an engraved marking on the float. Some manufacturers use a different standard.

Note that the “float” does not actually float in the fluid: it has to have a higher density than the fluid, otherwise it will float to the top even if there is no flow.