Rotameter

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.

Due to its use of gravity, a rotameter must always be vertically oriented and right way up, with the fluid flowing upward.
Due to its reliance on the ability of the fluid or gas to displace the float, graduations on a given rotameter will only be accurate for a given substance at a given temperature. The main property of importance is the density of the fluid; however, viscosity may also be significant. Floats are ideally designed to be insensitive to viscosity; however, this is seldom verifiable from manufacturers' specifications. Either separate rotameters for different densities and viscosities may be used, or multiple scales on the same rotameter can be used.
Rotameters normally require the use of glass (or other transparent material), otherwise the user cannot see the float. This limits their use in many industries however manufacturers have developed Metal Tube Rotameters wherein the floats are magnetic, no where glass parts are used. The position of the float is sensed by a rotating magnetic wheel or oscillating magnetic arm and is indicated on a dial.
Rotameters are not easily adapted for reading by machine; although magnetic floats that drive a follower outside the tube are available. Nowadays transmitters giving 4-20 mA out-puts are also available.