Rotameter is a device that measures the flow rate of liquid or gas in a closed tube. A Rotameter consists of a tapered tube, typically made of glass, with a float inside that is pushed up by flow and pulled down by gravity. At a higher flow rate, more area (between the float and the tube) is needed to accommodate the flow, so the float rises. Floats are made in many different shapes, with spheres and spherical ellipses the most common. The float is shaped so that it rotates axially as the fluid passes. This indicates if the float is stuck, since it will only rotate if it is free.