“A rotameter is a device that measures the flow rate of liquid or gas in a closed tube”.
A rotameter consists of a Conical Tube, typically made of glass with a ‘float’, actually a shaped weight, inside that is pushed up by the Drag Force of the flow and pulled down by Gravity. The position of the float indicates the flow rate on a marked scale.

Working of Rotameter:

A rotameter is basically constructed of a vertically oriented glass tapered tube and a metering float which is free to move within the tube. When there is no fluid flow, the float rests freely at the bottom of the tube. When liquid enters the bottom of the tube, the metering float begins to rise.
The position of the float changes directly with the flow rate. The accurate position of the float is at the point where the differential pressure between the upper and lower surfaces balances the weight of the float. The greater the flow, the higher the float gets lifted.


  • A rotameter requires no external power or fuel, it uses only the

inherent properties of the fluid, along with gravity, to measure

flow rate.

  • A rotameter is also a relatively simple device that can be mass

manufactured out of cheap materials, allowing for its

widespread use.

  • Since the area of the flow passage increases as the float moves

up the tube, the scale is approximately linear.

Simple and easy to use.

Railway rotameter

The New South Wales Government Railways constructed in 1903 a device for measuring the length of its lines of railway. That authority named the machine a Rotameter. It consisted of a four-wheel trolley with an additional large fifth wheel which traveled along the running surface of the rail. Its last recorded use was in the 1920s.



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