Rotameter is a flow measuring device commonly used in chemical and related industries. It is a simple piece of equipment that consists of a tapered tube and float. The float is placed inside the tube and nets are placed at both ends of the tube. This arrangement can be connected with the pipeline with the flanged or threaded connections. Rotameters are always installed vertically in the pipelines. A scale is marked on the tube to read the values of flow rate directly.

Read also: Flow measuring devices


In no flow condition, the float rests at the bottom of the rotameter. When fluid passes through the rotameter, the flow of fluid through the rotameter causes the float to move along with the fluid.

There are two primary forces involved,

  1. An upward drag force due to the motion of the fluid in the upward direction
  2. A downward force due to gravity is due to the weight of the float itself.

When these forces are balanced then the float moves to a particular location in the tube and it stays right there because it has achieved dynamic equilibrium. The scale on the rotameter will show the current flow reading.

The nets attached to either side of the rotameter ensure that the float does not get carried away in the pipeline due to either high flow or no flow condition. If it happens then it may get stuck near a valve in the pipeline and cause blockage or enter equipment down the line and cause it to malfunction.

Types of Rotameter

  1. Glass Tube Flowmeters
  2. Armoured Purgemeter
  3. Flanged Armoured Rotameter

Advantages of Rotameter

  • It is economical to install and maintain.
  • It has a linear scale over a large range of flow rates.
  • The pressure drop across the float is constant.
  • Rotameters are very versatile, they can be easily sized or their use can be changed for different systems.

Disadvantages of Rotameter

  • It requires a certain minimum magnitude of the flow rate of the fluid.
  • If the opaque fluid is used then the scale is not properly visible.
  • It cannot be installed in a horizontal position.
  • If the pressure of the fluid is very high then glass tubes may be subject to breakage.
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