Flow Measurement

What is a rotameter and how does it work

What is a rotameter and how does it work

A rotameter has rotating vanes and that’s how it got its name. It can be used to measure the flow of the fluid. The rota-meter has a metal float and a conical glass tube. So the float would rise according to the rate of flow through the tube. Rotameter has a tapered tube and its diameter is small at the bottom, the float in the rotameter can freely move and this float is situated at the bottom of the tube so when the fluid flows the float will rise. The float would only rise until its weight is balanced by the up-thrust of the fluid. So the flow rate is indicated by the position of the float. When the flow increases then the pressure drop in the tube increases so when the pressure drop is sufficient the float would rise and the amount of flow will be indicated.  The rotameter works by the principle of the upthrust force exerted by the fluid and force of gravity. In the variable area flowmeter, the drop rate is constant and the flow rate is the function of the area of constriction. The fluid density must be enough for the movement of the float otherwise the reading won’t be accurate.

How rotameters are constructed

Rotameter has a tapered tube with a small bottom diameter, the tube is designed to give a constant drop across the float by varying its cross-sectional area. The conical tube would be made up of glass, metal tubes, or plastic tubes, and each of them is used for separate applications. The glass type is used for gas and liquid, metal tubes are used for opaque liquids. The rotameter float could be constructed of metals of various densities it could be lead to aluminum, glass, and plastic types are also used. Mostly stainless steel floats are used. The float shape and proportion will be different for different applications.

How to calibrate a rotameter

At first, the storage tank must be filled, then open the bypass valve. Initially shut the valve of rotameter, now start the flow through rotameter. Use a stopwatch when the water starts to fill in the measuring tank. Increase the flow rate gently and repeat the process for different flow rates of water at different float position.

What are the types of rotameters

1 Glass tube rotameter

2.Metal tube rotameter

3.Plastic tube rotameters

4. Purgemeters

Glass tube rotameter

In this type, the tube is made up of glass, and the float will be visible. It is a simple rotameter and has high repeatability. The float is normally visible in the tube and the meter would show the readings on the glass surface.

Metal tube rotameter

It is also called armored meters and it is used in places were glass tube type is not applicable like for certain applications the temperature and pressure exceed the glass tube limit. The flow rate is shown by a pointer with the help of a magnet inside the float, the pointer is linked to the magnet. This type doesn’t need any external power.

Plastic tube

This type of rotameter is very cheap, and it can be used for a very wide variety of fluid measurements. This type of rotameter is widely used for many industrial applications.


This type is used for small flow measurements and it is used to measure liquids and gases and this type of rotameters is capable to accurately measure low flow rates.

How to install a rotameter

  • Before the installation of the rotameter check whether the device is damaged or not.
  • The float must be checked so that we could know it can freely move
  • During the installation, the rotameter valve should be partially open
  • Before installing the rotameter all the lines that are to be connected to the rotameter must be cleaned
  • It is required to install a shut-off valve or isolation valve to the system to allow the removal of meter for cleaning or servicing
  • The meter must be vertically mounted and it must be mounted properly otherwise it would badly affect the accuracy
  • The base mounting and panel mounting must be done properly

How to select a rotameter

  • Accuracy
  • It must be selected according to the minimum and maximum flow rate for the flow meter
  • Minimum and maximum process temperature
  • Different types of floats are available, so the float must be chosen according to the application
  • It must be used in applications where the movement of the float can be seen
  • Select the meter according to the fluid viscosity
  • Rotameters are available in different size so select the flow meter that suits the application
  • It must be selected according to the fluid temperature, pressure and specific gravity
  • Size of the pipe
  • Backpressure
  • Maximum process pressure

What are the characteristics of rotameter

  • It can be used to measure a wide variety of liquids
  • There are many types of tubes, float and fitting materials are available for different application
  • It can be used to meter liquid metals like mercury and liquid lead, inverted rotameters are used for this type of application because these metals are denser than the stainless steel float and the flow would be from top to bottom for this type.
  • Rotameter capacity can be altered by changing the float
  • It can be used to measure a higher flow rate

What are the advantages of rotameter

  • Visual index of the flow rate
  • It has a nearly linear scale
  • Constant head loss
  • No need for extra pipelines as the calming section is needed for stabilizing the flow
  • Low cost
  • No need for external power
  • Good rangeability
  • Compatible with electronic flow measurement devices
  • Repeatability
  • It can be used for small metering service

What are the disadvantages of rotameter

  • Impact of gravity
  • It must be installed vertically
  • It is mostly used for small pipes
  • Accuracy is low
  • It needs an upstream strainer
  • Its working is depended on gravity so that, it must be vertically placed and the right way up with the fluid flowing upward
  • Excessive wear could happen because of the continuous flow rate
  • Measurement error could happen because of leakage at very low flow rates

What are the applications of rotameter

  • Accurate level measurement
  • Purging for corrosive fluids
  • Continuous lubrication
  • Density measurement
  • Gas analyzers
  • Measurement and control of machinery
  • Control of gas burners
  • Industrial refrigeration flow control


post-graduate in Electronics & communication.

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