Rotameter is used to measure the volumetric flow rate of fluid in a closed tube. it is direct flow measuring device requires no external power source. Rotameter can be installed in vertical position only. It belongs to a class of meters called variable-area flow meters, which measure flow rate by allowing the cross-sectional area the fluid travels through to vary, causing a measurable effect.

Rotameter Working:-

A higher volumetric flow rate through a given area increases 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.


It is having tapered metering tube ,float with scale, guide wire and flange.

Metering tube: 

It is Tapered glass or metal tube set vertically in pipe. The metering tube is made of glass for low temp. & Pressure fluids. For high temp. & Pressure Metal tubes are used .In metal tubes the linear motion of the floats is converted into a rotary motion for the movement of a pointer on a calibrated scale. The float movement is converted in to proportional electrical signal also.



Rotameter floats are usually made of metals which have density higher than liquid .

Rotameter scale: 

Scale is marked on the glass tube or it is mounted close to the metering tube scale can be direct reading type or percentage scale is used.

Type of rotameter | Rotameter types:

Direct viewing rotamter :-

In the Direct viewing rotameter float is viewed directly & its displacement is measured on the scale calibrated in fluid flow rate.This type is used for free flowing & clear liquid.


Magnetically coupled rotameter:

In this type float movement is magnetically Coupled to the indicating, recording or controlling element. This is also used for electrical or pneumatic transmission of float movement to the remote point.


Advantages of Rotameter

  1. Minimum pressure drop.
  2. Range ability is high.
  3. Low in cost.
  4. Suitable for small flow rates.
  5. It can simply equipped with alarms and switches or any transmitting devices.
  6. It can also measures the flow rate of corrosive fluids.
  7. There is an availability of viscosity-immune floats.
  8. It can be used in light slurry services.

Disadvantages of Rota meter:

  1. Difficult to handle the glass tube type Rota meter.
  2. Must be mounted vertically.
  3. Not suitable for pulsating services.
  4. Limited to small pipe services.
  5. Limited to low temperatures.
  6. Accuracy is = ± 1/2 to 10%.
  7. Requires in-line mounting.

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