Rotameter:- mechanism and handling


Rotameter is a type of flowmeter which is also known as a variable area flowmeter.

Applications of rotameter

It is used to measure the velocity of air and liquid (water, oil, etc.) in industrial sectors. Besides that, it is also used in medical instruments such as oxygen concentrators, oxygen cylinders, and anesthesia machines.

Working principle

rotameter:- working mechanism

It consists of an outer vertical tapered glass tube with a narrow base and broad top. Since there is variation in the cross-sectional area of the tube, it is known as a variable area flowmeter. Inside the tube consists of a metering float. The glass tube consists of readings built in it.

When there is no flow of air through the rotameter, the float lies at the base of the tube. When the air flows to the tube through the base, it pushes the float upward. The displacement covered by the float from the base is directly proportional to the pressure exerted by the fluid to the float.

Three forces act on the float. They are as follows.

Force Direction of force Type of force
Weight (w) Downward Constant
Buoyancy (b) Upward Constant
Drag/ Blow force (d) Upward Variable


So, w = b + d (at equilibrium)

When the air pressure increases, the float rises. As the cross-sectional area of the glass tube, is more at the top than at the bottom, the air moving upward begins to escape from the sideways of the float. So, even on increasing the air pressure, the variable drag force comes back to its original stage. Thus the equilibrium is again maintained.

You will find labeling in the glass tube from 1 to 12 or more (based on the model) to indicate the flow rate of gas which is denoted by the rise of metering float in the rotameter.

Handling process

  • Since the rotameter is made up of a glass tube, it might break if you do not handle it properly.
  • You need to avoid parallax error while taking the reading.


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