What is Rotameter? Principle, Classification, Advantages, and Disadvantages.

Description of Rotameter

Rotameter is also called a variable area flowmeter. It is a type of flowmeter that measures flow rate or volumetric flow rate. It is defined as the amount of flow per unit of time that circulates through a section of a tube.

The rotameter is a conical tube, usually made of glass, metal, or plastic. A piece inside is called a float that moves vertically up and down depending on the flow of the fluid.

Rotameters are used to measure flows of both liquids and gases that work with a constant pressure drop.

They are based on the measurement of the vertical displacement of a “floating element”, whose equilibrium position depends on the circulating flow. It simultaneously leads to a change in the area of the fluid passage hole, in such a way that the pressure difference acting on the moving element remains practically constant.

Working Principle

The working principle of rotameters is based on the balance of forces acting on the float.

The flow is directed from the bottom to the top through the conical tube of the rotameter. By varying the flow rate, the float rises or falls, varying the annular area between the float and the tube until a balance is achieved between upward forces of the fluid and the weight of the float. The float remains stable at a height that corresponds to a certain value of circulating flow.

Float Material

Floats are available in a wide variety of shapes, sizes, and materials to meet the requirements of many applications. Large floats are made of corrosion-resistant materials.

The density of the float can be changed to adjust the meter range and resist corrosion from the measured fluid. PVC, Tantalum, and Teflon are available, SS 316 is the most commonly used material.

Basic Equation for Volumetric flow measurement

Q = kA √ gh

Q = Volumetric flow

k = a constant

A = Annual area between float and tube wall.

g = Acceleration due to gravity

h = Pressure differential

Tube Material

Plastic rotameters

They are inexpensive and last longer than other types of rotameters but are not useful for large flows.

Metal rotameter

They are used for hot, alkali solutions, slurries, hydrochloric acids, etc. It is used where general-purpose rotameters cannot be used. Suitable in applications where temperature and pressure exceed glass rotameters. With the help of a magnetic bar plus metal float, pointer movement can be arranged as can be visualized the flow.

Glass Rotameters

This type of rotameter metering tube is made of borosilicate glass. This type of rotameter lasts longer and allows visualization of the interior of the tube.

Parts of a Rotameter


It is a piece that is inside the rotameter that marks the value of the fluid flow. The float moves vertically through the rotameter depending on the flow rate of the fluid, and the higher the flow rate.

Floats are made of various materials such as Tantalum Hastelloy C, Monel, Teflon, and PVC. SS315 is common.


The value of the measured flow is indicated on it. The limit of the float will mark the line of the scale that corresponds to the flow of the fluid.


The part inside the rotameter through which the float moves. It serves to ensure that the float moves vertically.

Measuring tube

The tube through which the float moves, consists of the body of the rotameter.

Lower Limit

This is the lower limit of the float. When no flow is being measured, the float remains at rest in that position.

Upper limit

It is the upper limit of the float. It is recommended that the float does not reach the upper limit to avoid damage

Rotameter mounting options

  • Insertion flow meters
  • In-line flanged flow meters
  • In-line threaded flow meters


Tapered tube with a central guide

A common type in which the position of the float is indicated on an outer scale by the direct movement of an indicator on an electrical drive is shown in the figure. The inductance bridge is often used for the drive.

Tapered plug hole

With flow against the smaller diameter of the plug, it causes the plug to lift until the upward force of the fluid balances the downward force of gravity.

Tapered plug with free float

This type of rotameter uses a float of upwardly increasing cross-section and is provided with inclined slots to induce rotation and center the float in the tapered glass tube uniformly and is positioned vertically. Upward flow causes the float to seek its equilibrium height which is proportional to the flow.

Types of Rotameters

There are several kinds of rotameters depending on their use.

Purge Rotameter

Rotameter for fluids with small flow rates and densities.

Bypass rotameter

The cost of the rotameter increases with size, large-size diameter relatively costs high.

A bypass rotameter is helpful to measure flows in large-sized pipelines. Instead of using a pipeline-sized rotameter, an office or pitot tube is introduced in the pipeline.  This reduces the process pressure and small pipeline connected are diverted to an inexpensive rotameter.

In the main line, differential pressure is produced by the orifice causing corresponding flow through the rotameter. The main advantage of a bypass rotameter is that the flow measurement can be done at a low cost.  

Rotameter with electrical transducers

This type of rotameter uses a magnetic system to follow the changes generated in the flow input of the tube.

Rotameter with pneumatic transducers

This type of rotameter makes measurements through a magnetic mechanism.

Advantages of the Rotameter

Electrical power is not required to operate the rotameter. Rotameter operated on mechanical principles, they do not require any external energy to operate.

A very easy-to-use laboratory instrument.

The rotameter is an inexpensive device compared to other flow meters.

The measurement scale of the rotameter is practically linear, which makes it easy to read.

Disadvantages of Rotameters

The scale of a rotameter will only be effective for a certain fluid. If the fluid is changed, the rotameter will also require to be changed or recalibrated.

To make the measurement, a rotameter must be oriented completely vertically. This is because the operation of the rotameter is based on the force of gravity exerted on the float.

It is easy to make a mistake in reading the measurement since it is difficult for the float to stay completely still, it moves slightly up and down.

Flow measurement can be read manually, and cannot be automated using a machine.

If the fluid is very dark, reading the flow becomes a difficult task.

Applications of Rotameters

They are widely used in different areas such as

  • Chemical industries
  • Pharmaceutical industries
  • Laboratories
  • Food industries
  • Petrochemical industries

Leave a Reply