Updated: Nov 29, 2020

In this article, we will learn about variable area flowmeter extensively used in the process industries to measure the flowrate of gases and liquids.

As the name indicates that area is variable and the same you can look into the below image.

In the differential head flow meters (DP Transmitter), the flow restriction is of fixed size and the pressure difference across it changes with the flow rate, where the differential pressure in the variable area flowmeters (Rotameter) is made constant and the size of the restriction varies.

Therefore the flow rate is a function of the area of the opening through which the flow is passing. Rotameters are the most widely used variable area flowmeters in industrial flow measurement.

Basically, there are four types of Rotameter,

1. Glass tube rotameters

2. Metal tube rotameters

3. Bypass rotameters

4. Purge rotameter

Let, understand each in brief.


It works on the principle of variable area. The float moves freely up and down in the tapered borosilicate glass tube with fluid flow from bottom to top.

The float takes up a position where the buoyancy force and the float weight are balanced in proportion to flow rate.

The vertical position of the float as indicated by the scale is a measure of the flow rate. The float of the rotameter adjusts the size of the area by rising and falling in the tapered tube.

Depending on the rate of flow, the float takes a position in the tube that increases or decreases the size of the area, and thus keeps the differential pressure constant.

Rotameter is always mounted vertically in the pipeline. The inlet of the rotameter is at bottom and outlet at the top.

The rotameter is subject to error due to changes in the density or the viscosity of the flowing fluid. To overcome small changes in fluid density, the float should be of materials twice as dense as the flowing fluid.


The float moves freely up and down in the tapered measuring tube with fluid flow from bottom to top.

The vertical position of the float is a measure of the flow rate. The scales can be calibrated in mass or volume flow units.

A coupling magnet is fitted on the float to transmit float position from the measuring type to follow up magnet, which is attached to the indicating assembly.

Thus the pointer moves with the position of the float. This metal tube rotameter can be extended as a transmitter and this work on the principle of resistance to current converter using special servo potentiometer. Linearization is achieved by electronic circuitry.


Bypass Rotameters are suitable for the flow rate measurement in 2” NB or higher size pipelines.

The complete assembly consists of carrier rings, bypass line and a rotameter as an indicator. This is basically an inline rotameter either glass tube or a metal tube placed in a bypass piping.

The only difference is the range orifice placed in rotameter at the inlet. This range orifice is designed in such a way that the differential pressure through the bypass piping is equivalent to differential pressure across the main pipeline orifice.


Purge rotameters are used to measure the flow of liquids and gases in small quantities.

These rotameters are particularly well- suited for purging applications such as purging orifice legs, instrument cases, pump seals and other components to prevent corrosion or slurry sedimentation.

Other specialized applications include liquid level and specific gravity indication. These rotameters can also be used for the measurement and control of a variety of liquids and gases.

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