Rotameter is a one kind of variable area meter.

Blog content

  1. Diagram
  2. Features
  3. Advantages
  4. Disadvantages
  5. Calculations

Frequently asked questions

  1. Which forces work on Rotameter?
  2. What is Advantages and disadvantages of Rotameter?
  3. What are features of Rotameter?

A variable area meter is a meter that measures fluid flow by allowing the cross sectional area of the device to vary in response to the flow, causing some measurable effect that indicates the rate.

There are three forces acting on the float.
  1. Gravity force (Weight of the float)
  2. Drag force ( Due to fluid flow, in the direction of flow)
  3. Buoyant force ( Due to density difference between float and water)

Features of Rotameter

  •  In area meters "pressure drop" is constant.
  • The area through which the fluid flows varies with flow rate. The area is related through proper calibration to the flow rate.
  • These rotameters can be used for liquids and gases.

Rotameter consists of a gradually tapered tube, it is arranged in vertical position. The tube contains a float, which is used to indicate the flow of the fluid. 

This float will be suspended in the fluid while fluid flows from bottom of the tube to top portion. The entire fluid will flow through the annular space between the tube and float. 

The float is the measuring element. The tube is marked with the divisions and the reading of the meter is obtained from the scale reading at the reading edge of the float. Here to convert the reading to the flow rate a calibration sheet is needed.

For higher temperatures and pressure, where glass is not going to withstand, we use metalic tapered tubes. In metalic tubes, the float is not visible so we use a rod, which is called extension, which will be used as a indicator.

floats may be constructed using different types of materials from lead to aluminium or glass or plastic. Stainless steel floats are common. According to the purpose of the meter float shape will be selected.

The main property of importance is the density of the fluid; however, viscosity may also be significant. Floats are ideally designed to be insensitive to viscosity; however, this is seldom verifiable from manufacturers' specifications. 

Either separate rotameter for different densities and viscosity may be used, or multiple scales on the same rotameter can be used

Rotameter normally require the use of glass (or other transparent material), otherwise the user cannot see the float. 

This limits their use in many industries to benign fluids, such as water.

Rotameters are not easily adapted for reading by machine; although magnetic floats that drive a follower outside the tube are available.

Types of Rotameter

  1. Glass Tube Flowmeters
  2. Armoured Purgemeter
  3. Flanged Armoured Rotameter

1. Glass Tube Flowmeters:

They are the most common type of rotameters which are used extensively not only in industries but also in pilot plants and labs to measure flow rates of wide variety of fluids, both liquids and gases.

Glass Tube Flowmeters

2. Armoured Purgemeter:

One of the advantageous functionality is its ability to purge the fluid if the condition of the system is not proper. It is useful for low flow rate, high pressure and corrosive applications.

Armoured Purgemeter Rotameter

3. Flanged Armoured Rotameter

It is used in automated systems where the applications require quite opaque liquids at aggressive conditions. It is mostly used in high pressure applications.

Flanged Armoured Rotameter

Relation between flow and meter reading: ( Force balance in rotameter)

There are three forces acting on the float.
  1. Gravity force (Weight of the float)
  2. Drag force ( Due to fluid flow, in the direction of flow)
  3. Buoyant force ( Due to density difference between float and water)
By balancing the above forces,

FD = Drag force
g = Acceleration due to gavity
Vf = volume of float
rho-f = Density of float
rho = Density of fluid

In the above equation the right hand side is constant, so the drag force is constant. 

If flow rate increases the float position must change to keep the drag force as constant.

From the definition of drag coefficient,

As the fluid is flowing through the annulus region flow is directly proportional to annular area between the float and tube.

Where Df = Diameter of the float
           Dt = Diameter of the tube.

For linearly tapered tube with a diameter at the bottom equal to the float diameter, the area for flow is a quadratic function of the height of the float ‘h’.

By neglecting a2h2 term we get a linear relationship between flow and ‘h’.
In rotameter the flow is directly proportional to the square root of the reading on the tube.

Rotameter is a simple equipment which consists of a tapered tube and a float. 

The float is placed inside the tube and usually nets are placed at both the ends of the tube. 

This arrangement can be connected with a pipe line with flanged connections. 

Rotameters are always installed vertically in the pipelines. A scale is marked on the tube to read the values of flow rate directly.

Rotameter Working and Types

Rotameter Working

When the fluid is not flowing then the float rests at the bottom of the rotameter. 

The fluid is made to pass through the rotameter such that the direction of flow of the fluid is parallel to the axis of the rotameter.


The flow of fluid through the rotameter causes the float to move along with the fluid. 

There are two primary forces involved, an upward drag force due to the motion of the fluid in upward direction and a downward force due to gravity which is due to the weight of the float itself. 

When these forces are balanced then the float moves to a particular location in the tube and it stays right there because it has achieved dynamic equilibrium.

In case it happens that the flow rate of fluid flowing through the rotameter is very high then it may happen that the float may get swept along with the fluid. 

The nets attached to either side of the rotameter ensure that the float does not get carried away in the pipe line. If it happens then it may get stuck near a valve in pipeline and cause blockage or enter equipment down the line and cause it to malfunction. 

A down side of net is that if the flow rate of flowing fluid is very high then the float will get stuck near the net and act as a blockage for the fluid flow, this may cause the flanges to get weakened and the liquid may start showering at the site of rupture.

Advantages of Rotameter

It is simple to install and is easy and cheap to maintain.

It has a linear scale over large range of flow rates.

The pressure drop across the float is constant. Hence the pressure loss due to the float itself is quite small.

Rotameters are very versatile, they can be easily sized or their use can be changed for different systems.

No special fuel or external energy is required to pump.

Disadvantages of Rotameter

It requires a certain minimum magnitude of flow rate of fluid below which the float would fall and just stick to the rotameter.

If opaque fluid is used then the scale is not properly visible, it may cause misreading the meter.

It cannot be installed in a horizontal position.

If flow rate of fluid is very high then glass tubes may be subject to breakage.

Due to its use of gravity, a rotameter must always be vertically oriented and right way up, with the fluid flowing upward.

Due to its reliance on the ability of the fluid or gas to displace the float, graduations on a given rotameter will only be accurate for a given substance at a given temperature. 

Read more about "Industrial Equipments" from Industrial Guide - Click here

Thanks for reading - Rotameter
Naitik Patel
Industrial Guide

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