rotameter variable area flow meter
  • You Should Know the Basics of Variable Area Flow Meter or Rotameter

Featured Image Caption: Rotameter Variable Area Flow Meter

Rotameter is one of the two types of variable flow meters. The other one is the piston-type flow meter. However, the rotameter is more common. It’s the meter that comes to mind when you mention a variable area flow meter.

These meters have a tube and float. You can use them to gauge the flow rate of liquids and gases. The float within the tube gives a linear response to the change in flow rate.

How Variable Flow Meters Work

If you are looking for a versatile flow switch that’s simple and easy to use, you have a variable area flow switch solution. The standard type is a rotameter. Both types of variable area flowmeters have the same working principle.

You can use these meters to gauge all types of gases, liquids, and steam. The fluid moves through a tube and changes the position of the float. You will see a direct visual display of the change.

In 1908, Karl Kueppers invented the very first variable flow meter. The meter had a rotating float. The meters are popular to date.

Rotameters are very popular today. They have a linear scale that is easy to read—the float in the rotameter changes with the change in the fluid velocity. The float exerts pressure on the fluid. When the fluid passes, it exerts an upward force on the float. The float will stabilize when its force and the force of the fluid balance.

You read the flow meter from the scale at the side of the fluid. The scale reads from the bottom upwards. You calibrate the scale according to the fluid characteristics. The weight of the fluid has a direct effect on the accuracy of the fluid. The other factors that affect the accuracy of a rotameter are heat, pressure, and flow control at the first calibration. If you change the density of the fluid, you will affect the rotameter reading. Therefore, you should calibrate your rotameters annually to ensure that the readings are perfect.

Basic Designs

A basic rotameter has seven parts- meter tube, scale, float, o-ring, float stop, inlet and outlet fitting, and housing. Float stop regulates the float movement while the housing protects all the components of the rotameter.

If you have a glass meter tube, then a flow rate scale is better when applied to the meter tube. The general variable area flowmeter range is 1:12.5. You can translate this as 8:100%.

When using fluids with corrosive and abrasive properties, a glass tube would be inadequate. Then you can use metallic meter tubes. In this case, the float will be external to the meter tube. Sometimes you can use a flow rate converter to make the reading easier.

Types of Floats

There are several types of floats you can get for a rotameter. Each float depends on the kind of fluid you will use for your flow system. Here are the basic types of floats.

Ball floats

These flow switch floats are helpful in small flow rates. These floats have different materials to cater to the various fluid densities.

Pole Guided Floats

These floats have a pole firmly positioned within the meter tube. You use the float with a tapered meter tube with a hole drilled through its length axis.

Floats with Tail Guides

These floats beat the effect of viscosity. Therefore, you can get them in different materials and weights. For example, you reverse the floating head if you want to gauge 25%-30% flow rates. However, you may not be able to use the floats in fluids with extremely high viscosity.

Tapered Floats

You can use these floats with cylindrical tubes only- The type has to have an orifice. They have guide elements attached to the tube. The element helps move the guide rod to the center and guides the float.

Floats Tube “BL”

These floats are best when you use them with small cross-sectional areas and high flow rates. This is because their bead guide meter tubes have three ribs that guide the floats.

Low-Pressure Drop Floats

These rotameter floats are specific for fluids at low pressure. With such fluids, there is a high possibility of low-pressure drops. Therefore, you use these floats with a bead guide metering tube.

Floats with Guide Rods

These works are just like tapered floats. The only difference is that they work in tapered metal meter tubes.


As indicated above, you can select the best variable area flowmeter for your applications when you understand rotameter basics. The working principle is uniform in all types of flow switches. The difference in floats guides you in the best rotameter for your firm.

Naman Modi

By Naman Modi
who is a Professional Blogger, SEO Expert & Guest blogger at Proteus Industries Inc. He is an Award-Winning Freelancer & Web Entrepreneur helping new entrepreneur’s launches their first successful online business.

Member since April, 2019
View all the articles of Naman Modi.

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