what is a rotameter and how does it work?

You need a device that can offer an accurate flow rate when dealing with liquids and gases. Flawed flow rate monitoring may lead to huge losses. But the new technology has made everything easy and possible when it comes to measuring the flow rate of fluids. A Rotameter is among the variable-area flow meters you can rely on for accurate flow rate measurement. The device is made up of a tapered tube and a float inside it which helps to determine the volumetric flow rate of fluid passing through the meter.

Also, a change in weight and density of the float influences accuracy of a Rotameter. You’re supposed to calibrate your Rotameter every year to correct any change that occurs in the system. That way, your device will always give you accurate flow rate results. 

How Rotameter measures the flow rate

 A Rotameter measures the flow rate of fluids based on the variable area principle. The fluid you want to monitor enters into the Rotameter from the bottom of the tapered tube. Some fluid passes aside the float while others strike directly into the bottom of the float.

As the fluid flows into the Rota the force pushes up the float inside the tapered tube, increasing the area for fluid passage. The movement of the float due to fluid coming in creates two forces in the opposite direction, the gravitational force downward and the drag force upwards.

 The flow of fluid moves the floats upwards against the force of gravity. The flowing area created as the float moves up may reach a level where the pressure-induced force on the float matches the weight of the Rota float. Then, the float finds an equilibrium if the area around the float creates drag equal to the weight. That’s buoyancy.

I was wondering how the Rotameasures flow rate? You only need to look at the point where float weight and gravity are constant. The height at which the float is moved upwards is directly proportional to the flow velocity of the fluid that passes through the tapered tube. 

If the float is moved to a greater height, it shows the flow is higher. When measuring liquids, the float is moved up by fluid buoyancy and velocity head of fluid. But for gases, the float responds to velocity head only since buoyance is negligible.

The up or down movement of the float is relative to the flow rate of the fluid and annular area between the float and wall of the tube. If the upward force created by the fluid passing through the Rota is the equal downward gravitational force resulting from the weight of the float, then the float reaches a stable position.

Any increase or decrease in flow rate affects that equilibrium making the float move up or down. The annular area also changes as the float moves until the state of balance is achieved. 

Types of Rotameter

Many types of Rotameter flow meters are available in the market. But not all of them suit your specific needs. Ensure to inquire from the supplier about each type and how it works. That helps to make the right decision when it comes to Rotameter devices.

Here are various Rotameter types you may consider.

1. Glass tube Rotameter

Glass tube indicating-type of Rotameter is the basic device. While the tube of this Rotameter is made of borosilicate glass, its float is metallic and usually stainless steel to help prevent corrosion. But it may also be made of plastic or glass.

Float in this type of Rotameter has metering edges that point at specific reading on the scale. Your Rotameter can have connection or end-fitting based on use. Do you want to use a glass tube Rotameter to measure the flow rate of hair or water? You’re allowed to set the scale to get direct readings.

2. Metal Tube Rotameter

If you handle fluids with high pressure and temperature, a metal tube Rotameter will be an ideal device to use. Though the flow meter isn’t transparent, magnetic flowers help to determine the position of the float.

Metal tube Rota measures flow rate using the combination of spring and piston. You can use this Rotameter type to monitor the flow rate of corrosive fluids which attack glass tubes.

What you should look at when buying a Rotameter

  • Maximum pressure fluid
  • Level of accuracy needed
  • Size of your pipes
  • Minimum and maximum temperatures.
  • The highest and lowest flow rate of your Rota

Advantages of using a Rotameter

  • It offers affordable flow metering solutions
  • You can rely on Rota for accurate flow rate results
  • Low-pressure drop
  • You can use Rota to measure the flow rate of corrosive fluids
  • Uses linear scale
  • Rotameter is suitable for measuring the flow rate of gas and liquids
  • Rota has a simple and reliable design
  • You don’t need external power to run a flow meter 

Cons of a Rotameter

  • It may be challenging to handle the glass tube type of Rotameter
  • You’re required to mount the Rotameter vertically
  • Rotameter needs lining mounting.


After buying a Rota, ensure you consider gravity during installation since it affects flow measurement. Inquire about different Rota types and how they work. It helps to make the right choice of device. But you can always rely on a Rotameter for accurate flow rate results.