What is Rotameter and Does it Work?


Flow rate measuring devices come in many types. Rotameter is one of them. This kind of flow meter is used to measure the flow rate in gases and liquids. Rotameter is made from Glass, plastic, or metal. It also has a float that responds to the flow rate linearly to the fluid in the tube. A rotameter has many benefits when measuring flow rate. Some of them include low-pressure drop, long measurement range. This flow rate meter is also easy to set up and maintain.

What are the factors to consider when looking for a rotameter?

Operating conditions–Before buying a rotameter, you need to know the conditions you are going to apply. These conditions include the temperature, the pressure, and max & min flow rates.

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The size of the pipe –The user also needs to consider the size of the pipe that is going to be used in the installation process.

The type of rotameter– Before buying a rotameter, the user needs to know the type of rotameter needed in the application. Some rotameters require lookup tables, ones that direct readings, and meters that provide readings both ways.

Required accuracy– The kind of accuracy needed in the application is also important when choosing a rotameter.

Working Principle of Rotameters

Considering the benefits that we have mentioned above, rotameters are ideal flow rate meters for measuring variable-area. This kind of flow rate meter has a tapered tube. The float is raised when fluid goes through the tube. The float will go higher when more pressure is exerted on it by the great volumetric flow. To raise the float in liquids, buoyancy is combined with the velocity of a liquid. This is different in gases where buoyancy can be ignored, and only the velocity is used to set the float height.

Usually, the tube is mounted in a vertical position. The float will begin to rise when the fluid goes upwards from the bottom of the tube. The float will stay at the bottom when there is no flow. The annular area between the float and the tube wall and the flow rate is proportional to the heights that the float moves through. Differential pressure decreases when the float rises and the size of the annular opening increases across the float.

As the upward force is exerted and the float reaches a fixed position, the system reaches an equilibrium. This is due to the flow of the fluid, which levitates the float. Once this happens, the user then reads fluid viscosity and density flow rate. The application determines the make and the size of the rotameter. Based on the position of the gloat, the user can read the flow rate directly off the scale if the device is properly calibrated and has the right size.

We have rotameters that enable users to adjust the flow using a valve manually. The previous designs were made with free floats that rotated according to the fluid pressure and changing gas. They were called rotameters since they rotated. For commonly used fluids like water and air, rotameters come with direct reading scales and calibration data. If you are going to use the rotameter with other fluids, you will have to convert readings into standard formats. The water equivalent in liquids is GPM, while in gases, it is in standard cubic feet per minute (scfm).

Together with nomographs, slide rules, or the software used in getting the size of the rotameter, calibration tables for standard flow values, the manufacturer is the one to give this information.

Glass Tube Rotameter

A rotameter made from a glass tube is the basic type. The material used to make the Glass is called borosilicate. And to prevent corrosion, stainless steel is used to make the float. Sometimes the float is made from plastic or Glass. To help the float to point to specific readings in the scale, it will have sharp metering edges. Based on the use, the rotameter will have an end-fitting or a connection finishing. No matter the type of end-finish or case, both stainless steel and glass tubes can be used.

The most valuable part of the rotameter to standardize is the tube-float combination since it provides the measurement. Correlation tables for gases such as carbon dioxide, oxygen, nitrogen, argon, hydrogen, and helium, can be compared with correlation rotameter scales. Although it is going to be less convenient since you only get direct-reading from a scale of air or water, it’s going to be more accurate. It is also designed to work under specified pressures and temperatures.

Once the conversion is finished in correlation flow meters, it will give you the flow rate of a wide range of liquids in various conditions. Multiple floats are used to measure different flow rates simultaneously. Taking readings is made easy by fitting a glass-tube rotameter at eye level. Under ordinary conditions in industries, safety-shielded gas rotameters are used to measure flows of water or air. These rotameters can measure flows of up to 60 GPM. Metal or plastic caps are used based on the nature of the chemical being measured.

Glass can be used in some fluids such as water with temperatures above 90°C (194°F), as it will soften the rotameter Wet steam also has similar effects. Glass also can’t be used with caustic soda as it will devolve the Glass. This means that different tunings are needed with such fluids.


The density of the fluid determines the reading. Two floats are used if it is liable to change. One of the floats is used to correct density, and another one for volume. The fluid density matches the float density. Buoyancy will change the density. Hence it is important to change the position of the float. Fluids with low viscosity work well with mass-flow rotameters. Such fluids include light hydrocarbons, raw sugar, jet fuel, and gasoline.


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