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What Is a Rotameter?

By Jo Dunaway
Updated May 21, 2024
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A rotameter measures the flow rates of liquids or gases in industrial use. The basic design of a rotameter includes a vertical tube of gradually increasing diameter made of glass or another transparent material, within which a float will rise and fall according to the flow rate of the liquid or gas. The float rises when the flow is high, as it is forced to seek more diameter between itself and the outside of the tube in order to accommodate the force of the flow. When the flow is lower, gravity allows the float to find an equilibrium that can be matched against the internal linear scale for a reading. As the float rises and falls, it rotates on its axis, giving rise to the name rotameter.

Floats come in several shapes: circular, triangular, or elliptical. The reading from the internal linear scale is taken from the center of the float shape or the top of the float, according to the directions in the device's manual. Sometimes, a float is brightly colored so that an observer can easily determine if it is stuck or spinning freely. Though called a float, it doesn't actually float on the liquid. Instead, it is designed to take advantage of a scientific principle known as the variable area principle, which allows the float to find an equilibrium between the upward push of the liquid or gas flow and the downward pull of gravity.

When the flow rate changes, the float will either rise or fall in response and then settle to a new equilibrium of rate of flow versus gravity. The linear scale within exhibits a 0% to 100% range in most rotameters. Conversion charts for different types of liquids or gases can be used to determine the correct reading for the particular liquid or gas being examined. Some rotameters, however, are designed to give digital readouts of flow rate as well as temperatures. Others use a needle valve, to not only give a flow rate reading, but also a reading of flow under differing pressures.

The basic design of a rotameters, whether made of glass, plastic, or metal, requires no external power source, but operates under the basic laws of physics. As long as a rotameter is kept rigidly vertical, it will give good readings and can be used in many different applications. They can be used in pipelines for both liquids and gases, and can come equipped with high and low limit alarm systems. Research departments in several types of processing industries also use these devices. A rotameter can also be used as a purge-type flow meter to administer a constant rate of lubricants to bearings to reduce downtime in manufacturing.

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