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How does a Rotameter work?

Tuesday, August 20, 2019

A Rotameter is a form of variable area flow meter which has a simplistic operation whereby a liquid or gas passes through a tapered tube. In order for this gas to pass through the tube it must first raise a float held within the tube.

When a rotameter is used with a liquid the float rises because of a combination of the velocity head of the fluid and the buoyancy of the liquid.

With a gas the buoyancy is negligible and the float moves in the most part due to the velocity head of the gas. In both cases, the greater the flow then the higher up the tube the float moves. The float moves up and down the tapered tube in proportion to the flow rate and the annular area between the float and the tapered tube wall. As the float moves up through the tube because of its tapered nature the annular opening increases. As this increases the differential pressure across the float decreases. The float stabilises when the weight of the float is in equilibrium with the upward force being exerted by the fluid or gas.

Download the specifications and manual for our popular 7510 series rotameter

View 7510 series specifications

View 7510 series manual

The float can then be compared to a calibrated scale either printed onto the tube itself or placed next to the tube on the outside of the flow meter. The calibrated scale will commonly give a volumetric flow reading, for example litres per minute (LPM).

Reading the float at eye level

The importance of reading the float from the same height will give a true reading, Eye level = float level for reading.

Popular rotameters

Low cost acrylic tube variable area flow meter

Suitable for OEM applications

The King 7510 series is ideal for general water flow applications, for lower water flow rates down to 2 litres per hour (LPH) or higher water flow rates up to 75 litres per minute (LPM).

The 7520 and 7530 series offering panel mounting options as well as a control valve should your water or air process require fine tuning or flow adjustment.

Chemically-Resistant, Borosilicate Kinglass Flow Meter

Great value general purpose rotameter

Related article

King rotameters used in biostore process to monitor air flow

Premier Control Technologies offer a complete range of Rotameters – feel free to browse our Rotameters product range.

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