Variable-area flowmeters

Flow Measurement & Control

October 2001 Flow Measurement & Control

Variable-area flowmeters, or Rotameters, are simple, economical and accurate flow measuring instruments. They consist of a tapered tube mounted vertically in the fluid stream with its larger diameter at the top. Within the tube is a float, which is free to move up and down. As fluid or gas flows through the tube from bottom to top, the float rises until the area between tube wall and float is sufficient to sustain the weight of the float and thus give an indication of the flow rate.

Suitable for both liquids and gases, these flowmeters are manufactured by McCrometer, which is represented locally by Protea Automation Solutions.

The devices are produced in a multitude of configurations, and with different shaped floats for different applications. The shape of the float determines the effect of viscosity changes on the meter's accuracy. For example, a ball float (stainless steel, black glass, red sapphire, tantalum, tungsten carbide or brass) would be used for low flow rates where there is little variation in viscosity. But for applications demanding high accuracy, such as monitoring gases for anaesthetics, a streamlined shape of float would be used.

The fittings and casing surrounding the measuring tube are available in several materials such as stainless steel, brass and PVC, and there is an explosion-proof version with all the switches and magnetic indicators enclosed in an explosion-proof housing. The metering tube can be provided with either linear or log scale, either on the tube or detachable scale, and calibrated in mm increments or directly in flow units for a particular fluid. The tubes can be provided in glass or metal, and with optional high/low sensors. (The picture shows a selection of McCrometer variable-area flowmeters.)

Protea Automation Solutions

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Flow Measurement & Control

Variable-area flowmeters

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