Variable Area Flowmeters & Rotameters

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Variable area flow meters are used in pipelines to determine the flow of liquids or gases within. The flow meter consists of a conical measuring tube with a float inside it.

The measuring principle is based on the body being vertically deflected through the flowing medium. Various forces act on the float – the flow resistance, the buoyant force, as well as the weight force of the body.
In summary, if the volume flow rises, the float is lifted. The current flow is indicated on the scale at the top of the float.

We are able to supply variable area flow meters other than our standard range. If you require a glass tube, steel frame or any other variation then please let us know and we will be able to quote.

Typical Application for variable area flow meters:
Liquid flow meter or gas flow meter where high accuracy is not required
Measuring water and gas flow in plants or labs
Monitoring chemical lines
Purging instrument air lines (i.e. lines that use a valved meter)
Monitoring filtration loading
Monitoring flow in material-blending applications (i.e. lines that use a valved meter)
Monitor water for food & beverage plants

Key Points for variable area flow meters:
Variable area flow meters operate on the principle that the flow of the media will alter the position of a float in a tapered tube. The position of the float in the tapered tube is proportional to the volumetric flow rate
Typical accuracy of variable area flow meters is ±3% full scale
A variable area flow meter normally has a turn down ratio in the region of 10:1
A variable area flow meter normally requires vertical installation and a straight pipe run (minimum length of five times the diameter of the pipe) before the flow meters inlet

Advantages for variable area flow meters:
Low cost
Simple to install
Easy to use and virtually maintenance free
Wide choice of materials including all PTFE for aggressive fluids
Many control options including control valves, set-point switches and analogue outputs

Disadvantages for variable area flow meters:
Lower accuracy compared to many other flow meter technologies
Affected by pulsation
Sensitive to fluid changes such as viscosity, density and temperature


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