Skip to main content
Variable Area Flowmeters

Variable Area Flowmeters

They go by many different names; variable area flow meter, rotameters, float type flowmeters, and even carrot flow meters. They can be deployed in many varied applications – dosing, monitoring, and control applications are just a few examples.

They are also available in a wide range of materials; some are designed, manufactured and made suitable for measuring the flow of the most aggressive liquids and gasses.

Additionally, as a result of being very cost effective, robust and versatile, they are one of the most commonly used types of flowmeter in industry.

Principle of operation

The fluid or gas which is to be measured flows upward through a vertical conical tube (or sometimes a conical float). The diameter of the tube increases in an upwards direction. The flowing liquid or gas lifts the float in the tube to a height so that the annulus has an area that results in an equilibrium of the forces acting on the float.

See diagram below:

  • Af = Float area
  • Ac = Flow measuring tube area
  • Pf = Float weight
  • E = Fluid thrust
  • Free area of flow: Al = Ac-Af

This applies to plastic and glass tube variable area flowmeters. Stainless steel versions utilise a calibrated fixed orifice, with a conical float.

  • E = Force of the fluid flow
  • Pf = Weight of the float
  • Free area of flow : Al = Ao – Af


  • Ao = Calibrated orifice area
  • Af = Float area
  • In this case the area changes due to the conical shape of the float.

Advantages of Variable area Flow meters

Variable area flowmeters can be used on liquid, gas and also steam applications. Custom calibrations are available for changes in the following:

  • Density
  • Viscosity
  • Pressure
  • Temperature

They are an economical form of flow measurement, due to the cost-effective range of materials and construction.

They are of a simple design and thus easy to read, both close up and also from a distance. Because of this, they have the following attributes:

  • Easy installation
  • No requirement of a power supply (unless fitted with a transmitter or alarm)
  • Direct reading (visible float or pointer)
  • High repeatability

Variable area flow meters can be supplied in sizes from ¼” up to 6”. Connections can be threaded or plain ended, flanged & even hygienic. They can be installed with minimal straight upstream and downstream pipe and are suitable for both safe & ATEX hazardous areas.

A spring restrained versions is also available, suitable for mounting in horizontal & vertically downwards flow directions. Valves & flow regulators can be built into the units for controlling and regulating the flow of liquids and gasses.

A variable area flowmeter can be used for measuring very small flows (glass and plastic versions) and also large flows (stainless steel) of liquids, gasses and steam. Different materials of construction are available; plastics, glass, stainless steel and special versions made from hastelloy.

Custom scales can be produced in-house, using our specially developed scaling program. A choice of flow units such as cc’s, litres, M3, KG and percentage as examples, can be printed on the scales, as well as specific liquids, gasses at varying temperatures and pressures

As you can see, variable area flowmeters are very versatile when it comes to flow measurement. If you’d like any further information on the different types available or, have a specific application you’d like to discuss, please do not hesitate to contact us.