What are Variable Area Flow Meters?

How they work, where they excel, advantages, and limitations.

What are Variable Area Flow Meters

How they Work:

Known also as rotameters, or float-type meters, variable area technology has been around for more than 100 years. It is by far the most commonly quoted, sold, and installed flow meter in the world. They can be used to measure the flow rate of either gas or liquid in a tube. They are comprised of a few simple parts, a tube, a float, the scale, fittings, and any outputs or switches or protective external elements.

The theory of operation is straight-forward. There is a float that is able to move freely up and down within the flow tube. The bouyancy force acting on the float, the flow resistance, and the float weight are all in equilibrium. Guide rods are sometimes used to stabilize the float. Guides are common for gas meters or large flow liquid meters.

The float bouyancy is dependent on the density and mass of the float as well as its materials of construction and volume. The float must have a higher density than the media or it will move to the top even when there is no flow, resulting in an error of measurement. Essentially, the float must not float on top of the media.

Variable Area Flow Meter diagram


  • Linear scale
  • Typically cost less than other types
  • Simple, reliable design
  • Easy to install
  • Minimal to no straight run requirements
  • Easy start-up and minimal maintenance
  • Variety of materials possible
  • No external power needed
  • Good accuracy for low to medium viscosity media
  • Repeatable measurement
  • Can be used with corrosive liquids if compatible build material is selected
  • Low pressure drop that is usually constant
  • Visual flow indication with transparent tubes
  • Multiple instruments can be installed on a single line without significant pressure loss


  • Important to pick a meter that will operate in 60% to 80% of the meter’s range most of the time
  • Armored versions allow for use in extreme conditions


  • Usually limited to a 10:1 turndown
  • Cannot tolerate coating or crystalizing
  • Most do not handle large solids or debris
  • Has defined operating conditions
  • Changes in SG, pressure, and viscosity cause significant reading errors
  • Must be calibrated for density, viscosity, and compressed gases
  • Orientation must be vertical installation, with flow from bottom to top in most cases
  • Non-transparent liquids obscure reading the float
  • Not good for pulsating flow
  • Without protection, glass/plastic tube models can break

Common Applications:

  • Laboritories
  • Chemical Injection
  • Boiler Control
  • Purge Applications
  • Flitration Loading
  • Process Analyzers
  • Early warning for clogged systems
  • Lube Oil
  • Coolant Flow

Media Characteristics:

  • Liquids or gases
  • Suitable for a wide variety of media
  • Low to medium viscosity
  • Minimal large particulate matter
  • No significant coating or crystalizing
Variable Area Flow Meters