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.