A VA-flowmeter consists of a tapered tube, typically made of glass with a ‘float’, actually a shaped weight, inside that is pushed up by the drag force of the flow and pulled down by gravity. Drag force for a given fluid and float cross section is a function of flow speed squared only.
A higher volumetric flow rate through a given area increases flow speed and drag force, so the float will be pushed upwards. However, as the inside of the VA-flowmeter is cone shaped (widens), the area around the float through which the medium flows increases, the flow speed and drag force decrease until there is mechanical equilibrium with the float’s weight.
Floats are made in many different shapes, with spheres and ellipsoids being the most common.
The variable area flow meter for gases is highly dependent on the pressure in the measuring tube. The position of the control valve must be considered thereby (see also valve at the inlet or outlet).