Variable Area Flow Meter

The variable area flowmeter is a reliable, inexpensive and often used method for measuring the flow of liquids, gases and vapours in industrial applications. They are characterised by their high measuring accuracy and are suitable for a wide range of flow rates.
The measuring principle is based on the buoyancy of a float in a pipe, which is influenced by the flow of the liquid. The float usually has a spherical or cylindrical shape.

The basic design of a variable area flowmeter consists of a conical, vertically mounted measuring tube in which a "float" can move freely up and down.

Three forces act on the float.

• F_G=The weight force of the float.
  This depends on the mass of the float. In order to obtain different masses for the same dimensions of the float, a wide variety of materials are used, e.g. glass, metal, plastics, are used. When selecting the material, the chemical resistance to the fluid must of course be taken into account. A further weight reduction is achieved by using hollow floats.
• F_A= The buoyancy force of the float, which depends on the mass of the fluid displaced by the float, i.e. the volume of the float and the fluid.
• F_W= The resistance force which depends on the flow and the annular gap between the measuring tube and the float.

If the upward forces (F_W und F_A) and the downward force (F_G) are in equilibrium, the float "floats" in the measuring tube.

F_W = F_G - F _A

If the flow is changed, this equilibrium is altered and the float rises or falls. This changes the annular gap between the measuring tube and the float and thus the force(F_W). This continues until equilibrium is restored and the float is floating again. The level of the float is thus a direct measure of the flow, which can be read on a calibrated scale.
By combining measuring tubes of different diameters with floats of different weights and diameters, a wide range of different measuring ranges is obtained. Particularly in the laboratory, longer measuring tubes with a small angle of inclination are often used to increase the resolution of the scale and the measuring accuracy.