Variable Area Flow Meter

-Description of the measuring principle-

About the page

This is an information page about flow measurement with variable area flowmeters. My many years of experience in the field of measurement and control technology are recorded here. I accept no liability for incorrect information or errors and would be pleased to receive further suggestions and requests for additions.
Please also visit my information page about Pt100 temperature sensors:
https://pt100.de

Dipl.-Ing. Harald Peters

Suppliers

Links to selected flow meter manufacturers and suppliers.

Description of the Principle

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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.

  • FG=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.
  • FA= 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.
  • FW= The resistance force which depends on the flow and the annular gap between the measuring tube and the float.
If the upward forces (FW und FA) and the downward force (FG) are in equilibrium, the float "floats" in the measuring tube.

FW = FG - 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(FW). 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.