Variable area flow meters – reliable flow measurement for liquids and gases

What are variable area flow meters?

Variable area flow meters based on the variable area principle have been used successfully in industry for over 100 years.
They are considered a cost-effective, robust, and reliable solution for measuring the flow of liquids, gases, and vapors in pipelines.
Thanks to their simple mode of operation and high operational reliability, variable area flow meters are still indispensable in many industrial applications today.

How variable area flow meters work

Traditional variable area flow meters consist of a conical measuring tube made of glass or transparent plastic.
Inside is a freely movable variable area that rises in the tube depending on the flow rate. The current flow rate can be read directly and precisely on a scale.

Variable area flow meters for demanding applications

Variable area flow meters with conical metal measuring tubes are used for high operating pressures, high temperatures, or opaque media.
Here, the position of the float is transmitted to a display unit without contact via a magnetic coupling system.
These devices can be optionally equipped with electrical transmitters, for example for:

• 0(4)-20 mA / 0-10 V output
• HART
• PROFIBUS
• other industrial bus systems

This makes remote transmission of measured values possible without any problems.

Monitoring and additional functions

Variable area flow meters can be equipped with reed contacts or inductive contacts for monitoring flow limits.
These enable simple limit value monitoring and integration into existing automation systems.

Advantages of variable area flow meters

Variable area flow meters offer numerous advantages:

• Easy assembly
• Low pressure drop in the measuring tube
• High measuring accuracy and good repeatability
• Inexpensive (especially units made of plastic)
• No auxiliary energy is required for measurement
• Low maintenance costs
• With transparent measuring tubes can be used as sight glass at the same time

These properties make it a popular flow meter in chemistry, process engineering, water and wastewater technology, and many other industries.

Disadvantages of variable area flow meters

• Measurement depends on density, viscosity, pressure, temperature,
  i.e. the instruments must be calibrated for one fluid. A conversion for other fluids is possible.
• Standard units must be mounted vertically with flow from bottom to top.
• Low measuring range span (approx. 1:10)
• Not for fluids with dirt or solid content
• Not suitable for opaque fluids with transparent measuring tube.
• Only suitable for smaller nominal pipe diameters (up to approx. DN 150)

Future-proof flow measurement technology

Even though electronic flow meters are becoming increasingly important in the wake of automation and digitalization, the variable area flow meter remains an integral part of industrial measurement technology due to its simplicity, reliability, and cost-effectiveness—both today and in the future.

History

The first mention of the measurement of flow in a conical pipe with a float was made by the American Edmund A. Chameroy, who applied for a patent for his invention in 1868.
In the following 15 years, further patent applications were filed in England and Germany for designs with constructive improvements.
A decisive discovery was made in 1908 by Karl Küppers (1874-1933) in Aachen (Germany).
While building floats for monitoring burner gases, he discovered that a rotating float significantly improves the accuracy and reliability of the measurement. He applied for a patent for this discovery at the Reichspatentamt (Germany) in the same year with the number DE21225C (https://patents.google.com/patent/DE.
The rotation of the float was realised by inclined notches on the float head.
Felix Meyer recognised the importance of this invention and founded the "Deutsche Rotawerke GmbH" in Aachen (Germany). He optimised the float shape, produced and marketed the devices industrially as Rotameter®.
The company moved to Wehr in Baden in 1944 and still produces floats and other flowmeters there today under the name Rota-Yokogawa, as a subsidiary of the Japanese company Yokogawa.
Rotameter® is a registered term of the Rota-Yokogawa company.
In the meantime, variable area flowmeters are produced and marketed worldwide by many companies in a wide variety of designs.
Estimated market volume worldwide, several hundred thousand units per year.