What Is the Working Principle of Variable Area Flow Meter

In the constantly changing world of industry, precise flow measurement is essential. Whether monitoring liquids, gases, or steam, accurate data is crucial for efficiency and control. The variable area flow meter is a reliable and versatile tool that engineers and operators can trust from the many available flow measurement devices.

Understanding variable area flow meters is important for mastering fluid dynamics and process control. Let’s explore this essential instrument that has been valuable in various industrial settings for decades.

Variable area flow meters work at a constant delta pressure (p), and the area varies with flow rate. To maintain a constant Delta Pressure (p), the area will rise as the flow rate through the meter increases.

Variable Area Flow Meters

Variable Area Flow Meters

The cone-and-float design, often known as a rotameter, is the most popular type of variable area meter. A variable area meter is essentially a tapered tube (typically made of glass) containing a self-centering float that is pushed up by the flow and drawn down by gravity.

The float rises at greater flow rates to increase the area between the tube and the float and maintain a steady pace.

The flow rate is calculated by how far the float has risen up the tube: graduations are located on the side of the tube. Variable area meters are commonly used for metering gas, but several versions are available for a number of fluids. For liquids and thick fluids, a buoyancy correction term is necessary.

Variable area flow meters are simple yet adaptable flow measurement instruments that can be used on a wide range of liquids, gases, and steam. They work on the variable area concept, which states that a flowing fluid adjusts the position of a float, piston, or vane to open a greater area for the fluid to travel through. The flow rate is directly visible due to the position of the float, piston, or vane.


metal tube rotameter

A rotameter is a type of industrial flow meter used to monitor the rate of flow of liquids and gases. It works on the variable area concept, which states that fluid flow lifts a float in a tapered tube, increasing the area for fluid passage. The greater the float, the greater the flow.

The flow rate is in direct proportion to the height of the float. The float is raised by a combination of the buoyancy of the liquid and the fluid’s velocity head in liquids.

When using gases, buoyancy is insignificant, and the float responds solely to the velocity head. In the tube, the float travels up and down in response to the fluid flow rate and the annular area between the float and the tube wall.

When the upward force exerted by the flowing fluid matches the downward gravitational force exerted by the float’s weight, the float reaches a stable position in the tube. A change in flow rate disrupts this force equilibrium. The float then moves up or down, modifying the annular area until the forces are once again in equilibrium.

To satisfy the force equation, the rotameter float adopts a distinct location for each constant flow rate. However, because the float position is gravity-dependent, the rotameters must be vertically aligned and attached.

The tapered tube’s gradually increasing diameter provides a related increase in the annular area around the float and is designed by the basic equation for volumetric flow rate:


Q denotes the volumetric flow rate, such as gallons per minute.

k = a fixed value

A = the annular region formed by the float and the tube wall.

g = gravitational force

h denotes the pressure drop (head) across the float.

A is a direct function of flow rate Q when h is constant in a VA meter. As a result, the rotameter designer may establish the tube taper so that the height of the float in the tube can be used to calculate the flow rate.

Variable area flow meters are used primarily to set flow rates. The operator views the meter and adjusts the valve to bring the process flow to the proper flow rate. The meter’s ability to repeat or reproduce this flow rate is important. Rotameters are repeatable up to ±1 ⁄4% of the instantaneous flow rate. This function allows the operator to confidently reset or alter the flow.

glass tube rotameter


  • The rotameter is popular because it has a linear scale, a relatively long measuring range, and a low-pressure drop.
  • It is easy to set up and maintain.
  • It can be manufactured in various construction materials and designed to cover various pressures and temperatures.
  • The rotameter can easily be sized or converted from one type of service to another. It generally owes its wide use to its versatility in construction and applications.
  • The rotameter is an exceptionally practical flow measurement device because of its functional advantages.
  • The pressure drop across the float is low and remains constant as the flow rate changes. The reaction of the float to variations in flow rate is linear, and a 10-to-1 flow range or turndown is normal.
  • Variable area flow meters are commonly used to provide cost-effective local indication of small liquid or gas flows.


  • Low accuracy – uncertainty on volumetric flow rate is ~2% of reading
  • Generally small turn down
  • Float’s proclivity to stick at low flows
  • Requirement for buoyancy correction in liquids
  • Application cautions for variable area flow meters

Variable area flow meters should not be used with fluids that are opaque, filthy, or prone to coating the metering tube or float since this may render the flow meter inoperable.

Install variable area flow meters with floats in the vertical orientation because their operation is gravity-dependent. Variable area flow meters that need upward flow may be appropriate only in certain applications where the fluid flows by gravity.

A potential safety hazard can be created if a glass metering tube breaks, especially when dangerous fluids are in the flow meter. Be careful to install variable area flow meters with glass metering tubes where the glass cannot be damaged.

Also, the float can get stuck when the flow turns on suddenly or when high flow rates cause it to reach its highest mechanical position.


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