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What is a Variable Area Flow Meter?

What is a Variable Area Flow Meter

A variable area flow meter is a widely used instrument for measuring fluid flow rates in various industrial processes. It provides a simple and reliable solution for monitoring the flow of liquids, gases, and steam. This article will explore the working principle, types, advantages, limitations, applications, selection criteria, installation, maintenance, and calibration of variable area flow meters. So, let’s dive into the details and understand this essential flow measurement device.


Working Principle of Variable Area Flow Meters

Variable area flow meters operate based on the principle of fluid flow through a tapered tube or a cone-shaped device. As the fluid flows through the meter, it causes a displacement of a float, piston, or other movable element. The position of this element within the tapered tube is proportional to the flow rate. By observing the position of the element against a calibrated scale, the flow rate can be determined.


Types of Variable Area Flow Meters

There are several types of variable area flow meters available in the market, each with its own advantages and limitations. Let’s discuss some common types:



The rotameter, also known as a tapered tube flow meter, is one of the most commonly used variable area flow meters. It consists of a vertical tube with a float inside. As the fluid flows upward, the float rises, indicating the flow rate. Rotameters are easy to install, cost-effective, and suitable for a wide range of applications.


Cone Meter

A cone meter features a conical-shaped metering element placed within a pipe. As the fluid passes through the cone, it creates a pressure drop, which is used to determine the flow rate. Cone meters offer high accuracy and can handle demanding process conditions, making them suitable for applications where precise measurement is required.


Wafer Meter

The wafer meter is a compact and lightweight variable area flow meter that can be installed between flanges. It utilizes a movable piston or float to measure flow rates. Wafer meters are known for their ease of installation, low-pressure drop, and compatibility with various fluid types, making them popular in HVAC, water treatment, and chemical industries.


Piston Meter

Piston meters consist of a cylindrical metering chamber with a piston inside. As the fluid flows through the chamber, the piston moves, allowing the measurement of flow rates. These meters offer excellent accuracy and can handle high-pressure applications. They are commonly used in oil and gas, refining, and custody transfer applications.


Advantages of Variable Area Flow Meters

Variable area flow meters offer several advantages that contribute to their widespread use:


  • Simple Design: Variable area flow meters have a straightforward design, making them easy to understand and install.


  • Wide Flow Range: They can measure a broad range of flow rates, from low to high, making them versatile in various applications.


  • Good Accuracy: With proper calibration, variable area flow meters can provide accurate flow measurements.


  • Visual Indication: The position of the float or other movable element provides a visual indication of the flow rate, allowing operators to monitor the process in real-time.


  • Suitable for Wide Range of Fluids: These flow meters can handle a wide range of fluid types, including liquids, gases, and steam.


  • Cost-Effective: Variable area flow meters are cost-effective compared to other flow measurement technologies.


Limitations of Variable Area Flow Meters


While variable area flow meters have numerous advantages, they also have a few limitations to consider:


  • Pressure Drop: These meters can introduce a pressure drop in the system, which may not be desirable in certain applications.


  • Limited Turndown Ratio: The turndown ratio of variable area flow meters is limited compared to some other flow metering technologies.


  • Not Suitable for Viscous Fluids: Fluids with high viscosity may not provide accurate readings in variable area flow meters.


  • Flow Profile Sensitivity: The accuracy of these meters can be affected by non-uniform flow profiles, such as swirling or pulsating flows.


Applications of Variable Area Flow Meters

Variable area flow meters find applications in various industries, including:


  • Chemical Processing: Monitoring flow rates of chemicals and solvents.
  • Pharmaceuticals: Measuring flow rates of liquids and gases in drug manufacturing processes.
  • Water Treatment: Monitoring water flow rates in treatment plants.
  • HVAC: Controlling the flow of air and water in heating, ventilation, and air conditioning systems.
  • Oil and Gas: Measuring flow rates of oil, gas, and water in upstream and downstream processes.
  • Food and Beverage: Monitoring flow rates of ingredients and liquids in food and beverage production.


How to Select the Right Variable Area Flow Meter?

When choosing a variable area flow meter, several factors need to be considered:


  • Fluid Type: Determine the compatibility of the flow meter with the fluid being measured.
  • Flow Rate Range: Ensure that the selected meter can handle the expected flow rate range.
  • Process Conditions: Consider the temperature, pressure, and environmental conditions in which the flow meter will operate.
  • Accuracy Requirements: Evaluate the required accuracy for your specific application.
  • Installation Requirements: Assess space limitations, mounting options, and the need for additional accessories.
  • Budget: Consider the cost of the flow meter and its long-term maintenance requirements.


Installation and Maintenance of Variable Area Flow Meters

Proper installation and regular maintenance are crucial for optimal performance of variable area flow meters. Follow these guidelines:


  • Correct Orientation: Install the flow meter in the correct orientation as specified by the manufacturer.
  • Straight Pipe Length: Ensure an adequate straight pipe length before and after the flow meter to minimize disturbances.
  • Proper Grounding: Ground the flow meter to avoid electrical interference.
  • Regular Calibration: Periodically calibrate the flow meter to maintain accuracy.
  • Cleaning and Inspection: Clean the metering element and perform routine inspections to remove any debris or obstructions.


Calibration of Variable Area Flow Meters

Calibration is essential to maintain accurate flow measurements. It involves comparing the flow meter’s readings with a traceable standard. Calibration should be performed:


  • At Regular Intervals: Follow the manufacturer’s recommendations for calibration frequency.
  • After Maintenance: After any significant maintenance or repair work.
  • Whenever Accuracy is in Question: If the flow meter’s performance deviates from expected values.



Variable area flow meters are valuable instruments for measuring fluid flow rates in industrial processes. They offer simplicity, reliability, and visual indication of flow rates. With their wide range of applications, these flow meters continue to play a significant role in diverse industries. Proper selection, installation, calibration, and maintenance are essential to ensure accurate and consistent flow measurements.