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What is a Variable Area Flow Meter (Rotameter) and Why Use it to Measure Gas or Liquid Flow


What is a variable area flow meter or rotameter? A variable area flow meter, commonly referred to as a rotameter, is a flow meter that measures the volumetric flow of liquids and gases. 


What is a variable area flow meter? A variable area flow meter, commonly referred to as a rotameter, is a flow meter that measures the volumetric flow of liquids and gases. There is no difference between a variable area flow meter, rotameter, or a flow meter, and these terms are interchangeable. Inside a variable area flow meter, a gas or liquid moves a float until it reaches equilibrium - the technique to measure the flow rate. A user can easily read the gas or liquid flow rate from either a scale next to the tube or a scale on the tube.

Rotameter applications extend across industries because it was and still is an economical way of measuring very low to high flow rates, especially with the GT1600 Series glass tube rotameter. Brooks Instrument flow experts often ask users why they select flow meters. Their top six responses below explain why a rotameter continues to be successful.

  1. No external power required – Rotameters are mechanical devices that do not require power to provide gas or liquid flow measurement. Since power isn’t necessary, users install rotameters in hazardous and remote areas where it is expensive to supply power. The MT3809 Series flow meter is one such example.
  2. You can see the process – Users can easily read the flow measurement and look into their process. For example, does the process look dirty or cloudy? If so, the filters may need to be changed. Also, is the process the correct color? Are there bubbles in the liquid? A solution is the Sho-Rate™ Series rotameter.
  3. Rotameters are cost effective – Both rotameter price and function contribute to savings on the job. Rotameters can be installed with other flow measurement technologies and can also be used to complement each other at an economical price.
  4. Simple to install and maintain – Install rotameters by connecting the process line to the inlet and the outlet. Make sure the meter is vertical prior to full operation.
  5. Low pressure drop – Most small rotameters have only a few inches of water column pressure drop, meaning rotameters are often installed in many places in the process. Small pressure drops mean smaller pumps!
  6. Repeatability – Given the same process conditions, a rotameter will accurately repeat the flow measurement day after day.
These six features and benefits demonstrate how rotameters will continue to be important products to measure gas and liquid flow now and in the future. An application note on wastewater treatment plants demonstrates how variable area flow meters work in this process.

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