Variable Area Flow Meter

$ 1,368.00$ 3,050.00

SRS’s Variable Area Flow Meter, often referred to as “rotameters,” measure flow rate of a liquid or gas by relating linear displacement of an internal “float” or sharp-edged orifice tube to a corresponding flow rate.  As the flow rate increases, the orifice area that the flow moves through also increases – thus, the term “Variable Area”.  Variable Area meters either allow flow through a peripheral orifice formed between a tapered wall and a float as in the traditional rotameter, or an annular orifice and an internal tapered metering pin in the Device Head.  The attributes of the Variable Area flow meter are a linear relationship between flow rate, pressure differential and piston displacement.  Flow rate is read on the Variable Area meters in both Analog or Digital Display by aligning the position of the piston/float to an adjacent calibrated scale.

The Advantages of the Variable Area Flow Meter
  • Relative low cost and ease of installation
  • Robust and easy to maintain
  • No need for auxiliary power
  • Works with both liquids and gases


In the field of fluid dynamics, the variable area flow meter plays a crucial role in accurately measuring the flow rate of gases or liquids.  Also known as rotameters, these flow meters are based on a simple yet effective principle that makes them widely used in various industries.  This comprehensive description aims to provide an in-depth understanding of variable area flow meters, their construction, working principles, advantages, and applications.


Tube The heart of a variable area flow meter is a tapered tube typically made of glass or metal.  The tube contains a float or ball that moves freely within the tube in response to the fluid flowing through it.  The tube’s tapered shape ensures that the area through which the fluid can pass increases as the float moves upward, allowing for accurate flow rate measurements.


Mounted alongside the tube, a scale provides a visual indicator of the flow rate.  The scale is calibrated based on the specific fluid and flow conditions, allowing users to directly read the flow rate from the position of the float.

Working Principle

Variable area flow meters operate on the principle of fluid dynamics and gravity.  As the fluid enters the tapered tube, it exerts a force on the float, causing it to rise.  The float reaches an equilibrium position where the gravitational force acting on it is balanced by the upward force exerted by the fluid.

The position of the float within the tube indicates the flow rate, as the float’s position is directly related to the cross-sectional area available for the fluid to pass through.  A higher flow rate results in a higher position of the float, whereas a lower flow rate leads to a lower position.

Advantages of a Variable Area Flow Meter

Wide Rangeability Variable area flow meters offer excellent rangeability, allowing them to measure flow rates across a wide span.  They can accurately measure both high and low flow rates, making them versatile for various applications.

Simple and Reliable

With their straightforward design, variable area flow meters are easy to install, operate, and maintain.  They have few moving parts, reducing the risk of mechanical failures, and are known for their long-term reliability.

Visual Flow Indication

The presence of a float and scale affords an immediate visual indication of the flow rate, allowing for quick and intuitive monitoring and adjustment of the flow.


Variable area flow meters are compatible with a vast range of fluids, including aggressive and corrosive ones. The choice of tube material can be tailored to suit the specific application, ensuring compatibility with the fluid being measured.

Applications with a Variable Area Flow Meter

Process Industries Variable area flow meters find extensive use in process industries, such as chemical, pharmaceutical, and food processing, where the accurate measurement and control of fluid flow rates are critical.

Laboratory and Research

Their simplicity, reliability, and visual flow indication make variable area flow meters a popular choice in laboratories and research facilities. They are often utilized for experimentation, calibration, and general flow rate measurements.

HVAC Systems

These flow meters are also employed in heating, ventilation, and air conditioning (HVAC) systems to monitor and control fluid flows for optimal system performance and energy efficiency.

Oil and Gas

Oil and gas industries rely on variable area flow meters for various applications, including measuring flow rates in pipelines, monitoring additive injection, and managing the flow of gases and liquids.


Variable area flow meters are versatile and dependable instruments for measuring fluid flow rates. With their simple yet effective construction and wide rangeability, they find applications in diverse industries. From process control to laboratory research, and HVAC systems to oil and gas operations, variable area flow meters offer accurate flow rate measurement, visual indication, and reliability, contributing to improved efficiency and performance in countless applications.

Variable Area Flow Meter for liquids and gases

  • Modular design: from mechanical indicator to 4…20 mA / HART®, and Totalizer
  • Any installation position:  Vertical, horizontal, or in fall pipes
  • Flange: DN15…150 / 12…6″; also NPT, G, hygienic connections, etc.
  • -196…+400°C / -320…+752°F; max. 1500 psig

The SRS Direct Roto-Gear is the standard variable area (VA) flowmeter for the process and OEM industry.  additionally, the VA meter combines mechanical flow measurement of liquids or gases with state-of-the-art communication capabilities.  The device is modularly extendable.  Additionally, electronic modules can be added or replaced at any time without process interruption.  In this way, its functionality adapts to new requirements – from analog flow measurement without auxiliary power to digital integration into a field bus system.

In addition, the variable areas flow meter is available in various materials and in many different versions.  Moreover, the sturdy all-metal flowmeter can be also used in applications with high pressures (up to  1500 psig), temperatures (-196…+400°C / -320…+752°F), or aggressive media.  Also, it even allows installation in horizontal or fall pipes up to DN150 / 6″.   After that, a special version for use in hygienic applications is also available.  Finally,  the Variable Area Flow Meter is intrinsically safe and explosion-proof. 

Firstly, Product Highlights
  • Simple, low-cost installation:  Able to measure and display without an auxiliary power supply
  • Unique modularity and serviceability:  Limit switches, 2-wire 4…20mA, totalizer with LCD and pulse output, 
  • Universal ex-concept: Explosion-proof and intrinsically safe
  • Various stainless steel and alloy sensor materials
  • Optional stainless steel housing for corrosive atmospheres
  • Ingress protection IP66 and IP68 / NEMA4X and NEMA6
  • Full scale value for liquids:  10…120,000  l/h / 5…32,000 GPH
  • Full scale value for gases:  0.7…2800 m3/h / 25…100,000 SCFH
  • Turndown ratio 10:1 (optional 100:1)
  • Accuracy:  ±1.5% 
  • Optional hygienic design for use in the food and pharmaceutical industry
Secondly, Typical Applications

Chemical and other process industries:

  • Additives (e.g. catalysts, foam inhibitors, emulsifiers)
  • Acids, bases, phosgene, and other chlorides substances, sulfuric substances
  • Condensate and cooling water
  • Nitrogen inertia of vessels and tanks
  • Sample flow monitoring for process analysis
  • Gas sparging in liquids
Oil and gas industry
  • Injection skids for corrosion-, scale- and hydrate-inhibitors
  • Nitrogen, flue gases, etc.
  • In addition, Emulsifiers
Machinery and apparatus 
  • Gas seal monitoring for compressors
  • Also, Coolants and cooling water, lubricants, air supply
  • Dry-run prevention of pumps
  • Gas and oil measurement for industrial furnaces
Pharmaceutical industry
  • Demineralized water, alcohol, solvents, insulin, etc.
  • Also, Cleaning agents
  • Air measurement in clean rooms
Food and beverage industry
  • Vegetable oil, drinking water, milk, carbon dioxide
  • Cleaning agents
Water and wastewater industry
  • Oxygen sparging into aeration basins
  • Air injection to reduce iron content

Conventional and nuclear power industry

  • Heavy water, borated water, tritium etc.
  • Hydrazine
  • Sulfuric acid
  • Phosphate, nitrogen, hydrogen, Sulfur
  • Purified water, saline water, etc.
  • Steam, air, gases, boiler washing water etc.
  • Diesel, return fuel oil flows, etc.

Moreover, are you looking for another type of Flow Meter?







Additional information

Weight 7 lbs
Dimensions 24 × 14 × 14 in

1/2", 3/4", 1", 1-1/2", 2", 3", 4", 6"

Pressure Class

150LB, 300LB, 600LB


Flanged, Triclamp


24VDC, Battery


No, Yes

Communication Protocol



4-20 MA, 0-10 VDC


General Duty, Explosion Proof, Intrinsically Safe




SRS Variable Area Flow Meter Manual V3-1


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