What Is Variable Area Flow Meter?

 Variable Area Flow Meter

A Variable Area Flow Meter is a flow meter that can measure the volumetric flow of liquid, gas or steam. It is a type of differential pressure flowmeter. Also called a rotameter.

Within the Variable Area Flow Meter, gas or liquid moves the float until equilibrium is reached. The user can read the gas or liquid flow rate from the scale next to the tube or from the scale on the tube. Variable Area Flow Meter is a cost-effective flow measurement solution for small diameter, low flow gas, liquid, and steam.

variable area flow meter working principle

The operation of the variable area flow meter is based on the variable area principle:

The variable area flow meter consists of two parts. One part of the rotameter is a tapered tube that gradually expands from bottom to top; The other piece of the rotameter is a rotor placed in a tapered tube that can move freely up and down along the center line of the tube.

variable area flow meter working principle

When measuring the flow rate of a fluid, the fluid to be measured flows in from the lower end of the tapered tube. The flow of the fluid impacts the rotor and produces a force on it (the size of this force changes with the flow rate). When the flow rate is large enough, the The force generated lifts the rotor and raises it. At the same time, the measured fluid flows through the annular section between the rotor and the tapered tube wall and flows out from the upper end.

When the force exerted on the rotor by the measured fluid flows is exactly equal to the weight of the rotor in the fluid (called the displayed weight), the rotor is in a balanced state and stays at a certain height.

Analysis shows that there is a corresponding relationship between the position and height of the rotor in the tapered tube and the flow rate passing through it. Therefore, by observing the position and height of the rotor in the tapered tube, the corresponding flow value can be obtained.

Featured Variable Area Flow Meters

Advantages and disadvantages of variable area flow meter


  1. High accuracy: The rotameter adopts a mechanical structure and can achieve high measurement accuracy. Suitable for occasions with high flow requirements.
  2. Good stability: The rotameter has a simple and reliable structure, good stability, and is not easily affected by external interference. Effectively ensure the accuracy of measurement results.
  3. Wide range of applications: Rotameter can measure the flow of various media such as liquid, gas and steam. It has a wide range of applications and strong versatility.
  4. Easy to maintain: The main components of the rotameter are the rotor and bearings, and the structure is relatively simple. Easy to maintain and clean, no additional specialized equipment is required.
  5. Low cost: Rotameters are less expensive to manufacture than some other types of flowmeters.


  1. Large pressure loss: Since there is a circulation channel inside the rotameter, a certain resistance will be generated when the medium passes through, resulting in a certain pressure loss.
  2. Rigorous fluid requirements: Rotameters are sensitive to the properties of the medium fluid. Factors such as viscosity and temperature have a great influence on the measurement results. The parameters of the medium need to be strictly controlled.
  3. Limited flow range: The measurement range of the rotameter is relatively narrow. For larger or smaller flow measurements it may not be sufficient.
  4. Affected by impurities in the medium: If there are impurities such as particles and fibers in the medium, they will adhere to the rotor, resulting in low accuracy. Requires routine cleaning and maintenance.

variable area flow meter types

Common ones may include plastic tube variable area flow meters, glass tube variable area flow meters and metal rotor flow meters. This is distinguished by material. In fact, variable area flow meters can be divided into different categories according to different classification standards. The following content is for your reference.

Glass Tube Rotameter:
The most commonly used material for transparent conical tubes is glass. If the instrument without a guide structure is used to measure gas if it is not handled carefully, the glass tube can be easily broken;

Plastic tube rotor flow meter:
It is also made of transparent engineering plastics such as polystyrene, polycarbonate, plexiglass, etc., which has the advantage of not being easily broken.

Metal Tube Rotameter:
Compared with the transparent conical tube rotameter, it can be used at higher medium temperatures and pressures, and there is no potential danger of the cone tube of the glass tube rotameter being broken.

Local indication type rotameter

Some transparent tube rotor flowmeters mainly provide local indication. Equipped with a proximity switch for upper and lower flow limit alarm signal output.

Some local indication metal tube rotor flowmeters have the same appearance as remote signal output. It only transmits the rotor displacement through magnetic coupling, and then indicates it locally after being processed by linearization mechanisms such as connecting rod cams.

Remote signal output rotor flowmeter

The conversion part of the remote signal output type instrument converts the rotor displacement into current or air pressure analog signal output. They are respectively called electric remote rotor flowmeter and gas remote rotor flowmeter.

There are three types: liquid, gas and steam.

In fact, most rotameter flowmeters can be used for liquids and gases in the same instrument, and their structures are universal.

It’s just that my country’s rotameter industry standards (such as JB/T 6844-93) stipulate that the upper flow rate Qmax must meet the requirements of (1, 1.6, 2.5, 4 or 6) × 10nL/h (n is a positive or negative integer or zero). When the instrument designed for liquid (represented by water) is used for gas (represented by air), it does not meet the above requirements. The float and cone can only be designed separately for gas, which is divided into two series: liquid and gas.

To measure steam, you can only use a specially designed metal tube rotameter or add additional components to the standard instrument. For example, add a liquid damping component with a heat sink to reduce the float jump. The connection with the indication conversion part is separated by a heat sink.

Full flow type: that is, an instrument in which all the measured fluid flows through the rotameter.

Split flow type: Compared with the full flow type, only part of the measured fluid flows through the flow detection part such as the rotor. The split-flow rotameter is composed of a standard orifice plate (or velocity-average tube) on the main pipeline and a smaller-diameter float flowmeter. It is suitable for larger flow rates with a pipe diameter greater than 50mm and in places where local indication is required. It is low-priced. Split-flow rotameter is divided into two types: separate type and integrated type.

How is a variable area flow meter different from an orifice meter?

Simply put, the variable area flow meter uses the gravity of the rotor to measure the flow, and the orifice plate uses the pressure difference before and after the fluid to measure the flow.

  • Both orifice flowmeters and metal rotor flowmeters can be used to measure steam flow.
  • In terms of accuracy, the accuracy of the orifice flowmeter is higher than that of the metal rotor flowmeter.
  • Structurally speaking, the orifice flowmeter has no moving parts, while the metal rotor flowmeter has moving parts.
  • In terms of lifespan and maintenance, the orifice flowmeter is simple to use and easy to maintain.

Read more about: Orifice Plate Flow Meter.

Order Guide

  1. First consider the media properties. Chemical resistance to fluids The most common float material is 316 stainless steel, however, for some special applications, Hastelloy, aluminum and PVC can be used.
  2. What are the minimum flow rate and maximum flow rate of the flow meter?
  3. What are the minimum and maximum process temperatures?
  4. What are the pipe sizes?
  5. Do you want to use a direct-reading rotameter or are you okay with reading the results using a lookup table?
  6. How much precision is required?
  7. Is it necessary to use a valve to regulate flow?
  8. Is there back pressure?
  9. What is the maximum process pressure?

More Featured Flow Meters

Variable Area Flow Meter is the most commonly used flow meter in industry and laboratories. It is also called a rotameter. It can be widely used in flow measurement and process control in complex, harsh environments and various media conditions.

Our Sino-Inst mainly supplies metal Variable Area Flow Meters, which can be customized for high temperature, high pressure, corrosive media, etc. measurements. If you need to purchase Variable Area Flow Meter or have related technical questions, please feel free to contact us.

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