Rotameter- Working Principle, Uses, Pros, Cons, and Handling

Rotameter is a type of flowmeter which is also known as a variable area flowmeter. It is used to measure the velocity of air and liquid (water, oil, etc.) in industrial sectors. Besides that, it is also used in medical instruments such as oxygen concentrators, oxygen cylinders, and anesthesia machines. However, even for the same flow rate, the size, and the shape can alter for different models of these machines. Here we will be talking about the working principle of the rotameter.

Besides that, we will also be talking about the pros, cons, and handling process of the rotameter.

Rotameter Working Principle

Rotameter Working Principle

Rotameter consists of an outer vertical tapered glass tube with a narrow base and broad top. Since there is variation in the cross-sectional area of the tube, it is known as a variable area flowmeter. Inside the tube consists of a metering float. The glass tube consists of readings built in it.

When there is no flow of fluid through the rotameter, the float lies at the base of the tube. When the fluid flows to the tube through the base, it pushes the float upward. The displacement covered by the float from the base is directly proportional to the pressure exerted by the fluid to the float.

Three forces act on the float. They are as follows.

Force Direction of force Type of force
Weight (w) Downward Constant
Buoyancy (b) Upward Constant
Drag/ Blow force (d) Upward Variable

So, w = b + d (at equilibrium)

When the fluid pressure increases, the float rises. As the cross-sectional area of the glass tube, is more at the top than at the bottom, the fluid moving upward begins to escape from the sideways of the float. So, even on increasing the fluid pressure, the variable drag force comes back to its original stage. Thus the equilibrium is again maintained.

You will find labeling in the glass tube from 1 to 12 or more (based on the model) to indicate the flow rate of gas which is denoted by the rise of metering float in the rotameter.

Can rotameter be used horizontally?

No, it cannot be used horizontally. As we know the direction of weight (mg) lies vertically downward. So, it will have its horizontal component mgcos2700= 0. In other words, there will not be any force in the horizontal direction. So, equilibrium cannot be maintained between buoyancy and drag force.

Advantages of Rotameter

The variable area flowmeter can have a number of advantages. Some advantages of rotameter are stated below.

  • It is cheap and easily available in the market.
  • It provides a linear scale of measurement. Thus, taking a reading becomes simpler.
  • Rotameter is suitable for the measurement of fluids with a velocity of 1 LPM to 10 LPM. In other words, it is suitable for the measurement of fluid with small or medium velocity.
  • Even the corrosive fluids can be measured by the rotameter. This is because the metal and the glass cover are highly inert to various kinds of chemicals.
  • It provides higher accuracy and precision.

Disadvantages of Rotameter

Even though the rotameter is useful for multiple applications, it has got some drawbacks of its own. So, some of the disadvantages are stated below.

  • The outer glass layer can easily break due to fluid pressure. The glass layer can also break during maintenance or transportation of the machine.
  • Rotameter should be held upright. If you tilt it, it will cause an inaccuracy during measurement.
  • The fluid for which you are planning to measure the velocity should be transparent. Else, you will not be able to see the metering float properly. In that case, you will not be able to take the measurement.
  • It is not suitable for fluids having a high velocity.
  • If you do not position your eyes to the same level as that of the metering float, then there can be a case of parallax error.

Handling process

You need to take care of the following things while handling the rotameter.

  • Since the rotameter is made up of a glass tube, it might break if you do not handle it properly.
  • Do not set the flowmeter to ‘0′ while operating. This will obstruct the flow of the pressurized air. Hence, it can cause permanent damage to the rotameter. The flow rate should be at least 0.5 liters per min.
  • You need to avoid parallax errors while taking the reading.
  • You can use an alcohol swab to clean the outer layer of the glass.
  • Sometimes the float gets stuck at the base of the flowmeter. This will block the flow of air. Hence, you will not get any output from it. In such a case, invert the flowmeter upside down and try to move the float from the base. If you are unable to invert the flowmeter, then try using a gust of wind to remove it.



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