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.
It 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|
|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
- 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 the 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
- The outer glass layer can easily break due to the 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 the case of parallax error.
- Since the rotameter is made up of a glass tube, it might break if you do not handle it properly.
- You need to avoid parallax error while taking the reading.
- You can use an alcohol swab to clean the outer layer of the glass.