What is working principle of rotameter?

Rotameter is a variable area meter, i.e., it measures the area of flow, so as to produce a constant head differential. Therefore, rotameters are known as area meters.

Principle of rotameter

Rotameter consists of a vertical, tapered, and transparent tube in which a plummet is placed. During the fluid flow through the tube, the plummet rises and falls because of variation in flow. As a result, the area of the annular space between the plummet and the tube varies. The head loss across the annulus is equal to the weight of the plummet. The upper edge of the plummet is used as an index to note the reading on the tapered tube. This value indicates the flow of the fluid.

Construction of the Rotameter

Rotameter consists of a vertical tapered tube, which is mounted with a narrow end down (Figure 1.1). The tube is usually made of glass on which a linear scale is etched. A solid plummet is placed in the tube. The diameter of the plummet is smaller than the narrow end of the tube. Rotameters are available with electric and electronic transmitters for recording. Floats of different densities are available so that a 200-fold range of flow can be measured accurately. Floats may be made of lead, aluminum, glass, and plastic.

The Construction of a rotameter
Figure 1.1: The Construction of a rotameter

Working of rotameter

As the flow is upward through a tapered tube, the flow of fluid varies. The plummet, which is surrounded by fluid, rises and falls depending on the rate of flow. The greater the flow rate, the higher the plummet rises in the tube. In rotameters, the pressure drop is constant or nearly constant.

During the fluid flow, the area of the annular space between the plummet and the tube varies. Therefore, the head loss across the annulus is equal to the weight of the plummet. The flow may be read using the upper edge of the plummet as an index. The area is properly calibrated to the flow rate. The reading may be transmitted for recording, integrating, and controlling.

Normally, manufacturers supply the necessary data and charts along with the meter while purchasing the instruments.

Uses of Rotameter

Rotameters are extensively used in chemical industries, such as bulk drugs. In the fermenters, the supply of air is controlled through rotameters. Rotameters are satisfactory both for gases and for liquids at high and low pressures.


  1. The operator has a direct visual index of flow reading. It is satisfactory for manual control of processes for experimental work.
  2. It does not require the condition that straight pipes should run before and after the meter.
Make sure you also check our other amazing Article on : Laboratory Centrifuge
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