Rotameter Working Principle

The rotameter consists of a vertical tube with a hole in which the position of the buoy is considered vertical according to the flow rate through the tube (Fig. 3.41). For known flow rates, buoys remain stationary because of the vertical forces of differential pressure, gravity, viscosity, and the buoyancy will be balanced. So the ability to balance itself from a buoy hanging by a wire and depending on the area can be determined.

The down force (gravity minus the buoyant force) is constant and so the upward force (decreased pressure multiplied by the buoy) must also be constant. Assuming non compressive flow, the result is as follows:

Where,
Q = volume flow rate
C = emptying coefficient
A _t = area of ​​the tube
A _f = buoy area
V _f = volume of buoys
W _f = weight of buoy type
W _ff = the density of the fluid that flows

Figure 3.64 Rotameter

Buoys can be made from various materials to obtain the required density difference (W _f -W _ff ) to measure specific liquids or gases. Tube is often made from high-strength glass so that it can be done direct observation to the position of buoy.