The orificemeter and similar devices such as venturimeter and flow nozzle work on the principle of constant area variable pressure drop. Here the area of obstruction is constant, and the pressure drop changes with flow rate. On the other hand, rotameter works as a constant pressure drop variable area meter. It can only be used in a vertical pipeline. Its accuracy is also less than the other types of flowmeters. But it is simple in construction, ready to install, and the flow rate can be directly seen on a calibrated scale, without the help of any other device, e.g., differential pressure sensor, etc.
The construction of a rotameter is shown in Figure 18.8. It consists of a vertical pipe, tapered downward. The flow passes from the bottom to the top. There is cylindrical type metallic float inside the tube. The fluid flows upward through the gap between the tube and the float. As the float moves up or down there is a change in the gap, as a result changing the area of the orifice. In fact, the float settles down at a position, where the pressure drop across the orifice will create an upward thrust that will balance the downward force due to the gravity. The position of the float is calibrated with the flow rate.
Figure 18.8 Rotameter
From orificemeter: Volume of flow, 
Let us consider the upward and downward forces on the float are Fu and Fd, respectively. The apparent weight of float is W.
where ρ1 and ρ2 are specific weights of float and fluid; Vf and Af are volume of flow and area of flow respectively.
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