The Rotameter also known as variable area flowmeter is the most common among all flow measurement devices. Figure 2.12 shows the operation of a Rotameter. It basically consists of a tapered glass tube calibrated with a metering float that can move vertically up and down in the glass tube. Two stoppers one at the top and the other at the bottom of the tube prevent the float from leaving the glass tube. The fluid enters the tube through the inlet provided at the bottom. When no fluid is entering the tube, the float rests at the bottom of the tapered tube with one end of the float making contact with the lower stopper. The diameter of the float is selected in such a way that under conditions where there is no fluid entry into the tube, the float will block the small end of the tube completely.
When the fluid starts entering the tube through the inlet provided at the bottom, it forces the float to move upwards. This upward movement of the float will continue, until an equihbrium position is reached at which point the weight of the float is balanced by the upward force exerted by the fluid on the float. Greater the flow rate, higher is the float rise in the tube. The graduated tube allows direct reading of the flow rate.