The Water Rotameter
is a form of variable area flow meter that has a simple operation whereby a liquid or gas passes through a conical tube. In order for this gas to pass through the pipe, it must first raise the float fixed in the pipe. When the Plastic Rotameter
is used with liquid, the float will rise due to the combined action of the velocity of the fluid and the buoyancy of the liquid.
For gas, its buoyancy is negligible, and the float moves largely due to the velocity and lift of the gas. In both cases, the higher the flow, the higher the tube that the float moves up. The float moves up and down in proportion to the flow rate and the annular area between the float and the tapered tube wall. When the float moves upward through the tube due to its tapered nature, the annular opening increases. With this increase, the pressure difference across the float decreases. When the weight of the float is balanced with the upward force exerted by the fluid or gas, the float is stable.
The float can then be compared with a calibration ruler, which can be printed on the test tube itself or placed next to the test tube outside the flowmeter. The calibrated scale will usually give a volume flow reading, such as liters per minute (LPM).