click to enlarge
A rotameter, also sometimes called a 'variable area' flow meter, is simply a tapered plastic or glass tube with a float whose position in the tube is determined by the flow rate of fluid through the rotameter. The diagram at the left illustrates this with the tapering of the tube accentuated. Pictures of a couple of rotameters are shown
click to enlarge
at the right.
As a fluid flows through the rotameter at a greater flow rate, a larger cross-sectional area of flow is needed, so the 'float' rises until it reaches an equilibrium position where the upward force on it by the fluid is the same as the downward force of gravity. Note that the density of the 'float' must be greater than the density of the fluid or it would simply float to the top of the fluid at any flow rate. When used for pipe flow measurement with a particular fluid, the position of the float in the tube can be calibrated to read flow rate directly for that fluid.
Because of the way that gravity helps to determine the float position, a rotameter must be installed vertically, as shown in the diagrams. Also it should be mentioned that a rotameter works well for visually noting flow rate readings, but it doesn't lend itself well to machine reading and continuous recording of flow rate.
