A “variable area meter” measures fluid flow by allowing the cross sectional area of the device to vary in response to the flow, causing some measurable effect that indicates the rate.
A rotameter is an example of a variable area meter, where a weighted “float” rises in a tapered tube as the flow rate increases; the float stops rising when area between float and tube is large enough that the weight of the float is balanced by the drag of fluid flow.
A kind of rotameter used for medical gases is the Thorpe tube flowmeter. Floats are made in many different shapes, with spheres and spherical ellipses being the most common. Some are designed to spin visibly in the fluid stream to aid the user in determining whether the float is stuck or not.
Rotameters are available for a wide range of liquids but are most commonly used with water or air. They can be made to reliably measure flow down to 1% accuracy.
A rotameter is an industrial flowmeter used to measure the flowrate of liquids and gases. 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 rotameter consists of a tube and float. The float response to flowrate changes is linear, and a 10-to-1 flow range or turndown is standard.
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.
