rotameters

Rotameters, or variable area flowmeters, are simple devices used to measure the flow rate of liquids and gases in a closed conduit. They consist of a vertically oriented tapered tube with a float inside. Fluid flows from the bottom up, and the float rises until the upward flow force balances the downward weight and buoyancy of the float. The position of the float is read on a scale outside the tube, providing a direct indication of the flow rate. The tube’s variable cross-sectional area creates the flow characteristics that make the reading proportional to the flow.

The tube is typically made of glass or clear plastic, with the float made from metal or dense plastic. The scale is calibrated for a specific fluid (and usually temperature) because density and viscosity affect the buoyancy and flow. Rotameters require a vertical installation with the flow upward; the device provides no external power and has a relatively small pressure drop. They are widely used for both liquids and gases, with variants selected for the fluid and pressure range.

Advantages and limitations: Rotameters are inexpensive, simple, and highly reliable in appropriate conditions, and offer immediate, visual flow indication. Limitations include modest accuracy (often a few percent of reading), sensitivity to changes in fluid density or temperature, and reduced suitability for very high viscosities, pulsed flows, or very high-pressure applications. For gases, compressibility can affect the reading, and proper calibration is important. They remain common in laboratories, chemical and petrochemical processing, water treatment, and gas distribution systems.