Rotameter

Rotameter

A rotameter, or variable area flowmeter, is a device used to measure the flow rate of liquids and gases in a conduit. It consists of a tapered metering tube, typically vertical, with a float inside. As fluid flows upward, the float rises until the buoyancy, weight, and drag forces reach equilibrium. The float’s position along a calibrated scale indicates the instantaneous flow rate. The scale is usually specific to the fluid and temperature.

Construction and operation

The metering tube is usually made of glass or clear plastic, with an inlet at the bottom and an outlet at the top. The float is often metal or plastic and may differ in shape to suit the fluid. The tube’s taper creates a variable cross-sectional area through which the fluid passes, hence the term variable area flowmeter. Rotameters are designed to be installed in a vertical orientation to ensure correct operation.

Calibration and range

Readings are direct, but accuracy depends on the fluid’s density and viscosity, temperature, and whether the device is clean and free of entrained air. Each instrument is calibrated for specific liquids or gases, and sometimes for a particular viscosity range. Some models offer multiple scales for different fluids.

Advantages and limitations

Rotameters are inexpensive, robust, and require no external power source, delivering immediate, visual flow indications. Limitations include moderate accuracy compared with electronic meters, sensitivity to installation orientation and fluid properties, and reduced suitability for very high pressures, highly viscous fluids, or pulsating flows.

Applications

They are widely used in chemical processing, water treatment, laboratories, pharmaceuticals, HVAC systems, and medical gas lines, where a simple, local flow readout is desirable.