A rotameter – also known as a Variable Area flow meter – measures the flow rate of liquids and gases using a float that moves freely up and down in a conical measuring tube that widens upwards. The float's position in the tube reflects the flow rate: as the flow increases, the float is lifted until the buoyant force and gravity are in equilibrium. The reading is taken visually directly from a scale on the tube, or via magnetic coupling to an external indicator on metal tube versions. Rotameters are robust, typically do not require an external power supply, and are particularly well-suited for low flow rates. They are used in pharmaceutical laboratories and process plants for monitoring solvents and process gases, as well as in food and beverage production for controlling ingredient and cleaning medium flow. A limitation is that rotameters typically need to be mounted vertically with flow from bottom to top to ensure stable measurement.