What instrument uses a float in a tapered tube to indicate flow?

The instrument that uses a float in a tapered tube to indicate flow is a rotameter. A rotameter functions based on the principle of variable area flow measurement. As fluid flows through the tapered tube, the flow causes the float to rise to a position that is determined by the flow rate. The wider section of the tube allows for an increase in flow area as the flow increases, causing the float to move up, indicating higher flow rates. The position of the float corresponds directly to the flow rate, which is typically marked on a scale alongside the tube. This design allows for straightforward visual measurement of flow without the need for additional sensors or electronic components, making it a widely used instrument in various industries for monitoring fluid flow rates.