When a rotameter[https://apureinstrument.com/flow-meter/float-flow-meter/] is used with a liquid, the float rises due to the combination of the velocity head of the fluid and the buoyancy of the liquid.
For gases, buoyancy is negligible, and the movement of the float is mainly due to the speed of the gas indenter. In both cases, the higher the flow rate, the higher the float moves upwards. The float moves up and down on the conical tube in proportion to the flow rate and the annular area between the float and the conical tube wall. Due to the tapered nature of the float, the annular opening increases as the float passes through the tube. As the pressure increases, the differential pressure through the float decreases. When the weight of the float is balanced with the upward force applied by the fluid or gas, the float stabilizes.
The float can then be compared to the calibration scale printed on the tube itself or placed next to the tube outside the gauge. Calibrated scales usually give volumetric flow readings, such as liters per minute (LPM). This is the working principle of rotameter[https://apureinstrument.com/blogs/working-principle-of-rotameter/]