What device is used to measure the flow rate of gases or liquids?

The device used to measure the flow rate of gases or liquids is commonly known as a rotameter. A rotameter operates on the principle of variable area flow measurement, where the flow rate causes a float to rise within a tapered tube. The position of the float correlates directly with the flow rate, allowing for a visual reading of the measurement. This design enables accurate and continuous monitoring of flow, making rotameters particularly useful in various applications, including water treatment systems and industrial processes.

The other devices listed serve distinct purposes: a barometer measures atmospheric pressure, a piezometer is used to measure fluid pressure in a column of liquid, and a hydrometer determines the specific gravity of liquids. None of these are designed specifically for measuring flow rates like the rotameter.