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

The device used to measure the flow rate of gases is a rotameter. A rotameter operates on the principle of variable area flow measurement, where a float placed in a tapered tube rises or falls based on the flow rate of the gas. The position of the float gives a direct visual indication of the flow rate, which can be calibrated to provide precise measurements. This makes rotameters particularly suitable for low to moderate gas flow applications.

Other devices mentioned serve different purposes. A manometer is primarily used to measure pressure differences in fluids, not flow rates. A flowmeter encompasses a wide range of devices for measuring flow, some of which can measure gas flow, but it is a more general term that includes various types of meters, including rotameters. A pressure gauge measures pressure within a system and does not provide information regarding flow rates. Thus, the specificity of the rotameter makes it the correct choice for measuring gas flow rates.