Which sensor is typically used to measure air and gas flows?

The rotometer is the correct choice for measuring air and gas flows because it is specifically designed to measure the flow rates of gases and liquids. It operates on the principle of variable area flow measurement, where a float rises or falls within a tapered tube as the flow rate changes. This allows for a direct visual reading of the flow rate, making it particularly useful for monitoring air and gas systems.

In contrast, the manometer is primarily used to measure pressure differences, not flow rates. The Parshall flume and weirs are hydraulic structures typically employed for measuring the flow of liquids, particularly in open channels. They are not suited for air or gas flow measurements, hence making the rotometer the ideal sensor for the specific application of measuring air and gas flows.