Which of the following instruments is used for measuring air velocity?

The instrument used for measuring air velocity is the rotameter. A rotameter functions by allowing air to flow through a tapered tube, causing a float inside the tube to rise to a position that corresponds to the air velocity. The height of the float against a calibrated scale indicates the flow rate of the air, effectively providing a measurement of air velocity.

In the context of industrial hygiene and air quality management, accurate measurement of air velocity is critical for assessing ventilation effectiveness, designing air handling systems, and ensuring compliance with health and safety regulations. Utilizing a rotameter allows for quick and reliable measurements, making it an essential tool in environments where air movement is a key factor.

Other instruments, such as the micrometer, are used for measuring small distances or thicknesses and are not applicable for measuring air velocity. Similarly, manometers are used to measure pressure differences, and hydrometers are designed to measure the density of liquids. Hence, while those instruments serve important functions, they do not measure air velocity, reinforcing why the rotameter is the correct choice.