What device is used to measure flow volume by changing cross-sectional area?

The Venturi meter is a device specifically designed to measure the flow volume of liquids by utilizing the principle of changing cross-sectional area. As fluid flows through the meter, it enters a section of reduced diameter. According to Bernoulli's principle, as the cross-sectional area decreases, the velocity of the fluid increases, leading to a pressure drop. By measuring the pressure difference between the wider and narrower portions of the meter, the flow rate can be calculated.

This principle makes the Venturi meter highly effective for accurately measuring flow rates in various applications, particularly in water treatment processes. In contrast, while a flow meter might also measure flow volume, it encompasses a broader category of devices that may not necessarily rely on changes in cross-sectional area to obtain their measurements. Therefore, the Venturi meter is the correct choice, as it specifically operates based on that principle.