ANS:D - rotameter.

A rotameter is a type of flow meter used to measure the flow rate of a fluid within a closed conduit, such as a pipe. It consists of a tapered tube, typically made of glass or plastic, with a float inside. The float is usually spherical or cylindrical and is free to move up and down the tapered tube. Here's how a rotameter typically works:

1. Tapered Tube: The rotameter's tapered tube has a larger diameter at the bottom (where the fluid enters) and gradually narrows towards the top (where the fluid exits). This tapering creates a variable cross-sectional area along the length of the tube.
2. Float: Inside the tapered tube, there is a float that moves freely with the flow of the fluid. The float is designed to have a specific density that allows it to respond to the flow of the fluid. The float rises or falls in the tube in response to changes in the flow rate.
3. Flow Measurement: As the fluid flows through the rotameter, it exerts a force on the float. The force is a combination of the fluid's buoyancy and drag forces acting on the float. At a particular flow rate, the buoyancy force acting on the float balances the gravitational force, and the float achieves equilibrium, indicating a specific flow rate.
4. Readout: The flow rate is typically indicated by the position of the float relative to a scale printed on the outside of the tube. The scale is calibrated to provide a direct reading of the flow rate based on the position of the float.

Rotameters are commonly used for measuring the flow of gases and liquids in various industrial processes, laboratories, and research facilities. They are popular because they are relatively simple, cost-effective, and easy to install. However, their accuracy can be influenced by factors such as the fluid's viscosity, temperature, and pressure, as well as the orientation of the rotameter. Overall, rotameters provide a convenient and visual means of monitoring flow rates in many applications where precision measurement is not critical. They are particularly useful for applications where simplicity, reliability, and low cost are priorities.